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LT4254 Positive High Voltage Hot Swap Controller with Open-Circuit Detect
FEATURES
s s s s s s
DESCRIPTIO
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Allows Safe Board Insertion and Removal from a Live Backplane Controls Supply Voltage from 10.8V to 36V Foldback Current Limiting Open Circuit and Overcurrent Fault Detect Drives an External N-Channel MOSFET Automatic Retry or Latched Off Operation After Overcurrent Fault Programmable Supply Voltage Power-Up Rate Undervoltage and Overvoltage Protection Open MOSFET Detection
The LT(R)4254 is a high voltage Hot Swap controller that allows a board to be safely inserted and removed from a live backplane. An internal driver controls the high side N-channel MOSFET gate for supply voltages ranging from 10.8V to 36V. The part features an open-circuit detect (OPEN) output that indicates abnormally low load current conditions. The LT4254 features an adjustable analog foldback current limit. If the supply remains in current limit for more than a programmable time, the N-channel MOSFET shuts off, the PWRGD output goes low and the LT4254 either automatically restarts after a time-out delay or latches off until the UV pin is cycled low. The RETRY pin sets whether the part will automatically restart after an overcurrent fault or if it will latch off until the UV pin is cycled low. The PWRGD output indicates when the output voltage rises above a programmed level. An external resistor string from VCC provides programmable undervoltage and overvoltage protection. The LT4254 is available in a 16-lead SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation. Hot Swap is a trademark of Linear Technology Corporation.
APPLICATIO S
s s s s s
Hot Board Insertion Electronic Circuit Breaker/Power Bussing Industrial High Side Switch/Circuit Breaker 24V Industrial/Alarm Systems 12V and 24V Distributed Power Systems
TYPICAL APPLICATIO
VIN 24V (SHORT PIN) 16 R1 324k 1 UV LT4254 2 4 10 GND C2 33nF VCC 15 SENSE R5 0.025
24V, 2A Hot Swap Controller
Q1 IRF530 D1 CMPZ5241B 11V 13 R6 10 R7 100 C1 10nF R9 40.2k PWRGD UV = 20V OV = 40V PWRGD = 18V
4254 TA01
CL R8 140k R4 27k
VOUT 24V 1.5A
PWRGD 20V/DIV
GATE
VCC 20V/DIV
C3 0.1F
R2 40.2k R3 40.2k
OV OPEN TIMER
FB RETRY PWRGD GND 8
10 7 5
VOUT 20V/DIV GATE 20V/DIV 2.5ms/DIV
4245 TA02
U
TM
U
U
LT4254 Start-Up Behavior
CONTACT BOUNCE
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1
LT4254
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW UV OV NC OPEN PWRGD NC RETRY GND 1 2 3 4 5 6 7 8 16 VCC 15 SENSE 14 NC 13 GATE 12 NC 11 NC 10 FB 9 TIMER
Supply Voltage (VCC) .................................. - 0.3 to 44V Input Voltage (SENSE, PWRGD) ................. - 0.3 to 44V Input Voltage (GATE) .................................. - 0.3 to 50V Input Voltage (FB, UV, OPEN) ..................... - 0.3 to 44V Input Voltage (OV) ...................................... - 0.3 to 18V Input Voltage (RETRY) ............................... - 0.3 to 15V Minimum Input Voltage (TIMER) ......................... - 0.3V Maximum Input Current (TIMER) ....................... 100A Operating Temperature LT4254C ................................................. 0C to 70C LT4254I ............................................. - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LT4254CGN LT4254IGN
GN PART MARKING 4254 4254I
GN PACKAGE 16-LEAD PLASTIC SSOP
TJMAX = 125C, JA = 130C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 24V unless otherwise noted.
