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MIC2085/2086
Micrel
MIC2085/MIC2086
Single Channel Hot Swap Controllers
General Description
The MIC2085 and MIC2086 are single channel positive voltage hot swap controllers designed to allow the safe insertion of boards into live system backplanes. The MIC2085 and MIC2086 are available in 16-pin and 20-pin QSOP packages, respectively. Using a few external components and by controlling the gate drive of an external N-Channel MOSFET device, the MIC2085/86 provide inrush current limiting and output voltage slew rate control in harsh, critical power supply environments. Additionally, a circuit breaker function will latch the output MOSFET off if the current limit threshold is exceeded for a programmed period of time. The devices' array of features provide a simplified yet robust solution for many network applications in meeting the power supply regulation requirements and affords protection of critical downstream devices and components. All support documentation can be found on Micrel's web site at www.micrel.com.
Features
* MIC2085: Pin for pin functional equivalent to the LTC1642 * 2.3V to 16.5V supply voltage operation * Surge voltage protection to 33V * Operating temperature range -40C to 85C * Active current regulation limits inrush current independent of load capacitance * Programmable inrush current limiting * Analog foldback current limiting * Electronic circuit breaker * Dual-level overcurrent fault sensing * Fast response to short circuit conditions (< 1s) * Programmable output undervoltage detection * Undervoltage lockout protection * Power-on reset (MIC2085/86) and power-good (MIC2086) status outputs * /FAULT status output * Driver for SCR crowbar on overvoltage
Applications
* * * * * * RAID systems Cellular base stations LAN servers WAN servers InfiniBandTM Systems Industrial high side switching
Typical Application
Backplane PCB Edge Connector Connector
VIN 12V
Long Pin R1 3.3 C1 1F
3
1
RSENSE 0.007 2% 2
4
Q1 Si7884DP (PowerPAKTM SO-8)
VOUT 12V@5A
CLOAD 220F
Short Pin
16 15
*R6 10
R7 127k 1%
VLOGIC
GATE COMP+
14 11 10
VCC R5 47k R2 100k 1%
4
SENSE
R10 47k
R11 47k
ON /FAULT
COMPOUT
Output Signal (Power Good)
/FAULT
R3 Medium 1.82k 1% (or Short) Pin R4 10k 1%
6
C2 0.022F MIC2085 /POR FB
5 7
PWRGD LOGIC CONTROLLER /RESET
9 12 13
OV COMP-- REF CPOR
3
Power-On Reset Output R8 16.2k 1% C7 0.033F
CRWBR GND
8
1
Q2 2N4401
CFILTER
2
Q3 TCR22-4
GND
Long Pin
C3 0.1F
C4 0.1F
C5 8200pF
C6 0.01F
**R9 180
Overvoltage (Input) = 13.3V Undervoltage Lockout = 10.8V Undervoltage (Output) & Power-Good (Output) = 11.4V
POR/START-UP DELAY = 60ms Circuit-Breaker Response Time = 500s *R6 is an optional component used for noise filtering **R9 needed when using a sensitive gate SCR
InfiniBand is a trademark of InfiniBand Trade Association PowerPAK is a trademark of Vishay Intertechnology Inc. Micrel, Inc. * 1849 Fortune Drive * San Jose, CA 95131 * USA * tel + 1 (408) 944-0800 * fax + 1 (408) 944-0970 * http://www.micrel.com
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Micrel
Ordering Information
Part Number MIC2085-xBQS Fast Circuit Breaker Threshold x = J, 95mV x = K, 150mV* x = L, 200mV* x = M, Off x = J, 95mV x = K, 150mV* x = L, 200mV* x = M, Off Discharge Output NA Package 16-pin QSOP
MIC2086-xBQS
Yes
20-pin QSOP
*Contact factory for availability.
Pin Configuration
CRWBR 1 CFILTER 2 20 VCC 19 VCC 18 SENSE 17 GATE 16 REF 15 DIS 14 COMP- 13 COMP+ 12 COMPOUT 11 OV
CRWBR 1 CFILTER 2 CPOR 3 ON 4 /POR 5 /FAULT 6 FB 7 GND 8
16 VCC 15 SENSE 14 GATE 13 REF 12 COMP- 11 COMP+ 10 COMPOUT 9 OV
CPOR 3 ON 4 /POR 5 PWRGD 6 /FAULT 7 FB 8 GND 9 GND 10
MIC2085 16-Pin QSOP (QS)
MIC2086 20-Pin QSOP (QS)
Pin Description
Pin Number MIC2086 1 Pin Number MIC2085 1 Pin Name CRWBR Pin Function Overvoltage Timer and Crowbar Circuit Trigger: A capacitor connected to this pin sets the timer duration for which an overvoltage condition will trigger an external crowbar circuit. This timer begins when the OV input rises above its threshold as an internal 45A current source charges the capacitor. Once the voltage reaches 470mV, the current increases to 1.5mA. Current Limit Response Timer: A capacitor connected to this pin defines the period of time (tOCSLOW) in which an overcurrent event must last to signal a fault condition and trip the circuit breaker. If no capacitor is connected, then tOCSLOW defaults to 5s. Power-On Reset Timer: A capacitor connected between this pin and ground sets the start-up delay (tSTART) and the power-on reset interval (tPOR). When VCC rises above the UVLO threshold, the capacitor connected to CPOR begins to charge. When the voltage at CPOR crosses 1.24V, the start-up threshold (VSTART), a start cycle is initiated if ON is asserted while capacitor CPOR is immediately discharged to ground. When the voltage at FB rises above VFB, capacitor CPOR begins to charge again. When the voltage at CPOR rises above the power-on reset delay threshold (VTH), the timer resets by pulling CPOR to ground, and /POR is deasserted. If CPOR = 0, then tSTART defaults to 20s.
2
2
CFILTER
3
3
CPOR
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Pin Description (Cont.)
Pin Number MIC2086 4 Pin Number MIC2085 4 Pin Name ON Pin Function ON Input: Active high. The ON pin, an input to a Schmitt-triggered comparator used to enable/disable the controller, is compared to a VTH reference with 100mV of hysteresis. Once a logic high is applied to the ON pin (VON > 1.24V), a start-up sequence is initiated as the GATE pin starts ramping up towards its final operating voltage. When the ON pin receives a low logic signal (VON < 1.14V), the GATE pin is grounded and /FAULT is high if VCC is above the UVLO threshold. ON must be low for at least 20s in order to initiate a start-up sequence. Additionally, toggling the ON pin LOW to HIGH resets the circuit breaker. Power-On Reset Output: Open drain N-Channel device, active low. This pin remains asserted during start-up until a time period tPOR after the FB pin voltage rises above the power-good threshold (VFB). The timing capacitor CPOR determines tPOR. When an output undervoltage condition is detected at the FB pin, /POR is asserted for a minimum of one timing cycle, tPOR. The /POR pin has a weak pull-up to VCC. Power-Good Output: Open drain N-Channel device, active high. When the voltage at the FB pin is lower than 1.24V, the PWRGD output is held low. When the voltage at the FB pin is higher than 1.24V, then PWRGD is asserted. A pull-up resistor connected to this pin and to VCC will pull the output up to VCC. The PWRGD pin has a weak pull-up to VCC. Circuit Breaker Fault Status Output: Open drain N-Channel device, active low. The /FAULT pin is asserted when the circuit breaker trips due to an overcurrent condition. Also, this pin indicates undervoltage lockout and overvoltage fault conditions. The /FAULT pin has a weak pull-up to VCC. Power-Good Threshold Input: This input is internally compared to a 1.24V reference with 3mV of hysteresis. An external resistive divider may be used to set the voltage at this pin. If this input momentarily goes below 1.24V, then /POR is activated for one timing cycle, tPOR, indicating an output undervoltage condition. The /POR signal de-asserts one timing cycle after the FB pin exceeds the power-good threshold by 3mV. A 5s filter on this pin prevents glitches from inadvertently activating this signal. Ground Connection: Tie to analog ground. OV Input: When the voltage on OV exceeds its trip threshold, the GATE pin is pulled low and the CRWBR timer starts. If OV remains above its threshold long enough for CRWBR to reach its trip threshold, the circuit breaker is tripped. Otherwise, the GATE pin begins to ramp up one POR timing cycle after OV drops below its trip threshold. Uncommitted Comparator's Open Drain Output. Comparator's Non-Inverting Input. Comparator's Inverting Input. Discharge Output: When the MIC2086 is turned off, a 550 internal resistor at this output allows the discharging of any load capacitance to ground. Reference Output: 1.24V nominal. Tie a 0.1F capacitor to ground to ensure stability. Gate Drive Output: Connects to the gate of an external N-Channel MOSFET. An internal clamp ensures that no more than 13V is applied between the GATE pin and the source of the external MOSFET. The GATE pin is immediately brought low when either the circuit breaker trips or an undervoltage lockout condition occurs.
