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MIC2587/MIC2587R
Single-Channel, Positive High-Voltage Hot Swap Controller
General Description
The MIC2587 and MIC2587R are single-channel positive voltage hot swap controllers designed to provide safe insertion and removal of boards for systems that require live (always-powered) backplanes. These devices use few external components and act as controllers for external Nchannel power MOSFET devices to provide inrush current control and output voltage slew rate control. Overcurrent fault protection is provided via programmable analog foldback current limit circuitry equipped with a programmable overcurrent filter. These protection circuits combine to limit the power dissipation of the external MOSFET to insure that the MOSFET is in its SOA during fault conditions. The MIC2587 provides a circuit breaker function that latches the output MOSFET off if the load current exceeds the current limit threshold for the duration of the programmable timer. The MIC2587R provides a circuit breaker function that automatically attempts to restart power after a load current fault at a low duty cycle to prevent the MOSFET from overheating. Each device provides either an active-HIGH (-1BM) or an active-LOW (-2BM) "Power-is-Good" signal to indicate that the output load voltage is within tolerance. Data sheets and support documentation can be found on Micrel's web site at www.micrel.com.
Features
* MIC2587: Pin-for-pin functional equivalent to the LT1641 * Operates from +10V to +80V with 100V ABS MAX operation * Industrial temperature specifications at VCC = +24V and VCC = +48V * Programmable current limit with analog foldback * Active current regulation minimizes inrush current * Electronic circuit breaker for overcurrent fault protection - Output latch off (MIC2587) or - Output auto-retry (MIC2587R) * Fast responding circuit breaker (< 2s) to short circuit loads * Programmable input undervoltage lockout * Fault Reporting: Open-drain "Power-is-Good" output for enabling DC/DC converter(s) - Active-HIGH: MIC2587-1/MIC2587R-1 - Active-LOW: MIC2587-2/MIC2587R-2
Applications
* * * * * * General-purpose hot board insertion High-voltage, high-side electronic circuit breaker +12V/+24V/+48V Distributed Power Systems +24V/+48V Industrial/Alarm Systems Telecom Systems Medical Systems
Ordering Information
Part Number Standard MIC2587-1BM MIC2587-2BM MIC2587R-1BM MIC2587R-2BM Pb-Free MIC2587-1YM MIC2587-2YM MIC2587R-1YM MIC2587R-2YM Active-HIGH Active-LOW Active- HIGH Active-LOW Latched Latched Auto-retry Auto-retry 8 pin SOIC 8 pin SOIC 8 pin SOIC 8 pin SOIC PWRGD Polarity Circuit Breaker Function Package
Power, Connect, and Protect is a trademark of Micrel Inc. Micrel, Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax +1 (408) 474-1000 * http://www.micrel.com
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MIC2587/MIC2587R
Typical Application
MIC2587/87R Typical Application Circuit
Pin Configuration
8-pin SOIC (M) MIC2587-1BM MIC2587R-1BM
8-pin SOIC (M) MIC2587-2BM MIC2587R-2BM
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Pin Description
Pin Number Pin Name Pin Function Enable Input: When the voltage at the ON pin is higher than the VONH threshold, a start cycle is initiated. An internal current source (IGATEON) is activated which charges the GATE pin, ramping up the voltage at this pin to turn on an external MOSFET. Whenever the voltage at the ON pin is lower than the VONL threshold, an undervoltage lockout condition is detected and the IGATEON current source is disabled while the GATE pin is pulled low by another internal current source (IGATEOFF). After a load current fault, toggling the ON pin LOW will reset the circuit breaker then back HIGH (ON pin) will initiate another start cycle. Output Voltage Feedback Input: This pin is connected to an external resistor divider that is used to sample the output load voltage. The voltage at this pin is measured against an internal comparator whose output controls the PWRGD (or /PWRGD) signal. PWRGD (or /PWRGD) asserts when the FB pin voltage crosses the VFBH threshold. When the FB pin voltage is lower than its VFBL threshold, PWRGD (or /PWRGD) is de-asserted. The FB comparator exhibits a typical hysteresis of 80mV. The FB pin voltage also affects the MIC2587/MIC2587R's foldback current limit operation (see the "Functional Description" section for further information). Power-is-Good (PWRGD or /PWRGD), Open-drain Output: This pin remains de-asserted during start up while the FB pin voltage is below the VFBH threshold. Once the voltage at the FB pin rises above the VFBH threshold, the Power-is-Good