5
5
/POR
6
N/A
PWRGD
7
6
/FAULT
8
7
FB
9,10 11
8 9
GND OV
12 13 14 15 16 17
10 11 12 NA 13 14
COMPOUT COMP+ COMPDIS REF GATE
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Pin Description (Cont.)
Pin Number MIC2086 18 Pin Number MIC2085 15 Pin Name SENSE Pin Function Circuit Breaker Sense Input: A resistor between this pin and VCC sets the current limit threshold. Whenever the voltage across the sense resistor exceeds the slow trip current limit threshold (VTRIPSLOW), the GATE voltage is adjusted to ensure a constant load current. If VTRIPSLOW (48mV) is exceeded for longer than time period tOCSLOW, then the circuit breaker is tripped and the GATE pin is immediately pulled low. If the voltage across the sense resistor exceeds the fast trip circuit breaker threshold, VTRIPFAST, at any point due to fast, high amplitude power supply faults, then the GATE pin is immediately brought low without delay. To disable the circuit breaker, the SENSE and VCC pins can be tied together. The default VTRIPFAST for either device is 95mV. Other fast trip thresholds are available: 150mV, 200mV, or OFF(VTRIPFAST disabled). Please contact factory for availability of other options. 19,20 16 VCC Positive Supply Input: 2.3V to 16.5V. The GATE pin is held low by an internal undervoltage lockout circuit until VCC exceeds a threshold of 2.18V. If VCC exceeds 16.5V, an internal shunt regulator protects the chip from VCC and SENSE pin voltages up to 33V.
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Absolute Maximum Ratings(1)
(All voltages are referred to GND) Supply Voltage (VCC) ..................................... -0.3V to 33V SENSE Pin .......................................... -0.3V to VCC + 0.3V GATE Pin ....................................................... -0.3V to 22V ON, DIS, /POR, PWRGD, /FAULT, COMP+, COMP-, COMPOUT ....................... -0.3V to 20V CRWBR, FB, OV, REF ..................................... -0.3V to 6V Maximum Currents Digital Output Pins ..................................................... 10mA (/POR, /FAULT, PWRGD, COMPOUT) DIS Pin ....................................................................... 30mA ESD Rating: Human Body Model ................................................... 2kV Machine Model ........................................................ 200V
Operating Ratings(2)
Supply Voltage (VCC) .................................... 2.3V to 16.5V Operating Temperature Range .................. -40C to +85C Junction Temperature (TJ) ........................................ 125C Package Thermal Resistance R(J-A) 16-pin QSOP ..................................................... 112C/W 20-pin QSOP ....................................................... 91C/W
Electrical Characteristics(3)
VCC = 5.0V, TA = 25C unless otherwise noted. Bold indicates specifications over the full operating temperature range of -40C to +85C. Symbol VCC ICC VUV VUVHYST VFB VFBHYST VOV VOV VOVHYST IOV VTH ICPOR ITIMER VCR VCR ICR Parameter Supply Voltage Supply Current Undervoltage Lockout Threshold UV Lockout Hysteresis FB (Power-Good) Threshold Voltage FB rising FB Hysteresis OV Pin Threshold Voltage OV Pin Threshold Voltage Line Regulation OV Pin Hysteresis OV Pin Current POR Delay and Overcurrent (CFILTER) VCPOR, VCFILTER rising Timer Threshold Power-On Reset Timer Current Current Limit /Overcurrent Timer Current (CFILTER) CRWBR Pin Threshold Voltage CRWBR Pin Threshold Voltage Line Regulation CRWBR Pin Current Timer on Timer off Timer on Timer off 2.3V < VCC < 16.5V 2.3V < VCC < 16.5V CRWBR On, VCRWBR = 0V CRWBR On, VCRWBR = 2.1V CRWBR Off, VCRWBR = 1.5V VTRIP = VCC -VSENSE 2.3V VCC 16.5V VTRIPSLOW VTRIPFAST x=J x=K x=L -60 1.19 -2.5 -30 445 1.24 -2.0 5 -20 2.5 470 4 -45 -1.5 3.3 48 95 150 200 8 12 21-VCC OV pin rising 2.3V < VCC < 16.5V 1.19 1.19 VCC rising VCC falling 2.05 1.85 Condition Min 2.3 1.6 2.18 2.0 180 1.24 3 1.24 5 3 0.2 1.29 -1.5 -15 495 15 -30 -1.0 55 110 1.29 15 1.29 Typ Max 16.5 2.5 2.28 2.10 Units V mA V V mV V mV mV mV mV A V A mA A mA mV mV A mA mA mV mV mV mV V V V
VTRIP
Circuit Breaker Trip Voltage (Current Limit Threshold)
40 80
VGS
External Gate Drive
VGATE - VCC
VCC < 3V 5V < VCC < 9V 9V < VCC < 15.0V
4 11 4.5
9 13 13
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Electrical Characteristics (Cont.)
Symbol IGATE Parameter GATE Pin Pull-up Current Condition Start cycle, VGATE = 0V VCC =16.5V VCC = 2.3V /FAULT = 0, VGATE>1V VCC = 16.5V VCC = 2.3V ON rising ON falling Min -22 -20 25 12 1.19 1.09 Typ -16 -14 50 20 1.24 1.14 100 VON = VCC VCPOR rising IOUT = 1.6mA (PWRGD for MIC2086 only) /FAULT, /POR, PWRGD = GND (PWRGD for MIC2086 only) ILOAD = 0mA; CREF = 0.1F 2.3V < VCC < 16.5V IOUT = 1mA VREF= 0V VCM = VREF VCM = VREF ON pin toggles from HI to LOW 100 -5 3 550 1000 1.21 -20 1.24 5 2.5 3.5 5 1.27 10 7.5 1.19 1.24 0.5 1.29 0.4 1.29 1.19 Max -8 -8 Units A A mA mA V V mV A V V A V mV mV mA mV mV
IGATEOFF
GATE Pin Sink Current
VON VONHYST ION VSTART VOL IPULLUP VREF VLNR VLDR IRSC VCOS VCHYST RDIS
ON Pin Threshold Voltage ON Pin Hysteresis ON Pin Input Current Undervoltage Start-up Timer Threshold /FAULT, /POR, PWRGD Output Voltage Output Signal Pull-up Current /FAULT, /POR, PWRGD, COMPOUT Reference Output Voltage Reference Line Regulation Reference Load Regulation Reference Short-Circuit Current Comparator Offset Voltage Comparator Hysteresis Discharge Pin Resistance
AC Electrical Characteristics(4)
Symbol tOCFAST Parameter Fast Overcurrent Sense to GATE Low Trip Time Slow Overcurrent Sense to Gate Low Trip Time ON Delay Filter FB Delay Filter Condition VCC = 5V VCC -VSENSE = 100mV CGATE = 10nF, See Figure 1 VCC = 5V VCC -VSENSE = 50mV CFILTER = 0, See Figure 1 Min Typ 1 Max Units s
tOCSLOW
5
s
tONDLY tFBDLY
Notes:
20 20
s s
1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Specification for packaged product only. 4. Specification for packaged product only.