output asserts with minimal delay (typically 5s). PWRGD (MIC2587-1) (MIC2587R-1) Active-HIGH /PWRGD (MIC2587-2) (MIC2587R-2) Active-LOW For the (-1) options, the PWRGD output pin will be high-impedance when the FB pin voltage is higher than VFBH and will pull down to GND when the FB pin voltage is less than VFBL. For the (-2) options, the /PWRGD output pin will be high-impedance when the FB pin voltage is lower than VFBL and will pull down to GND when the FB pin voltage is higher than VFBH. The Power-is-Good output pin is connected to an open-drain, N-channel transistor implemented with high-voltage structures. These transistors are capable of operating with pull-up resistors to supply voltages as high as 100V. To use this signal as a logic control in low-voltage dc-dc conversion applications, an external pull-up resistor between this pin and the logic supply voltage is recommended, unless an internal pull-up impedance is provided by the dc-dc module or other device (load). 4 GND Tie this pin directly to the system's analog GND plane Current Limit Response Timer: A capacitor connected from this pin to GND provides overcurrent filtering to prevent nuisance "tripping" of the circuit breaker by setting the time (tFLT) for which the controller is allowed to remain in current limit. Once the MIC2587 circuit breaker trips, the output latches off. Under normal (steady state) operation, the TIMER pin is held to GND by an internal 3.5A current source (ITIMERDN). When the voltage across the external sense resistor exceeds the VTRIP threshold, an internal 65A current source (ITIMERUP) is activated to charge the capacitor connected to the TIMER pin. When the TIMER pin voltage reaches the VTIMERH threshold, the circuit breaker is tripped pulling the GATE pin low, the ITIMERUP current source is disabled, and the TIMER pin capacitor is discharged by the ITIMERDN current source. When the voltage at the TIMER pin is less than 0.5V, the MIC2587 can be restarted by toggling the ON pin LOW then HIGH. For the MIC2587R, the capacitor connected to the TIMER pin sets the period of auto-retry where the duty cycle is fixed at a nominal 5%.
1
ON
2
FB
3
5
TIMER
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Pin Description
Pin Number Pin Name Pin Function Gate Drive Output: This pin is the output of an internal charge pump connected to the gate of an external, N-channel power MOSFET. The charge pump has been designed to provide a minimum gate drive (VGATE = VGATE - VCC) of +7.5V over the input supply's full operating range. When the ON pin voltage is higher than the VONH threshold, a 16A current source (IGATEON) charges the GATE pin. When in current limit, the output voltage at the GATE pin is adjusted so that the voltage across the external sense resistor is held equal to VTRIP while the capacitor connected to the TIMER pin charges. If the current limit condition goes away before the TIMER pin voltage rises above the VTIMERH threshold, then steady-state operation resumes. The GATE output pin is shut down whenever: (1) the input supply voltage is lower than the VUVL threshold, (2) the ON pin voltage is lower than the VONL threshold, (3) the TIMER pin voltage is higher than the VTIMERH threshold, or (4) the difference between the VCC and SENSE pins is greater than VTRIP while the TIMER pin is grounded. For cases (3) and (4) - overcurrent fault conditions - the GATE is immediately pulled to ground by IGATEFLT, a 30mA (minimum) pull-down current. Circuit Breaker Sense Input: This pin is the (-) Kelvin sense connection for the output supply rail. A low-valued resistor (RSENSE) between this pin and the VCC pin sets the circuit breaker's current limit trip point. When the current limit detector circuit is enabled (as well as the current limit timer), while the FB pin voltage remains higher than 1V, the voltage across the sense resistor (VCC-VSENSE) will be regulated to VTRIP (47mV, typically) to maintain a constant current into the load. When the FB pin voltage is less than 0.5V, the voltage across the sense resistor decreases linearly to a minimum of 12mV (typical) when the FB pin voltage is at 0V. To disable the circuit breaker (and defeat all current limit protections), the SENSE pin and the VCC pin can be tied together. Positive Supply Voltage Input: This pin is the (+) Kelvin sense connection for the output supply rail. The nominal operating voltage range for the MIC2587 and the MIC2587R is +10V to +80V, and VCC can withstand input transients up to +100V. An undervoltage lockout circuit holds the GATE pin low whenever the supply voltage to the MIC2587 and the MIC2587R is less than the VUVH threshold.
6
GATE
7
SENSE
8
VCC
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MIC2587/MIC2587R
Absolute Maximum Ratings(1)
(All voltages are referred to GND) Supply Voltage (VCC) pin .............................. -0.3V to +100V GATE pin...................................................... -0.3V to +100V ON, SENSE pins .......................................... -0.3V to +100V PWRGD, /PWRGD pins ............................... -0.3V to +100V FB pin ........................................................... -0.3V to +100V TIMER pin ........................................................ -0.3V to +6V ESD Rating Human Body Model ................................................. 2kV Machine Model ......................................................200V Lead Temperature (Soldering) Standard Package (-xBM) IR Reflow..............................240C + 0C/-5C Lead-Free Package (-xYM) IR Reflow..............................260C + 0C/-5C
Operating Ratings(2)
Supply Voltage (VCC)...................................... +10V to +80V Ambient Temperature Range (TA) ............... -40C to +85C Junction Temperature (TJ) ....................................... +125C Package Thermal Resistance (JA) 8-pin SOIC...................................................... 160 C/W
DC Electrical Characteristics(4)
VCC = +24V and +48V, TA = 25C, unless otherwise noted. BOLD indicates specifications apply over the full operating temperature range of -40C to +85C. Symbol VCC ICC VUVH VUVL VHYSLO VFBH VFBL VHYSFB VFB IFB VTRIP VGATE IGATEON IGATEFLT VCC Undervoltage Lockout Hysteresis Feedback Pin Voltage High Threshold Feedback Pin Voltage Low Threshold Feedback Voltage Hysteresis FB Pin Threshold Line Regulation FB Pin Input Current Circuit Breaker Trip Voltage, VCC-VSENSE MOSFET Gate Drive, VGATE-VCC GATE Pin Pull-up Current GATE Pin rapid pull-down current (during a fault condition, until VGATE = VGATE[TH]) (VGATE[TH] is the MOSFET threshold) IGATEOFF ITIMERUP GATE Pin Turn-off Current TIMER Pin Charging Current Normal turn-off, or from VGATE[TH] (MOSFET) to 0V after a fault condition (VCC - VSENSE) > VTRIP VTIMER = 0V -24 1.8 -65 -120 mA A 10V VCC 80V 0V VFB 3V VFB = 0V (See Fig. 1) VFB = 1V (See Fig. 1) +10V VCC +80V Start cycle, VGATE = 7V (VCC - VSENSE) = (VTRIP + 10mV) VGATE = 5V -0.05 -1 5 39 7.5 -10 30 -16 80 12 47 FB Low-to-High transition FB High-to-Low transition 1.280 1.208 Parameter Supply Voltage Supply Current Supply Voltage Undervoltage Lockout VCC rising VCC falling 7.5 7.0 Condition Min 10 2 8.0 7.5 500 1.313 1.233 80 0.05 1 17 55 18 -22 200 1.345 1.258 Typ Max 80 5 8.5 8.0 Units V mA V V mV V V mV mV/V A mV mV V A mA
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DC Electrical Characteristics(4)
VCC = +24V and +48V, TA = 25C, unless otherwise noted. BOLD indicates specifications apply over the full operating temperature range of -40C to +85C. Symbol ITIMERDN VTIMERH VTIMERL VONH VONL VHYSON ION VOL Parameter TIMER Pin Pull-down Current TIMER Pin High Threshold Voltage TIMER Pin Low Threshold Voltage ON Pin High Threshold Voltage ON Pin Low Threshold Voltage ON Pin Hysteresis ON Pin Input Current Power-Good Output Voltage 0V VON 80V PWRGD or /PWRGD = LOW IOL = 1.6 mA IOL = 4 mA IOFF Power-Good Leakage Current PWRGD or /PWRGD = Open-Drain VPG = VCC, VON = 1.5V 0.4 0.8 10 V V A ON Low-to-High transition ON High-to-Low transition Condition (VCC - VSENSE) < VTRIP VTIMER = 0.6V 1.280 0.4 1.280 1.208 1.313 0.49 1.313 1.233 80 2 1.345 0.6 1.355 1.258 V V V V mV A Min 1.5 Typ 3.5 Max 5 Units A
AC Electrical Characteristics
VCC = +24V and +48V, TA = 25C unless otherwise noted. BOLD indicates specifications apply over the full operating temperature range of -40C to +85C. Symbol tPONLH tPONHL tPFBLH tPFBHL tPFBHL tPFBLH tOCSENSE
Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. 4. Specification for packaged product only.