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Timing Diagrams
VTRIPFAST 48mV (VCC VSENSE) 0 tOCSLOW tOCFAST
VGATE 0
1V
1V
1.24V CFILTER 0
Figure 1. Current Limit Response
1.24V FB 0 tPOR 1.24V CPOR 0
/POR 0
Figure 2. Power-On Reset Response
tONDLY Arm Fast Comparator 1.24V ON 0 tSTART 1.24V CPOR 0 tPOR Arm Slow Comparator
GATE 0 1.24V FB 0
/POR 0
Figure 3. Power-On Start-Up Delay Timing
Current Limit Threshold (mV)
50
20
0
200
400
600
800
1000
FB Voltage (mV)
Figure 4. Foldback Current Limit Response January 2004 7
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Typical Characteristics
Supply Current vs. Temperature Power-On Reset Timer Current vs. Temperature Power-On Reset Timer (Off) Current vs. Temperature
10 9 8 ICPOR (mA) 7 6 5 4 3 2 VCC = 2.3V VCC = 16.5V
4.0 SUPPLY CURRENT (mA) 3.5 3.0 2.5 2.0 1.5 1.0 0.5
2.6 2.4
ICPOR (A)
VCC = 16.5V VCC = 5V
2.2 2.0 1.8 1.6
VCC = 5V
VCC = 16.5V
VCC = 5V
VCC = 2.3V
VCC = 2.3V
0.0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.4 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
34 30
Overcurrent Timer Current vs. Temperature
Overcurrent Timer (Off) Current vs. Temperature
5 4
ITIMER (mA)
30 25
Gate Pull-Up Current vs. Temperature
ITIMER (A)
22 18 14
VCC = 16.5V
3 2 1 VCC = 2.3V VCC = 5V
IGATE (A)
26
VCC = 16.5V
20 15 10 5
VCC = 16.5V
VCC = 5V
VCC = 2.3V
VCC = 2.3V
VCC = 5V
10 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
25 20
Gate Pull-Up Current vs. VCC
16 14 12 VGS (V)
External Gate Drive vs. Temperature
VCC = 5V
IGATE ( A)
8 6 4 2 VCC = 2.3V
10 5 0
2
4
6
8
10 12 14 16 18 VCC
(V)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
VGS (V)
15
10
VCC = 16.5V
22 20 18 16 14 12 10 8 6 4 2 0 2
External Gate Drive vs. VCC
4
6
8
10 12 14 16 18 VCC (V)
100 90 80
Gate Sink Current vs. Temperature
600 500
IGATEOFF (mA)
Gate Sink Current vs. Gate Voltage
1.25 1.24 VTH (mV)
POR Delay/Overcurrent Timer Threshold vs. Temperature
VCC = 16.5V
IGATEOFF (mA)
70 60 50 40 30 20 VCC = 5V
400 12VCC 300 200 100 0 0 2 4 6 5VCC
1.23 VCC = 2.3V 1.22 1.21 VCC = 5V
VCC = 16.5V
VCC = 2.3V
10 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
8
10
12
14
VGATE (V)
1.20 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
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Typical Characteristics
Current Limit Threshold (Fast Trip) vs. Temperature
55 53 VTRIPSLOW (mV) VCC = 2.3V 51 49 VCC = 5V 47 VCC = 16.5V VCC = 2.3V
120 115 110
Current Limit Threshold (Slow Trip) vs. Temperature
UVLO THRESHOLD (V)
2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8
UVLO Threshold vs. Temperature
UVLO+
VTRIPFAST (mV)
105 100 95
90 V = 5V CC 85
VCC = 16.5V
UVLO-
80 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
45 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.7 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.30 ON THRESHOLD (V)
ON Pin Threshold (Rising) vs. Temperature
1.20 ON THRESHOLD (V)
ON Pin Threshold (Falling) vs. Temperature
ON PIN INPUT CURRENT (nA)
40 35 30 25 20 15 10 5
ON Pin Input Current vs. Temperature
VCC = 16.5V 1.25 VCC = 2.3V
VCC = 2.3V 1.15
VCC = 5V 1.20
VCC = 16.5V VCC = 2.3V
1.10
VCC = 16.5V VCC = 5V
VCC = 5V
1.15 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.05 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.30
FB THRESHOLD (V)
OVERVOLTAGE PIN THRESHOLD (V)
FB (Power-Good) Threshold vs. Temperature
1.30
Overvoltage Pin Threshold vs. Temperature
26
Output Signal Pull-Up Current vs. Temperature
VCC = 16.5V VCC = 5V
VCC = 16.5V 1.25 VCC = 2.3V
VCC = 16.5V
IPULLUP (A)
22
1.25 VCC = 2.3V 1.20
18
VCC = 2.3V
1.20
VCC = 5V
14
1.15 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
1.15 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
10 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
COMPARATOR OFFSET VOLTAGE (V)
0.5 0.4 0.3 0.2 0.1
Comparator Offset Voltage vs. Temperature
Discharge Pin Resistance vs. Temperature
1000 900 800
VCC = 5V
RDIS ()
700 600 500 400 300
2.3V
5V
16.5V
VCC = 16.5V VCC = 2.3V
0.0 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
200 -40 -20 0 20 40 60 80 100 TEMPERATURE (C)
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Test Circuit
VIN 12V R2 154kW 1% SW1 ON/OFF R3 20kW 1%
4
IIN C1 0.47mF
3
RSENSE 0.01W 1 5% 2
4
Q1 Si7892DP (PowerPAKTM SO-8) R1 10W GATE FB
17
IOUT VOUT R4 97.6kW 1% RLOAD CLOAD
VCC ON
19,20
SENSE
18
C2 0.022mF R5 12.4kW 1% Downstream Signal
8
MIC2086
/POR
5
CPOR 3 C3 0.047mF Not all pins shown for clarity.
DIS GND CFILTER 9,10 2 C4 0.047mF
15
SW2 DIS
R6 4.4kW R7 1.5kW C5 0.033mF Q2 ZTX788A R8 330W Q3 TCR22-4
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Functional Characteristics
12V Hot Insert Response
12V Turn On Response
VCC CPOR ON 1V/div 1V/div 5V/div
CPOR 1V/div
ON 1V/div
IIN =IOUT 1A/div
VIN = 12V RLOAD = 4.8 CLOAD = 1000F
TIME (20ms/div.)
/POR 10V/div
VOUT 5V/div
VIN = 12V RLOAD = 4.8 CLOAD = 1000F
TIME (20ms/div.)
Inrush Current Response
Power-Good Response
VOUT /FAULT VCC 10V/div 10V/div 5V/div
ON 1V/div
IIN 1A/div
TIME (10ms/div.)
PG FB 10V/div 1V/div
VIN = 12V RLOAD = 3.4 CLOAD = 5700F
VOUT 5V/div
VIN = 12V RLOAD = 4.8 CLOAD = 1000F
TIME (10ms/div.)
Turn Off -- Normal Discharge
Turn Off -- Crowbar Discharge
ON IIN = IOUT GATE 20V/div 1V/div 2A/div
VOUT 5V/div
TIME (2.5ms/div.)
VOUT 5V/div
VIN = 12V RDIS(External) = 0 RLOAD = 4.8 CLOAD = 1000F SW2 = HIGH
ON IIN = IOUT GATE 2A/div 20V/div 1V/div
VIN = 12V RLOAD = 4.8 CLOAD = 1000F SW2 = LOW
TIME (2.5ms/div.)
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Functional Characteristics (continued)
Turn On Into Short Circuit
/FAULT 10V/div
CFILTER ON 1V/div 1V/div
VIN = 12V RLOAD = 0 CLOAD = 1000F
VOUT 5V/div
TIME (10ms/div.)
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Functional Block Diagram
MIC2086
SENSE VCC
18 (15) 19,20 (16)
Charge Pump 48mV
+
17 (14)
GATE
1.24V REG2
+
21V
16 (13) 12 (10) 13 (11) 14 (12)
13V
95mV REG1 VCC1 20mA
+
VCC1
REF COMPOUT COMP+ COMP DIS*
550W VCC1 UVLO 2.2V Circuit Breaker Response or UVLO 20mA VCC1 20mA VCC1 VCC1 20mA VCC1 45mA
15
7 (6)
/FAULT
CFILTER
2 (2)
1.24V
9,10 (8) 8 (7)
+
Logic
5 (5)
/POR
GND FB
1.24V VCC1 2.5mA
+
Glitch Filter
1.5mA
6
PWRGD*
1 (1)
CPOR
3 (3)
CRWBR
1.24V
+
+
0.45V
ON
4 (4)
1.24V
+
Glitch Filter
Glitch Filter
+
11 (9)
1.24V
OV
*DIS and PWRGD are not available on MIC2085. Pin numbers for MIC2085 are in parenthesis ( ) where applicable.
MIC2086 Block Diagram
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where VTRIPSLOW is the current limit slow trip threshold found in the electrical table and RSENSE is the selected value that will set the desired current limit. There are two basic start-up modes for the MIC2085/86: 1)Start-up dominated by load capacitance and 2)start-up dominated by total gate capacitance. The magnitude of the inrush current delivered to the load will determine the dominant mode. If the inrush current is greater than the programmed current limit (ILIM), then load capacitance is dominant. Otherwise, gate capacitance is dominant. The expected inrush current may be calculated using the following equation:
Functional Description
Hot Swap Insertion When circuit boards are inserted into live system backplanes and supply voltages, high inrush currents can result due to the charging of bulk capacitance that resides across the supply pins of the circuit board. This inrush current, although transient in nature, may be high enough to cause permanent damage to on-board components or may cause the system's supply voltages to go out of regulation during the transient period which may result in system failures. The MIC2085/86 acts as a controller for external N-Channel MOSFET devices in which the gate drive is controlled to provide inrush current limiting and output voltage slew rate control during hot plug insertions. Power Supply VCC is the supply input to the MIC2085/86 controller with a voltage range of 2.3V to 16.5V. The VCC input can withstand transient spikes up to 33V. In order to help suppress transients and ensure stability of the supply voltage, a capacitor of 1.0F to 10F from VCC to ground is recommended. Alternatively, a low pass filter, shown in the typical application circuit, can be used to eliminate high frequency oscillations as well as help suppress transient spikes. Start-Up Cycle When the voltage on the ON pin rises above its threshold of 1.24V, the MIC2085/86 first checks that its supply (VCC) is above the UVLO threshold. If so, the device is enabled and an internal 2A current source begins charging capacitor CPOR to 1.24V to initiate a start-up sequence (i.e., start-up delay times out). Once the start-up delay (tSTART) elapses, CPOR is pulled immediately to ground and a 15A current source begins charging the GATE output to drive the external MOSFET that switches VIN to VOUT. The programmed startup delay is calculated using the following equation:
INRUSH IGATE x
CLOAD CGATE
15A x
CLOAD CGATE
(3)
where IGATE is the GATE pin pull-up current, CLOAD is the load capacitance, and CGATE is the total GATE capacitance (CISS of the external MOSFET and any external capacitor connected from the MIC2085/86 GATE pin to ground).
Load Capacitance Dominated Start-Up
In this case, the load capacitance, CLOAD, is large enough to cause the inrush current to exceed the programmed current limit but is less than the fast-trip threshold (or the fast-trip threshold is disabled, `M' option). During start-up under this condition, the load current is regulated at the programmed current limit value (ILIM) and held constant until the output voltage rises to its final value. The output slew rate and equivalent GATE voltage slew rate is computed by the following equation:
Output Voltage Slew Rate, dVOUT /dt =
ILIM CLOAD
(4)
t START = CPOR x
VTH 0.62 x CPOR (F) ICPOR
(1)
where VTH, the POR delay threshold, is 1.24V, and ICPOR, the POR timer current, is 2A. As the GATE voltage continues ramping toward its final value (VCC + VGS) at a defined slew rate (See "Load Capacitance"/"Gate Capacitance Dominated Start-Up" sections), a second CPOR timing cycle begins if: 1)/FAULT is high and 2)CFILTER is low (i.e., not an overvoltage, undervoltage lockout, or overcurrent state). This second timing cycle, tPOR, starts when the voltage at the FB pin exceeds its threshold (VFB) indicating that the output voltage is valid. The time period tPOR is equivalent to tSTART and sets the interval for the /POR to go Low-to-High after "power is good" (See Figure 2 of "Timing Diagrams"). Active current regulation is employed to limit the inrush current transient response during start-up by regulating the load current at the programmed current limit value (See "Current Limiting and Dual-Level Circuit Breaker" section). The following equation is used to determine the nominal current limit value: ILIM =
M0235-121903
where ILIM is the programmed current limit value. Consequently, the value of CFILTER must be selected to ensure that the overcurrent response time, tOCSLOW, exceeds the time needed for the output to reach its final value. For example, given a MOSFET with an input capacitance CISS = CGATE = 4700pF, CLOAD is 2200F, and ILIMIT is set to 6A with a 12V input, then the load capacitance dominates as determined by the calculated INRUSH > ILIM. Therefore, the output voltage slew rate determined from Equation 4 is:
Output Voltage Slew Rate, dVOUT /dt =
6A V = 2.73 2200F ms
and the resulting tOCSLOW needed to achieve a 12V output is approximately 4.5ms. (See "Power-On Reset, Start-Up, and Overcurrent Timer Delays" section to calculate tOCSLOW.)
GATE Capacitance Dominated Start-Up
In this case, the value of the load capacitance relative to the GATE capacitance is small enough such that the load current during start-up never exceeds the current limit threshold as determined by Equation 3. The minimum value of CGATE that will ensure that the current limit is never exceeded is given by the equation below:
I CGATE (min) = GATE x CLOAD ILIM
(5)
VTRIPSLOW 48mV = RSENSE RSENSE
(2) 14 January 2004
MIC2085/2086
where CGATE is the summation of the MOSFET input capacitance (CISS) and the value of the external capacitor connected to the GATE pin of the MOSFET. Once CGATE is determined, use the following equation to determine the output slew rate for gate capacitance dominated start-up.
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Output Undervoltage Detection The MIC2085/86 employ output undervoltage detection by monitoring the output voltage through a resistive divider connected at the FB pin. During turn on, while the voltage at the FB pin is below the threshold (VFB), the /POR pin is asserted low. Once the FB pin voltage crosses VFB, a 2A current source charges capacitor CPOR. Once the CPOR pin voltage reaches 1.24V, the time period tPOR elapses as the CPOR pin is pulled to ground and the /POR pin goes HIGH. If the voltage at FB drops below VFB for more than 10s, the /POR pin resets for at least one timing cycle defined by tPOR (see Applications Information for an example). Input Overvoltage Protection The MIC2085/86 monitors and detects overvoltage conditions in the event of excessive supply transients at the input. Whenever the overvoltage threshold (VOV) is exceeded at the OV pin, the GATE is pulled low and the output is shut off. The GATE will begin ramping one POR timing cycle after the OV pin voltage drops below its threshold. An external CRWBR circuit, as shown in the typical application diagram, provides a time period that an overvoltage condition must exceed in order to trip the circuit breaker. When the OV pin exceeds the overvoltage threshold (VOV), the CRWBR timer begins charging the CRWBR capacitor initially with a 45A current source. Once the voltage at CRWBR exceeds its threshold (VCR) of 0.47V, the CRWBR current immediately increases to 1.5mA and the circuit breaker is tripped, necessitating a device reset by toggling the ON pin LOW to HIGH. Power-On Reset, Start-Up, and Overcurrent Timer Delays The Power-On Reset delay, tPOR, is the time period for the /POR pin to go HIGH once the voltage at the FB pin exceeds the power-good threshold (VTH). A capacitor connected to CPOR sets the interval, tPOR, and tPOR is equivalent to the start-up delay, tSTART (see Equation 1). A capacitor connected to CFILTER is used to set the timer which activates the circuit breaker during overcurrent conditions. When the voltage across the sense resistor exceeds the slow trip current limit threshold of 48mV, the overcurrent timer begins to charge for a period, tOCSLOW, determined by CFILTER. If no capacitor is used at CFILTER, then tOCSLOW defaults to 5s. If tOCSLOW elapses, then the circuit breaker is activated and the GATE output is immediately pulled to ground. The following equation is used to determine the overcurrent timer period, tOCSLOW.
V t OCSLOW = CFILTER x TH 0.062 x CFILTER (F) (7) I
TIMER
dVOUT /dt (output) =
IGATE CGATE
(6)
Table 1 depicts the output slew rate for various values of CGATE.