Parameter ON High to GATE High ON Low to GATE Low FB Valid to PWRGD High (MIC2587/87R-1) FB Invalid to PWRGD Low (MIC2587/87R-1) FB Valid to /PWRGD Low (MIC2587/87R-2) FB Invalid to /PWRGD High (MIC2587/87R-2) Overcurrent Sense to GATE Low Trip Time
Condition IRF530, CGATE = 10nF VIN = 48V, IRF530, CGATE = 10nF RPG = 50k pull-up to 48V CL=100pF RPG = 50k pull-up to 48V CL=100pF RPG = 50k pull-up to 48V CL=100pF RPG = 50k pull-up to 48V CL=100pF (VCC - VSENSE) = (VTRIP + 10mV) Figure 7
Min
Typ 3 1 2 4 4 2 1
Max
Units ms ms s s s s
2
s
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MIC2587/MIC2587R
Timing Diagrams
Figure 1. Foldback Current Limit Transfer Characteristic
Figure 2. ON to GATE Timing
Figure 3. MIC2587/87R-1 FB to PWRGD Timing
Figure 4. MIC2587/87R-2 FB to /PWRGD Timing
Figure 5. Overcurrent Sense to GATE Timing
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MIC2587/MIC2587R
Function Diagram
MIC2587/MIC2587R Block Diagram
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MIC2587/MIC2587R Zener diode (18-V) connected from the source to the gate as shown in the "Typical Applications" circuit is highly recommended. A good choice for an 18-V Zener diode in this application is the MMSZ5248B, available in a small SOD123 package. CGATE is used to adjust the GATE voltage slew rate while R3 minimizes the potential for high-frequency parasitic oscillations from occurring in M1. However, note that resistance in this part of the circuit has a slight destabilizing effect upon the MIC2587/MIC2587R's current regulation loop. Compensation resistor R4 is necessary for stabilization of the current regulation loop. The current through the power transistor during initial inrush is given by:
IINRUSH = CLOAD x IGATEON CGATE
Functional Description
Hot Swap Insertion When circuit boards are inserted into systems carrying live supply voltages ("hot swapped"), high inrush currents often result due to the charging of bulk capacitance that resides across the circuit board's supply pins. These current spikes can cause the system's supply voltages to temporarily go out of regulation causing data loss or system lock-up. In more extreme cases, the transients occurring during a hot swap event may cause permanent damage to connectors or on-board components. The MIC2587/MIC2587R is designed to address these issues by limiting the maximum current that is allowed to flow during hot swap events. This is achieved by implementing a constant-current control loop at turn-on. In addition to inrush current control, the MIC2587 and MIC2587R incorporate input voltage supervisory functions and user-programmable overcurrent protection, thereby providing robust protection for both the system and the circuit board. Input Supply Transient Suppression and Filtering The MIC2587/MIC2587R is guaranteed to withstand transient voltage spikes up to 100V. However, voltage spikes in excess of 100V may cause damage to the controller. In order to suppress transients caused by parasitic inductances, wide (and short) power traces should be utilized. Alternatively, a heavier trace plating will help minimize inductive spikes that may arise during events (e.g., short circuit loads) that can cause a large di/dt to occur. External surge protection, such as a clamping diode, is also recommended as an added safeguard for device (and system) protection. Lastly, a 0.1F decoupling capacitor from the VCC pin to ground is recommended to assist in noise rejection. Place this filter capacitor as close as possible to the VCC pin of the controller. Start-Up Cycle When the power supply voltage to the MIC2587/MIC2587R is higher than the VUVH and the VONH threshold voltages, a start cycle is initiated. When the controller is enabled, an internal 16A current source (IGATEON) is turned on and the GATE pin voltage rises from 0V with respect to ground at a rate equal to:
dVGATE IGATEON = dt CGATE
(2)
The drain current of the MOSFET is monitored via an external current sense resistor to ensure that it never exceeds the programmed threshold, as described in the "Circuit Breaker Operation" section. A capacitor connected to the controller's TIMER pin sets the value of overcurrent detector delay, tFLT, which is the time for which an overcurrent event must last to signal a fault condition and to cause an output latch-off. These devices will be driving a capacitive load in most applications, so a properly chosen value of CTIMER prevents false-, or nuisance-, tripping at turn-on as well as providing immunity to noise spikes after the start-up cycle is complete. The procedure for selecting a value for CTIMER is given in the "Circuit Breaker Operation" section. Overcurrent Protection The MIC2587 and the MIC2587R use an external, lowvalue resistor in series with the drain of the external MOSFET to measure the current flowing into the load. The VCC connection (Pin 8) and the SENSE connection (Pin 7) are the (+) and (-) inputs, respectively, of the device's internal current sensing circuits. Kelvin sense connections are strongly recommended for sensing the voltage across these pins. See the "Applications Information" for further details. The nominal current limit is determined by the following equation.