IGATE = 15A CGATE 0.001F 0.01F 0.1F 1F dVOUT/dt 15V/ms 1.5V/ms 0.150V/ms 0.015V/ms
Table 1. Output Slew Rate Selection for GATE Capacitance Dominated Start-Up Current Limiting and Dual-Level Circuit Breaker Many applications will require that the inrush and steady state supply current be limited at a specific value in order to protect critical components within the system. Connecting a sense resistor between the VCC and SENSE pins sets the nominal current limit value of the MIC2085/86 and the current limit is calculated using Equation 2. However, the MIC2085/86 exhibits foldback current limit response. The foldback feature allows the nominal current limit threshold to vary from 24mV up to 48mV as the FB pin voltage increases or decreases. When FB is at 0V, the current limit threshold is 24mV and for FB 0.6V, the current limit threshold is the nominal 48mV. (See Figure 4 for Foldback Current Limit Response characteristic). The MIC2085/86 also features a dual-level circuit breaker triggered via 48mV and 95mV current limit thresholds sensed across the VCC and SENSE pins. The first level of the circuit breaker functions as follows. Once the voltage sensed across these two pins exceeds 48mV, the overcurrent timer, its duration set by capacitor CFILTER, starts to ramp the voltage at CFILTER using a 2A constant current source. If the voltage at CFILTER reaches the overcurrent timer threshold (VTH) of 1.24V, then CFILTER immediately returns to ground as the circuit breaker trips and the GATE output is immediately shut down. For the second level, if the voltage sensed across VCC and SENSE exceeds 95mV at any time, the circuit breaker trips and the GATE shuts down immediately, bypassing the overcurrent timer period. To disable current limit and circuit breaker operation, tie the SENSE and VCC pins together and the CFILTER pin to ground.
where VTH, the CFILTER timer threshold, is 1.24V and ITIMER, the overcurrent timer current, is 20A. Tables 2 and 3 provide a quick reference for several timer calculations using select standard value capacitors.
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CPOR tPOR = tSTART
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Using some basic algebra and simplifying Equation 8 to isolate R5, yields:
V OUT(Good) R5 = R6 - 1 VFB(MAX)
0.01F 6ms 0.02F 12ms 0.033F 18.5ms 0.05F 30ms 0.1F 60ms 0.33F 200ms Table 2. Selected Power-On Reset and Start-Up Delays
(8.1)
CFILTER 1800pF 4700pF 8200pF 0.010F 0.020F
tOCSLOW 100s 290s 500s 620s 1.2ms
0.033F 2.0ms 0.050F 3.0ms 0.1F 6.2ms 0.33F 20.75ms Table 3. Selected Overcurrent Timer Delays
where VFB(MAX) = 1.29V, VOUT(Good) = 11V, and R6 is 13.3k. Substituting these values into Equation 8.1 now yields R5 = 100.11k. A standard 100k 1% is selected. Now, consider the 11.4V minimum output voltage, the lower tolerance for R6 and higher tolerance for R5, 13.17k and 101k, respectively. With only 11.4V available, the voltage sensed at the FB pin exceeds VFB(MAX), thus the /POR and PWRGD (MIC2086) signals will transition from LOW to HIGH, indicating "power is good" given the worse case tolerances of this example. Input Overvoltage Protection The external CRWBR circuit shown in Figure 5 consists of capacitor C4, resistor R7, NPN transistor Q2, and SCR Q3. The capacitor establishes a time duration for an overvoltage condition to last before the circuit breaker trips. The CRWBR timer duration is approximated by the following equation: t OVCR
(C4 x VCR ) 0.01x C4(F)
ICR
(9)
Applications Information
Output Undervoltage Detection For output undervoltage detection, the first consideration is to establish the output voltage level that indicates "power is good." For this example, the output value for which a 12V supply will signal "good" is 11V. Next, consider the tolerances of the input supply and FB threshold (VFB). For this example, the 12V supply varies 5%, thus the resulting output voltage may be as low as 11.4V and as high as 12.6V. Additionally, the FB threshold has 50mV tolerance and may be as low as 1.19V and as high as 1.29V. Thus, to determine the values of the resistive divider network (R5 and R6) at the FB pin, shown in Figure 5, use the following iterative design procedure. 1) Choose R6 so as to limit the current through the divider to approximately 100A or less.
where VCR, the CRWBR pin threshold, is 0.47V and ICR, the CRWBR pin current, is 45A during the timer period (see the CRWBR timer pin description for further description). A similar design approach as the previous undervoltage detection example is recommended for the overvoltage protection circuitry, resistors R2 and R3 in Figure 5. For input overvoltage protection, the first consideration is to establish the input voltage level that indicates an overvoltage triggering a system (output voltage) shut down. For this example, the input value for which a 12V supply will signal an "output shut down" is 13.2V (+10%). Similarly, from the previous example: 1) Choose R3 to satisfy 100A condition.
1.29V 12.9k . 100A 100A R6 is chosen as 13.3k 1%. 2) Next, determine R5 using the output "good" voltage of 11V and the following equation: R6
(R5 + R6) VOUT(Good) = VFB R6 (8)
VFB(MAX)
1.19V 11.9k . 100A 100A R3 is chosen as 13.7k 1%. 2) Thus, following the previous example and substituting R2 and R3 for R5 and R6, respectively, and 13.2V overvoltage for 11V output "good", the same formula yields R2 of 138.3k. The next highest standard 1% value is 140k. Now, consider the 12.6V maximum input voltage (VCC +5%), the higher tolerance for R3 and lower tolerance for R2, 13.84k and 138.60k, respectively. With a 12.6V input, the voltage sensed at the OV pin is below VOV(MIN), and the MIC2085/86 will not indicate an overvoltage condition until VCC exceeds at least 13.2V. R3
VOV(MIN)
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RSENSE 0.012 1 2% 2
3 4
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Q1 IRF7822 (SO-8) CLOAD 220F R4 10 GATE
14
VIN 12V R1 100k C1 1F
VOUT 12V@3A
R2 140k 1%
16
15
VCC
SENSE
R5 100k 1%
4
C2 0.022F ON FB MIC2085
7
R6 13.3k 1% /POR /FAULT
5 6 1
9
OV
Downstream Signals Q2 2N4401 Q3 TCR22-4
R3 13.7k 1%
CPOR
3
GND
8
CRWBR
C4 0.01F C5 0.033F *R7 180
C3 0.05F Overvoltage (Input) = 13.3V Undervoltage (Output) = 11.0V POR/START-UP Delay = 30ms *R7 needed when using a sensitive gate SCR. Additional pins omitted for clarity.
Figure 5. Undervoltage/Overvoltage Circuit
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PCB Connection Sense There are several configuration options for the MIC2085/86's ON pin to detect if the PCB has been fully seated in the backplane before initiating a start-up cycle. In the typical applications circuit, the MIC2085/86 is mounted on the PCB with a resistive divider network connected to the ON pin. R2 is connected to a short pin on the PCB edge connector. Until the connectors mate, the ON pin is held low which keeps the GATE output charge pump off. Once the connectors mate, the resistor network is pulled up to the input supply, 12V in this example, and the ON pin voltage exceeds its threshold (VON)
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of 1.24V and the MIC2085/86 initiates a start-up cycle. In Figure 6, the connection sense consisting of a logic-level discrete MOSFET and a few resistors allows for interrupt control from the processor or other signal controller to shut off the output of the MIC2085/86. R4 keeps the GATE of Q2 at VIN until the connectors are fully mated. A logic LOW at the /ON_OFF signal turns Q2 off and allows the ON pin to pull up above its threshold and initiate a start-up cycle. Applying a logic HIGH at the /ON_OFF signal will turn Q2 on and short the ON pin of the MIC2085/86 to ground which turns off the GATE output charge pump.
Backplane PCB Edge Connector Connector
VIN 12V
Long Pin
3
RSENSE 0.008 1 2% 2 C1 1F
4
Q1 Si7860DP (PowerPAKTM SO-8)
VOUT 12V@5A
CLOAD 220F
Short Pin R4 10k
4 16 15
R5 10
R6 127k 1%
VCC ON
SENSE GATE
14
R1 20k R2 20k MIC2085 *Q2 /POR PCB Connection Sense CPOR
3 5 1
C2 0.01F FB
7
/ON_OFF
R3 100
R7 16.2k 1%
/FAULT GND
8
Downstream Signals
GND
Long Pin
C2 0.05F
Undervoltage (Output) = 11.4V POR/START-UP DELAY = 30ms *Q2 is TN0201T (SOT-23) Additional pins omitted for clarity.
Figure 6. PCB Connection Sense with ON/OFF Control
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MIC2085/2086
Higher UVLO Setting Once a PCB is inserted into a backplane (power supply), the internal UVLO circuit of the MIC2085/86 holds the GATE output charge pump off until VCC exceeds 2.18V. If VCC falls below 2V, the UVLO circuit pulls the GATE output to ground and clears the overvoltage and/or current limit faults. For a higher UVLO threshold, the circuit in Figure 7 can be used to delay the output MOSFET from switching on until the desired input voltage is achieved. The circuit allows the charge pump
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R1 x 1.24V . The GATE to remain off until VIN exceeds 1 + R2 drive output will be shut down when VIN falls below R1 x 1.14V . In the example circuit (Figure 7), the rising 1+ R2 UVLO threshold is set at approximately 11V and the falling UVLO threshold is established as 10.1V. The circuit consists of an external resistor divider at the ON pin that keeps the GATE output charge pump off until the voltage at the ON pin exceeds its threshold (VON) and after the start-up timer elapses.
VIN 12V
3
RSENSE 0.010 1 2% 2
4
Q1 IRF7822 (SO-8) CLOAD 220F R3 10 GATE
14
VOUT 12V@4A
C1 1F R1 392k 1% R4 127k 1%
16
15
VCC
SENSE
4
C2 0.01F ON MIC2085 FB
7
R2 49.9k 1% CPOR
3
R5 16.2k 1% /POR
5
GND
8
Downstream Signal
C3 0.1F Undervoltage Lockout (Rising) = 11.0V Undervoltage Lockout (Falling) = 10.1V Undervoltage (Output) = 11.4V POR/START-UP Delay = 60ms Additional pins omitted for clarity.
Figure 7. Higher UVLO Setting
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MIC2085/2086
Fast Output Discharge for Capacitive Loads In many applications where a switch controller is turned off by either removing the PCB from the backplane or the ON pin is reset, capacitive loading will cause the output to retain voltage unless a `bleed' (low impedance) path is in place in order to discharge the capacitance. The MIC2086 is equipped with an internal MOSFET that allows the discharging of any load capacitance to ground through a 550 path. The discharge feature is configured by wiring the DIS pin to the output (source) of the external MOSFET and becomes active
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(DIS pin output is low) once the ON pin is deasserted. Figure 8(a) illustrates the use of the discharge feature with an optional resistor (R5) that can be used to provide added resistance in the output discharge path. For an even faster discharge response of capacitive loads, the configuration of Figure 8(b) can be utilized to apply a crowbar to ground through an external SCR (Q3) that is triggered when the DIS pin goes low which turns on the PNP transistor (Q2). See the different "Functional Characteristic" curves for a comparison of the discharge response configurations.
VIN 12V R1 47kW ON Signal
4
RSENSE 0.007W 1 5% 2 C1 1mF
3 4
Q1 Si7892DP (PowerPAKTM SO-8) R2 10W GATE FB
17
VCC ON
19,20
SENSE
18
R3 110kW 1%
CLOAD 1500mF
VOUT 12V@5A
C2 0.022mF R4 14.7kW 1%
*R5
8
MIC2086
DIS
15
CPOR 3 C3 0.01mF
/POR 5 PWRGD 6 GND CFILTER 9,10 2 C4 0.01mF
(a)
Downstream Signals
VIN 12V R1 47kW ON Signal
4
RSENSE 0.007W 1 5% 2 C1 1mF
3 4
Q1 Si7892DP (PowerPAKTM SO-8) R2 10W GATE FB
17
VCC ON
19,20
SENSE
18
R3 110kW 1%
CLOAD 1500mF
VOUT 12V@5A
C2 0.022mF R4 14.7kW 1% Downstream Signal R6
4.4kW
8
MIC2086
/POR
5
Undervoltage (Output) = 11V POR/START-UP Delay = 6ms Circuit Breaker Response Time = 620ms *R5 of Figure 8(a) is optional to combine in series with internal 550W. Additional pins omitted for clarity.
CPOR 3 C3 0.01mF
DIS 15 GND CFILTER 9,10 2 C4 0.01mF
(b)
R5 1.5kW C5 0.022mF
Q2 ZTX788A R7 220W
Q3 TCR22-4
Figure 8. MIC2086 Fast Discharge of Capacitive Load
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MIC2085/2086
Auto-Retry Upon Overcurrent Faults The MIC2085/86 can be configured for automatic restart after a fault condition. Placing a diode between the ON and /FAULT pins, as shown in Figure 9, will enable the autorestart capability of the controller. When an application is configured for auto-retry, the overcurrent timer should be set to minimize the duty cycle of the overcurrent response to prevent thermal runaway of the power MOSFET. See "MOSFET Transient Thermal Issues" section for further detail. A limited duty cycle is achieved when the overcurrent timer duration (tOCSLOW) is much less than the start-up delay timer duration (tSTART) and is calculated using the following equation:
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The circuit in Figure 10 distributes 12V from the backplane to the MIC2182 DC/DC converter that steps down +12V to +3.3V for local bias. The pass transistor, Q1, isolates the MIC2182's input capacitance during module plug-in and allows the backplane to accommodate additional plug-in modules without affecting the other modules on the backplane. The two control input signals are VBxEn_L (active LOW) and a Local Power Enable (active HIGH). The MIC2085 in the circuit of Figure 10 performs a number of functions. The gate output of Q1 is enabled by the two bit input signal VBxEn_L, Local Power Enable = [0,1]. Also, the MIC2085 limits the drain current of Q1 to 7A, monitors VB_In for an overvoltage condition greater than 16V, and enables the MIC2182 DC/DC converter downstream to supply a local voltage rail. The uncommitted comparator is used to monitor VB_In for an undervoltage condition of less than 10V, indicated by a logic LOW at the comparator output (COMPOUT). COMPOUT may be used to control a downstream device such as another DC/DC converter. Additionally, the MIC2085 is configured for auto-retry upon an overcurrent fault condition by placing a diode (D1) between the /FAULT and ON pins of the controller.
Auto - Retry Duty Cycle =
t OCSLOW t START
x 100%
(10)
An InfiniBandTM Application Circuit The circuit in Figure 10 depicts a single 50W InfiniBandTM module using the MIC2085 controller. An InfiniBandTM backplane distributes bulk power to multiple plug-in modules that employ DC/DC converters for local supply requirements.
VIN 5V
3
RSENSE 0.012 1 5% 2
4
Q1 IRF7822 (SO-8) CLOAD 220F R3 10 GATE
14
VOUT 5V@2.5A
C1 1F R1 47k R4 34k 1%
16
15
VCC R2 33k ON SIGNAL D1 1N914 /FAULT OUTPUT
6
SENSE
4
C2 0.022F ON MIC2085 /FAULT /POR CPOR
3 5
FB
7
R5 14.7k 1% Downstream Signal
GND
8
CFILTER
2
Undervoltage (Output) = 4.27V POR/START-UP Delay = 12ms Circuit Breaker Response Time = 290s Auto-Retry Duty Cycle = 2.5% Additional pins omitted for clarity.