ILIMIT = VTRIP(TYP) RSENSE
(3)
(1)
The internal charge pump has sufficient output drive to fully enhance commonly available power MOSFETs for the lowest possible DC losses. The gate drive is guaranteed to be between 7.5V and 18V over the entire supply voltage operating range (10V to 80V), so 60V BVDSS and 30V BVDSS N-channel power MOSFETs can be used for +48V and +24V applications, respectively. However, an external October 2004 9
where VTRIP(TYP) is the typical current limit threshold specified in the datasheet and RSENSE is the value of the selected sense resistor. As the MIC2587 and the MIC2587R employ a constant-current regulation scheme in current limit, the charge pump's output voltage at the GATE pin is adjusted so that the voltage across the external sense resistor is held equal to VTRIP while the capacitor connected to the TIMER pin is being charged. If
M9999-102204 (408) 955-1690
Micrel the current-limit condition goes away before the TIMER pin voltage rises above the VTIMERH threshold, then steadystate operation resumes. To prevent excessive power dissipation in the external MOSFET under load current fault conditions, the FB pin voltage is used as the control element in a circuit that lowers the current limit as a function of the output voltage. When the load current increases to the point where the output voltage at the load approaches 0V (likewise, the MIC2587/MIC2587R's FB pin voltage also approaches 0V), the result is a proportionate decrease in the maximum current allowed into the load. This foldback current limit subcircuit's transfer characteristic is shown in Figure 1. Under excessive load conditions (output and FB voltage equals 0V), the foldback current limiting circuit controls the MIC2587/MIC2587R's GATE drive to force a constant 12mV (typical) voltage drop across the external sense resistor. Circuit Breaker Operation The MIC2587/MIC2587R employ an electronic circuit breaker that protects the external N-channel power MOSFET and other system components against largescale output current faults, both during initial card insertion or during steady-state operation. The current limit threshold is set via an external resistor, RSENSE, connected between the circuit's VCC pin and SENSE pin. For the MIC2587/MIC2587R, a fault current timing circuit is set via an external capacitor (CTIMER) that determines the length of the time delay (tFLT) for which the controller remains in current limit before the circuit breaker is tripped. Programming the response time of the overcurrent detector helps to prevent nuisance tripping of the circuit breaker because of high inrush currents charging bulk and distributed capacitive loads. The nominal overcurrent response time is calculated using the following equation:
tFLT = CFILTER x VTIMERH ITIMERUP
MIC2587/MIC2587R GATE pin is immediately pulled low by a 30mA (minimum) internal current sink. This operation turns off the MOSFET quickly and disconnects the input from the load. The value of CTIMER should be selected to allow the circuit's minimum regulated output current (IOUT) to equal ILIMIT for somewhat longer than the time it takes to charge the total load capacitance. An initial value for CTIMER is found by calculating the time it will take for the MIC2587/MIC2587R to completely charge up the output capacitive load. Assuming the load is enabled by the PWRGD (or /PWRGD) signal of the IC, the turn-on delay time is derived from the following expression, I = C x (dV/dt):
t TURN-ON = CLOAD x VCC(MAX) ILIMIT
(5)
Using parametric values for the MIC2587/MIC2587R, an expression relating a worse-case design value for CTIMER, using the MIC2587/MIC2587R specification limits, to the circuit's turn-on delay time is:
CTIMER(MAX) = t TURN-ON x ITIMERUP(MAX) VTIMERH(MIN)
120 A CTIMER(MAX) = t TURN -ON x 1.280V
F CTIMER (MAX) = t TURN -ON x 94 x 10 - 6 sec
(6)
tFLT (ms) = 20 x CFILTER ( F)
(4)
For example, in a system with a CLOAD = 1000F, a maximum VCC = +72V, and a maximum load current on a nominal +48V buss of 1.65A, the nominal circuit design equations steps are: 1. Choose ILIMIT = IHOT_SWAP(nom) = 2A (1.65A + 20%); 2. Select an RSENSE (Closest 1% standard value is 19.6m); 3. Using ICHARGE = ILIMIT = 2A, the application circuit turn-on time is calculated using Equation 5:
t TURN-ON =
Whenever the voltage across RSENSE exceeds the MIC2587/MIC2587R's nominal circuit breaker threshold voltage of 47mV during steady-state operation, two things occur: 1. A constant-current regulation loop will engage within 1s after an overcurrent condition is detected by RSENSE, and the control loop is designed to hold the voltage across RSENSE equal to 47mV. This feature protects both the load and the MIC2587/MIC2587R circuits from excessively high currents. 2. Capacitor CTIMER is then charged up to the VTIMERH threshold (1.313V) by an internal 65A current source (ITIMERUP). If the excessive current persists such that the voltage across CTIMER crosses the VTIMERH threshold, the circuit breaker trips and the October 2004 10