C3 0.02F
C4 4700pF
Figure 9. Auto-Retry Configuration
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InfiniBandTM Application
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InfiniBand Backplane VB_In (12V)
Long
3
1
RSENSE 0.007W 5% 2
InfiniBand MODULE
Q1 IRF7822 (SO-8) R7 174kW 1% C2 0.022mF
MIC2182 DC/DC Converter VIN
4
R2 165kW 1% Short VBxEn_L R3 13.3kW 1% R4 78.7kW 1%
9 11
VCC OV
16
SENSE
15
R6 10W GATE
14
R5 11kW 1%
COMP+ CPOR COMP
COMPOUT 10 /POR 5
MIC2085
/UV
3 12
FB /FAULT ON CRWBR
1
7 6 4
Power-On Reset RUN/SS Output R8 25.5kW D1 1N914 1% 3.3V @ 4A
Long VB_Ret
C1 0.01mF
C5 0.033mF
REF C3 CFILTER GND 0.1mF 2 8 C4 0.022mF R1 10kW
13
LocalEnable Power
GND Overvoltage (Input) = 16.0V Undervoltage (Input) = 10.0V Undervoltage (Output) & Power-Good (Output) = 10.0V Circuit Breaker Response Time = 1.2ms POR/START-UP Delay = 18.5ms Auto-Retry Duty Cycle = 6.5%
Figure 10. A 50W InfiniBandTM Application Sense Resistor Selection The next lowest standard value is 6.0mW. At the other set The MIC2085 and MIC2086 use a low-value sense resistor to of tolerance extremes for the output in question: measure the current flowing through the MOSFET switch (and therefore the load). This sense resistor is nominally 56.7mV ILOAD(CONT,MAX) = = 9.45A , valued at 48mV/ILOAD(CONT). To accommodate worst-case 6.0m tolerances for both the sense resistor (allow 3% over time almost 10A. Knowing this final datum, we can determine and temperature for a resistor with 1% initial tolerance) and the necessary wattage of the sense resistor, using P = I2R, still supply the maximum required steady-state load current, where I will be ILOAD(CONT, MAX), and R will be a slightly more detailed calculation must be used. (0.97)(RSENSE(NOM)). These numbers yield the following: The current limit threshold voltage (the "trip point") for the PMAX = (10A)2 (5.82m) = 0.582W. MIC2085/86 may be as low as 40mV, which would equate to In this example, a 1W sense resistor is sufficient. a sense resistor value of 40mV/ILOAD(CONT). Carrying the MOSFET Selection numbers through for the case where the value of the sense resistor is 3% high yields: Selecting the proper external MOSFET for use with the MIC2085/86 involves three straightforward tasks: 40mV 38.8mV RSENSE(MAX) = = * Choice of a MOSFET which meets minimum (1.03) ILOAD(CONT) ILOAD(CONT) (11) voltage requirements. Once the value of RSENSE has been chosen in this manner, * Selection of a device to handle the maximum it is good practice to check the maximum ILOAD(CONT) which continuous current (steady-state thermal the circuit may let through in the case of tolerance build-up in issues). the opposite direction. Here, the worst-case maximum cur* Verify the selected part's ability to withstand any rent is found using a 55mV trip voltage and a sense resistor peak currents (transient thermal issues). that is 3% low in value. The resulting equation is: MOSFET Voltage Requirements 55mV 56.7mV The first voltage requirement for the MOSFET is that the drainILOAD(CONT,MAX) = = (0.97) RSENSE(NOM) RSENSE(NOM) (12) source breakdown voltage of the MOSFET must be greater than VIN(MAX). For instance, a 16V input may reasonably be As an example, if an output must carry a continuous 6A expected to see high-frequency transients as high as 24V. without nuisance trips occurring, Equation 11 yields: Therefore, the drain-source breakdown voltage of the MOSFET 38.8mV must be at least 25V. For ample safety margin and standard RSENSE(MAX) = = 6.5m . 6A availability, the closest minimum value should be 30V.
(
)
(
)
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The second breakdown voltage criterion that must be met is a bit subtler than simple drain-source breakdown voltage. In MIC2085/86 applications, the gate of the external MOSFET is driven up to a maximum of 21V by the internal output MOSFET. At the same time, if the output of the external MOSFET (its source) is suddenly subjected to a short, the gate-source voltage will go to (21V - 0V) = 21V. Since most power MOSFETs generally have a maximum gate-source breakdown of 20V or less, the use of a Zener clamp is recommended in applications with VCC 8V. A Zener diode with 10V to 12V rating is recommended as shown in Figure 11. At the present time, most power MOSFETs with a 20V gate-source voltage rating have a 30V drain-source breakdown rating or higher. As a general tip, choose surface-mount devices with a drain-source rating of 30V or more as a starting point. Finally, the external gate drive of the MIC2085/86 requires a low-voltage logic level MOSFET when operating at voltages lower than 3V. There are 2.5V logic level MOSFETs available. Please see Table 4, "MOSFET and Sense Resistor Vendors" for suggested manufacturers. MOSFET Steady-State Thermal Issues The selection of a MOSFET to meet the maximum continuous current is a fairly straightforward exercise. First, arm yourself with the following data: * The value of ILOAD(CONT, MAX.) for the output in question (see "Sense Resistor Selection" ). * The manufacturer's data sheet for the candidate MOSFET. * The maximum ambient temperature in which the device will be required to operate.
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* Any knowledge you can get about the heat sinking available to the device (e.g., can heat be dissipated into the ground plane or power plane, if using a surface-mount part? Is any airflow available?). The data sheet will almost always give a value of on resistance given for the MOSFET at a gate-source voltage of 4.5V, and another value at a gate-source voltage of 10V. As a first approximation, add the two values together and divide by two to get the on-resistance of the part with 8V of enhancement. Call this value RON. Since a heavily enhanced MOSFET acts as an ohmic (resistive) device, almost all that's required to determine steady-state power dissipation is to calculate I2R. The one addendum to this is that MOSFETs have a slight increase in RON with increasing die temperature. A good approximation for this value is 0.5% increase in RON per C rise in junction temperature above the point at which RON was initially specified by the manufacturer. For instance, if the selected MOSFET has a calculated RON of 10m at a TJ = 25C, and the actual junction temperature ends up at 110C, a good first cut at the operating value for RON would be: RON 10m[1 + (110 - 25)(0.005)] 14.3m The final step is to make sure that the heat sinking available to the MOSFET is capable of dissipating at least as much power (rated in C/W) as that with which the MOSFET's performance was specified by the manufacturer. Here are a few practical tips: 1. The heat from a surface-mount device such as an SO-8 MOSFET flows almost entirely out of the drain leads. If the drain leads can be soldered down to one square inch or more, the copper will act as the heat sink for the part. This copper must be on the same layer of the board as the MOSFET drain.
Q1 IRF7822 (SO-8)
4
VIN 12V
3
RSENSE 0.007 1 2% 2 C1 1F R1 47k
*D1 1N5240B 10V CLOAD 220F
VOUT 12V@5A
16
15
VCC
SENSE GATE
R3 10
14
R4 100k 1%
4
C2 0.01F ON FB MIC2085
7
R2 33k CPOR
3
R5 13.3k 1% /FAULT /POR
6 5
Downstream Signals
GND
8
C3 0.1F
Undervoltage (Output) = 11.0V POR/START-UP Delay = 60ms *Recommended for MOSFETs with gate-source breakdown of 20V or less (IRF7822 VGS(MAX) = 12V) for catastrophic output short circuit protection. Additional pins omitted for clarity.
Figure 11. Zener Clamped MOSFET GATE January 2004 23
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2. Airflow works. Even a few LFM (linear feet per minute) of air will cool a MOSFET down substantially. If you can, position the MOSFET(s) near the inlet of a power supply's fan, or the outlet of a processor's cooling fan. 3. The best test of a surface-mount MOSFET for an application (assuming the above tips show it to be a likely fit) is an empirical one. Check the MOSFET's temperature in the actual layout of the expected final circuit, at full operating current. The use of a thermocouple on the drain leads, or infrared pyrometer on the package, will then give a reasonable idea of the device's junction temperature. MOSFET Transient Thermal Issues Having chosen a MOSFET that will withstand the imposed voltage stresses, and the worse case continuous I2R power dissipation which it will see, it remains only to verify the MOSFET's ability to handle short-term overload power dissipation without overheating. A MOSFET can handle a much higher pulsed power without damage than its continuous dissipation ratings would imply. The reason for this is that, like everything else, thermal devices (silicon die, lead frames, etc.) have thermal inertia. In terms related directly to the specification and use of power MOSFETs, this is known as "transient thermal impedance," or Z(J-A). Almost all power MOSFET data sheets give a Transient Thermal Impedance Curve. For example, take the following case: VIN = 12V, tOCSLOW has been set to 100msec, ILOAD(CONT. MAX) is 2.5A, the slow-trip threshold is 48mV nominal, and the fast-trip threshold is 95mV. If the output is accidentally connected to a 3 load, the output current from the MOSFET will be regulated to 2.5A for 100ms (tOCSLOW) before the part trips. During that time, the dissipation in the MOSFET is given by: P = E x I EMOSFET = [12V-(2.5A)(3)] = 4.5V PMOSFET = (4.5V x 2.5A) = 11.25W for 100msec. At first glance, it would appear that a really hefty MOSFET is required to withstand this sort of fault condition. This is where the transient thermal impedance curves become very useful. Figure 12 shows the curve for the Vishay (Siliconix) Si4410DY, a commonly used SO-8 power MOSFET.
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Taking the simplest case first, we'll assume that once a fault event such as the one in question occurs, it will be a long time - 10 minutes or more - before the fault is isolated and the channel is reset. In such a case, we can approximate this as a "single pulse" event, that is to say, there's no significant duty cycle. Then, reading up from the X-axis at the point where "Square Wave Pulse Duration" is equal to 0.1sec (=100msec), we see that the Z(J-A) of this MOSFET to a highly infrequent event of this duration is only 8% of its continuous R(J-A). This particular part is specified as having an R(J-A) of 50C/W for intervals of 10 seconds or less. Thus: Assume TA = 55C maximum, 1 square inch of copper at the drain leads, no airflow. Recalling from our previous approximation hint, the part has an RON of (0.0335/2) = 17m at 25C. Assume it has been carrying just about 2.5A for some time. When performing this calculation, be sure to use the highest anticipated ambient temperature (TA(MAX)) in which the MOSFET will be operating as the starting temperature, and find the operating junction temperature increase (TJ) from that point. Then, as shown next, the final junction temperature is found by adding TA(MAX) and TJ. Since this is not a closedform equation, getting a close approximation may take one or two iterations, but it's not a hard calculation to perform, and tends to converge quickly. Then the starting (steady-state)TJ is: TJ TA(MAX) + TJ TA(MAX) + [RON + (TA(MAX) - TA)(0.005/C)(RON)] x I2 x R(J-A) TJ 55C + [17m + (55C-25C)(0.005)(17m)] x (2.5A)2 x (50C/W) TJ (55C + (0.122W)(50C/W) 61.1C Iterate the calculation once to see if this value is within a few percent of the expected final value. For this iteration we will start with TJ equal to the already calculated value of 61.1C: TJ TA + [17m + (61.1C-25C)(0.005)(17m)] x (2.5A)2 x (50C/W) TJ ( 55C + (0.125W)(50C/W) 61.27C
Normalized Thermal Transient Impedance, Junction-to-Ambient
2 1 Duty Cycle = 0.5
Normalized Effective Transient Thermal Impedance
0.2
Notes:
0.1 0.1 0.05
PDM t1 t2 t1 1. Duty Cycle, D = t2 2. Per Unit Base = RthJA = 50 C/W
0.02 Single Pulse 0.01 10-4 10-3 10-2 10-1 Square Wave Pulse Duration (sec) 1
3. TJM - TA = PDMZthJA(t) 4. Surface Mounted
10
30
Figure 12. Transient Thermal Impedance
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So our original approximation of 61.1C was very close to the correct value. We will use TJ = 61C. Finally, add (11.25W)(50C/W)(0.08) = 45C to the steadystate TJ to get TJ(TRANSIENT MAX.) = 106C. This is an acceptable maximum junction temperature for this part. PCB Layout Considerations Because of the low values of the sense resistors used with the MIC2085/86 controllers, special attention to the layout must be used in order for the device's circuit breaker function to operate properly. Specifically, the use of a 4-wire Kelvin connection to measure the voltage across RSENSE is highly recommended. Kelvin sensing is simply a means of making sure that any voltage drops in the power traces connecting to the resistors does not get picked up by the traces themselves. Additionally, these Kelvin connections should be isolated from all other signal traces to avoid introducing noise onto these sensitive nodes. Figure 13 illustrates a recommended,
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multi-layer layout for the RSENSE, Power MOSFET, timer(s), overvoltage and feedback network connections. The feedback and overvoltage resistive networks are selected for a 12V application (from Figure 5). Many hot swap applications will require load currents of several amperes. Therefore, the power (VCC and Return) trace widths (W) need to be wide enough to allow the current to flow while the rise in temperature for a given copper plate (e.g., 1 oz. or 2 oz.) is kept to a maximum of 10C ~ 25C. Also, these traces should be as short as possible in order to minimize the IR drops between the input and the load. For a starting point, there are many trace width calculation tools available on the web such as the following link: http://www.aracnet.com/cgi-usr/gpatrick/trace.pl Finally, plated-through vias are utilized to make circuit connections to the power and ground planes. The trace connections with indicated vias should follow the example shown for the GND pin connection in Figure 13.
Current Flow to the Load *SENSE RESISTOR (2512)
D D D D
Current Flow to the Load *POWER MOSFET (SO-8)
G S S S
W
W
**R4 109 **CGATE R3 13.7k9 1%
R2 140k9 1%
Via to GND Plane
16
15
14
13
12
10
Via to POWER (VCC) Plane
11
VCC
REF
COMPOUT
7
GATE
COMP-
COMP+
SENSE
MIC2085
CFILTER
CRWBR
/FAULT
CPOR
/POR
1
2
3
4
5
6
8
GND
ON
FB
OV
9
**CFILTER
**CPOR
R5 100k9 1% R6 13.3k9 1%
Current Flow from the Load
Via to GND Plane
W
DRAWING IS NOT TO SCALE *See Table 4 for part numbers and vendors **Optional components Trace width (W) guidelines given in "PCB Layout. Recommendations" section of the datasheet.
Figure 13. Recommended PCB Layout for Sense Resistor, Power MOSFET, and Feedback/Overvoltage Network January 2004 25
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MOSFET and Sense Resistor Vendors Device types and manufacturer contact information for power MOSFETs and sense resistors is provided in Table 4. Some of the recommended MOSFETs include a metal heat sink on the bottom side of the package. The recommended trace for
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the MOSFET Gate of Figure 13 must be redirected when using MOSFETs packaged in this style. Contact the device manufacturer for package information.
MOSFET Vendors Vishay (Siliconix)
Key MOSFET Type(s) Si4420DY (SO-8 package) Si4442DY (SO-8 package) Si3442DV (SO-8 package) Si7860DP (PowerPAKTM SO-8) Si7892DP (PowerPAKTM SO-8) Si7884DP (PowerPAKTM SO-8) SUB60N06-18 (TO-263) SUB70N04-10 (TO-263) IRF7413 (SO-8 package) IRF7457 (SO-8 package) IRF7822 (SO-8 package) IRLBA1304 (Super220TM) FDS6680A (SO-8 package) FDS6690A (SO-8 package) PH3230 (SOT669-LFPAK) HAT2099H (LFPAK)
*Applications IOUT 10A IOUT = 10A-15A, VCC 5V IOUT 3A, VCC 5V IOUT 12A IOUT 15A IOUT 15A IOUT 20A, VCC 5V IOUT 20A, VCC 5V IOUT 10A IOUT 10A IOUT = 10A-15A, VCC 5V IOUT 20A, VCC 5V IOUT 10A IOUT 10A, VCC 5V IOUT 20A IOUT 20A
Contact Information www.siliconix.com (203) 452-5664
International Rectifier
www.irf.com (310) 322-3331
Fairchild Semiconductor Philips Hitachi
www.fairchildsemi.com (207) 775-8100 www.philips.com www.halsp.hitachi.com (408) 433-1990
* These devices are not limited to these conditions in many cases, but these conditions are provided as a helpful reference for customer applications.
Resistor Vendors Vishay (Dale) IRC
Contact Information www.vishay.com/docswsl_30100.pdf (203) 452-5664 "OARS" Series www.irctt.com/pdf_files/OARS.pdf "LR" Series www.irctt.com/pdf_files/LRC.pdf (second source to "WSL") (828) 264-8861 Table 4. MOSFET and Sense Resistor Vendors
Sense Resistors "WSL" Series
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Micrel
Package Information
PIN 1
0.157 (3.99) 0.150 (3.81)
DIMENSIONS: INCHES (MM)
0.009 (0.2286) REF 0.025 (0.635) BSC 0.0098 (0.249) 0.0040 (0.102)
0.012 (0.30) 0.008 (0.20)
0.0098 (0.249) 0.0075 (0.190)
45
8 0
SEATING 0.0688 (1.748) PLANE 0.0532 (1.351)
0.196 (4.98) 0.189 (4.80)
0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.229 (5.82)
Rev. 04
16-Pin QSOP (QS)
0.344 (8.74) 0.337 (8.56)
0.0575 REF
0.157 (3.99) 0.150 (3.81)
8 0
0.244 (6.20) 0.229 (5.82) 0.025 BSC (0.635) 0.012 (0.305) 0.008 (0.203)
0.009 (0.229) 0.007 (0.178)
Rev. 04
0.068 (1.73) 0.053 (1.35)
0.010 (0.254) 0.004 (0.102)
Note: 1. All Dimensions are in Inches (mm) excluding mold flash. 2. Lead coplanarity should be 0.004" max. 3. Max misalignment between top and bottom. 4. The lead width, B to be determined at 0.0075" from lead tip.
7 BSC 0.050 (1.27) 0.016 (0.40)
20-Pin QSOP (QS)
January 2004
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MICREL, INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
USA
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser's use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. (c) 2003 Micrel, Incorporated. M0235-121903
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January 2004

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