(1000F x 72V)
2A
= 36ms
Allowing for capacitor tolerances and a nominal 36ms turnon time, an initial worse-case value for CTIMER is:
F CTIMER(MAX) = 0.036s x 94 x 10 - 6 = 3.38 F sec
The closest standard 5% tolerance capacitor value is 3.3F and would be a good initial starting value for prototyping. Whenever the MIC2587 is not in current limit, CTIMER is discharged to GND by an internal 3.5A current sink (ITIMERDN). For the MIC2587R, the circuit breaker automatically resets after (20) tFLT_AUTO time constants. If the fault condition still exists, capacitor CTIMER will begin to charge up to the
M9999-102204 (408) 955-1690
Micrel VTIMERH threshold, and if exceeded, trip the circuit breaker. Capacitor CTIMER will then be discharged by ITIMERDN until the voltage across CTIMER drops below the VTIMERL threshold, at which time another start cycle is initiated. This will continue until either of the following occurs: a) the fault condition is removed, b) the input supply voltage power is removed/recycled, or c) the ON pin is toggled LOW then HIGH. The duty cycle of the auto-restart function is therefore fixed at 5% and the period of the auto-restart cycle is given by:
t AUTO -RESTARt = 20 x tFLT_AUTO
MIC2587/MIC2587R Power-is-Good Output Signals For the MIC2587-1 and MIC2587R-1, the power-good output signal (PWRGD) will be high impedance when the FB pin voltage is higher than the VFBH threshold and will pull down to GND when the FB pin voltage is lower than the VFBL threshold. For the MIC2587-2 and MIC2587R-2, the power-good output signal (/PWRGD) will pull down to GND when the FB pin voltage is higher than the VFBH threshold and will be high impedance when the FB pin voltage is lower than the VFBL threshold. Hence, the (-1) parts have an active-HIGH PWRGD signal and the (-2) parts have an active-LOW /PWRGD output. PWRGD (or /PWRGD) may be used as an enable signal for one or more following DC/DC converter modules or for other system uses as desired. When used as an enable signal, the time necessary for the PWRGD (or /PWRGD) signal to pull-up (when in high impedance state) will depend upon the (RC) load at the Power-is-Good pin. The Power-is-Good output pin is connected to an opendrain, N-channel transistor implemented with high-voltage structures. These transistors are capable of operating with pull-up resistors to supply voltages as high as 100V.
t AUTO -RESTART = 20 x
(CTIMER ) x (VTIMERH - VTIMERL )
ITIMERUP
ms t AUTO-RESTART = CTIMER x 250 F
(7)
The auto-restart period for the example above where the worse-case CTIMER was calculated to be 3.3F is: tAUTO-RESTART = 825ms Input Undervoltage Lockout The MIC2587/MIC2587R have an internal undervoltage lockout circuit that inhibits operation of the controller's internal circuitry unless the power supply voltage is stable and within an acceptable tolerance. If the supply voltage to the controller, with respect to ground is greater than the VUVH threshold voltage (8V typical), then the controller's internal circuits are enabled and the controller is then ready for normal operation pending the state of the ON pin voltage. Once in steady-state operation, the controller's internal circuits remain active so long as the supply voltage with respect to ground is higher than the controller's internal VUVL threshold voltage (7.5V typical).
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a similar approach as setting the circuit's ON/OFF input voltage. The equations to set the trip points are shown below. For the following +48V telecom application, poweris-good output signal PWRGD (or /PWRGD) is to be deasserted when the output supply voltage is lower than +48V-10% (+43.2V).
R5 + R6 VOUT(NOT GOOD) = VFBL x R6
Applications Information
External ON/OFF Control The MIC2587/MIC2587R have an ON pin input that is used to enable the controller to commence a start-up sequence upon card insertion or to disable controller operation upon card removal. In addition, the ON pin can be used to reset the MIC2587/MIC2587R's internal electronic circuit breaker in the event of a load current fault. To reset the electronic circuit breaker, the ON pin is toggled LOW then HIGH. The ON pin is internally connected to an analog comparator with 80mV of hysteresis. When the ON pin voltage falls below its internal VONL threshold, the GATE pin is immediately pulled low. The GATE pin will be held low until the ON pin voltage is above its internal VONH threshold. The external circuit's ON threshold voltage level is programmed using a resistor divider (R1 & R2) as shown in the "Typical Application" circuit. The equations to set the trip points are shown below. For the following example, the external circuit's ON threshold is set to VONH(EX) = +37V, a value commonly used in +48V Central Office power distribution applications.
R1+ R2 VONH(EX) = VONH x R2
(9)
Given VFBL and R6, a value for R5 can be determined. A suggested value for R6 is that which will provide approximately 100A of current through the voltage divider chain at VOUT(NOT GOOD) = VFBL. This yields the following equation as a starting point:
R6 = VFBL(TYP) 100A
= 1.233V = 12.33 k 100A
The closest standard 1% value for R6 is 12.4k. Now, solving for R5 yields:
V 43.2V OUT(NOT GOOD) - 1 = 12.4k x R5 = R6 x - 1 = 422 k V 1.233V FBL(TYP)
(8)
Given VONH and R2, a value for R1 can be determined. A suggested value for R2 is that which will provide approximately 100A of current through the voltage divider chain at VCC = VONH. This yields the following as a starting point:
R2 = VONH(TYP) 100 A
= 1.313V = 13.13k 100 A
The closest standard 1% value for R2 is 13k. Now, solving for R1 yields:
V 37V ONH(EX) - 1 = 13k x R1= R2 x - 1 = 353.3 k V 1.313V ONH(TYP)
The closest standard 1% value for R5 is 422k. Using standard 1% resistor values, the external circuit's nominal "power-is-good" and "power-is-not-good" output voltages are: VOUT(GOOD) = +46V VOUT(NOT GOOD) = +43.2V In solving for VOUT(GOOD), substitute VFBH for VFBL in Equation 9. Sense Resistor Selection The sense resistor is nominally valued at:
R SENSE(NOM) = VTRIP(TYP) IHOT_SWAP(NOM)
(10)
The closest standard 1% value for R1 is 357k. Using standard 1% resistor values, the external circuit's nominal ON and OFF thresholds are: VON(EX) = +36V VOFF(EX) = +34V In solving for VOFF(EX), replace VONH with VONL in Equation 8. Output Voltage Power-is-Good Detection The MIC2587/MIC2587R includes an analog comparator used to monitor the output voltage of the controller through an external resistor divider as shown in the "Typical Application" circuit. The FB input pin is connected to the non-inverting input and is compared against an internal reference voltage. The analog comparator exhibits a hysteresis of 80mV. Setting the "Power-is-Good" threshold for the circuit follows
where VTRIP(TYP) is the typical (or nominal) circuit breaker threshold voltage (47mV) and IHOT_SWAP(NOM) is the nominal inrush load current level to trip the internal circuit breaker. To accommodate worse-case tolerances in the sense resistor (for a 1% initial tolerance, allow 3% tolerance for variations over time and temperature) and circuit breaker threshold voltages, a slightly more detailed calculation must be used to determine the minimum and maximum hot swap load currents. As the MIC2587/MIC2587R's minimum current limit threshold voltage is 39mV, the minimum hot swap load current is determined where the sense resistor is 3% high:
IHOT_SWAP(MIN) =
(1.03 x R SENSE(NOM) )
39mV
=
37.9mV R SENSE(NOM)
Keep in mind that the minimum hot swap load current should be greater than the application circuit's upper steady-state load current boundary. Once the lower value of RSENSE has been calculated, it is good practice to check
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IHOT_SWAP(MAX) =
MIC2587/MIC2587R
across RSENSE is sampled in such a way that the high currents through the power traces will not introduce significant parasitic voltage drops in the sense leads. It is recommended to connect the hot swap controller's sense leads directly to the sense resistor's metalized contact pads. The Kelvin sense signal traces should be symmetrical with equal length and width, kept as short as possible and isolated from any noisy signals and planes. Additionally, for designs that implement Kelvin sense connections that exceed 1" in length and/or if the Kelvin (signal) traces are vulnerable to noise possibly being injected onto these signals, the example circuit shown in Figure 7 can be implemented to combat noisy environments. This circuit implements a 1.6 MHz lowpass filter to attenuate higher frequency disturbances on the current sensing circuitry. However, individual system analysis should be used to determine if filtering is necessary and to select the appropriate cutoff frequency for each specific application. Other Layout Considerations Figure 8 is a recommended PCB layout diagram for the MIC2587-2BM. 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., 1oz. or 2oz.) is kept to a maximum of 10C to 25C. Also, these traces should be as short as possible in order to minimize the IR drops between the input and the load. The feedback network resistor values in Figure 8 are selected for a +24V application. The resistors for the feedback (FB) and ON pin networks should be placed close to the controller and the associated traces should be as short as possible to improve the circuit's noise immunity. The input "clamping diode" (D1) is referenced in the "Typical Application Circuit" on Page 1. If possible, use high-frequency PCB layout techniques around the GATE circuitry (shown in the "Typical Application Circuit") and use a dummy resistor (e.g., R3 = 0) during the prototype phase. If R3 is needed to eliminate high-frequency oscillations, common values for R3 range between 4.7 to 20 for various power MOSFETs. Finally, the use of plated-through vias will be needed to make circuit connection to the power and ground planes when utilizing multi-layer PCBs. MOSFET and Sense Resistor Vendors Device types, part numbers, and manufacturer contacts for power MOSFETs and sense resistors are provided in Table 1.
(0.97 x R SENSE(NOM) )
55mV
=
56.7mV R SENSE(NOM)
In this case, the application circuit must be sturdy enough to operate over a ~1.5-to-1 range in hot swap load currents. For example, if an MIC2587 circuit must pass a minimum hot swap load current of 4A without nuisance trips, RSENSE should be set to:
R SENSE(NOM) = 39mV = 9.75m 4A
where the nearest 1% standard value is 9.76m. At the other tolerance extremes, IHOT_SWAP(MAX) for the circuit in question is then simply:
IHOT_SWAP(MAX) = 56.7mV = 5.8A 9.76m
With a knowledge of the application circuit's maximum hot swap load current, the power dissipation rating of the sense resistor can be determined using P = I2 x R. Here, The current is IHOT_SWAP(MAX) = 5.8A and the resistance RSENSE(MIN) = (0.97)(RSENSE(NOM)) = 9.47m. Thus, the sense resistor's maximum power dissipation is: PMAX = (5.8A)2 x (9.47m) = 0.319W A 0.5W sense resistor is a good choice in this application. When the MIC2587/MIC2587R's foldback current limiting circuit is engaged in the above example, the current limit would nominally fold back to 1.23A when the output is shorted to ground.
PCB Layout Recommendations
4-Wire Kelvin Sensing Because of the low value typically required for the sense resistor, special care must be used to accurately measure the voltage drop across it. Specifically, the measurement technique across RSENSE must employ 4-wire Kelvin sensing. This is simply a means of ensuring that any voltage drops in the power traces connected to the resistors are not picked up by the signal conductors measuring the voltages across the sense resistors. Figure 6 illustrates how to implement 4-wire Kelvin sensing. As the figure shows, all the high current in the circuit (from VCC through RSENSE and then to the drain of the N-channel power MOSFET) flows directly through the power PCB traces and through RSENSE. The voltage drop
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MIC2587/MIC2587R
Figure 6. 4-Wire Kelvin Sense Connections for RSENSE
Figure 7. Current Limit Sense Filter for Noisy Systems
Figure 8. Recommended PCB Layout for Sense Resistor, Power MOSFET, Timer and Feedback Network.
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MIC2587/MIC2587R
MOSFET Vendors
Key MOSFET Type(s) SUM75N06-09L (TO-263) SUM70N06-11 (TO-263) SUM50N06-16L (TO-263) SUP85N10-10 (TO-220AB) SUB85N10-10 (TO-263) SUM110N10-09 (TO-263) SUM60N10-17 (TO-263) IRF530 (TO-220AB) IRF540N (TO-220AB) 2SK1298 (TO-3PFM) 2SK1302 (TO-220AB) 2SK1304 (TO-3P) Sense Resistors "WSL" and "WSR" Series
Breakdown Voltage (VDSS) 60V 60V 60V 100V 100V 100V 100V 100V 100V 60V 100V 100V Contact Information
Contact Information www.siliconix.com (203) 452-5664 www.siliconix.com (203) 452-5664
Vishay - Siliconix
International Rectifier Renesas
www.irf.com (310) 322-3331 www.renesas.com (408) 433-1990
Resistor Vendors Vishay - Dale IRC
www.vishay.com/docswsl_30100.pdf
(203) 452-5664 www.irctt.com/pdf_files/OARS.pdf "OARS" Series www.irctt.com/pdf_files/LRC.pdf "LR" Series (828) 264-8861 second source to "WSL" Table 1. MOSFET and Sense Resistor Vendors
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MIC2587/MIC2587R
Package Information
8-Pin SOIC (M)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 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 a Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. (c) 2004 Micrel, Incorporated.
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