SYMBOL VCC ICC VUVLH VUVHYS IINUV VOVHL VOVHYS IINOV VOPEN VOLOPEN IINOPEN VSENSETRIP IINSNS IPU IPD VGATE VFB VFBHYS VOLPGD PARAMETER Operating Voltage Operating Current Undervoltage Threshold Hysteresis UV Input Current Overvoltage Threshold Hysteresis OV Input Current Open-Circuit Voltage Threshold (VCC - VSENSE) OPEN Output Low Voltage Leakage Current SENSE Pin Trip Voltage (VCC - VSENSE) SENSE Pin Input Current GATE Pull-Up Current GATE Pull-Down Current External N-Channel Gate Drive (Note 2) FB Voltage Threshold FB Hysteresis Voltage PWRGD Output Low Voltage IO = 1.6mA IO = 5mA Charge Pump On, VGATE = 7V Any Fault, VGATE = 3V VGATE - VCC, 12V VCC 20V 20V VCC 36V FB High-to-Low Transition FB Low-to-High Transition
q q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
q
MIN 10.8
TYP 1.9
MAX 36 3 4.04 0.55 -1 -3 4.04 0.55 1 5 0.5 1.3 1 25 60 70 - 55 80 12.5 12.5 4.04 4.65 0.60 0.4 1.0
UNITS V mA V V A A V V A mV V V A mV mV A A mA V V V V V V V
4254f
VCC Low-to-High Transition UV = 4.5V UV = 0V VCC Low-to-High Transition 0V OV < 6.5V
q
3.96 0.25
4 0.4 -0.1 -1.5
q
3.96 0.25
4 0.4 0.1 3.5 0.20 0.75 0.1
q
2
IO = 2mA IO = 5mA VOPEN = 5V FB = 0V FB 2V
q q
5.5 40 -15 40 4.5 10 3.96 4.20 0.3
12 50 40 - 35 60 8.8 11 4 4.45 0.45 0.25 0.63
2
U
W
U
U
WW
W
LT4254
The q denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VCC = 24V unless otherwise noted.
SYMBOL IPWRGD IINFB ITIMERPU ITIMERPD DTIMER VRETRY(TH) IINRTR tPHLUV tPLHUV tPHLFB tPLHFB tPHLSENSE PARAMETER PWRGD Pin Leakage Current FB Input Current TIMER Pull-Up Current TIMER Pull-Down Current Duty Cycle (RETRY Mode) RETRY Threshold RETRY Input Current UV Low to GATE Low UV High to GATE High FB Low to PWRGD Low FB High to PWRGD High (VCC - VSENSE) High to GATE Low VCC - VSENSE = 275mV RETRY = GND CGATE = 100pF CGATE = 100pF CONDITIONS VPWRGD = 36V FB = 4.5V
q q q q
ELECTRICAL CHARACTERISTICS
MIN -1 - 60 1 1.5 0.4 -120
TYP 0.1 -0.1 - 120 3 3 0.8 -85 1.7 6 0.8 3.2 2.5
MAX 10 -180 5 4.5 1.2 -40
UNITS A A A A % V A s s s s
4
s
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired.
Note 2: An internal clamp limits the GATE pin to a maximum of 11V above VCC (under normal operating conditions). Driving this pin to a voltage beyond the clamp voltage may damage the part.
TYPICAL PERFOR A CE CHARACTERISTICS
SENSE Pin Regulation Voltage vs Temperature, FB = 0V
17
SENSE REGULATION VOLTAGE (mV)
15 13 11 9 7 5 -50 -25
SENSE REGULATION VOLTAGE (mV)
50
ICC (mA)
50 25 75 0 TEMPERATURE (C)
UW
100
4254 G01
SENSE Pin Regulation Voltage vs Temperature, FB > 2V
60
3.0
ICC vs VCC
TA = 25C
55
2.5
2.0
45
1.5
125
40 -50 -25
75 0 25 50 TEMPERATURE (C)
100
125
1.0 10.8
15.8
25.8 20.8 VCC (V)
30.8
36
4254 G03
4254 G02
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3
LT4254 TYPICAL PERFOR A CE CHARACTERISTICS
ICC vs Temperature
PWRGD THRESHOLD VOLTAGE LOW-TO-HIGH (V)
2.0 VCC = 24V VCC = 36V
4.46 4.44 4.42 4.40 4.38 4.36 4.34 -50 -25
PWRGD THRESHOLD VOLTAGE HIGH-TO-LOW (V)
1.8
ICC (mA)
1.6
VCC = 10.8V
1.4
1.2
1.0 -50 -25
50 25 0 75 TEMPERATURE (C)
FB Pin Hysteresis vs Temperature
480 460 0 -5
PWRGD HYSTERESIS (mV)
440 420 400 380 360 340 320 300 -50 -25 0 25 75 50 TEMPERATURE (C) 100 125
IGATE PULL-UP CURRENT (A)
-15 -20 -25 -30 -35 -40 -45 -50 -25 0 25 75 50 TEMPERATURE (C) 100 125
IGATE DOWN (mA)
VGATE - VCC vs Temperature
12 11 10 9 8 7 6 5 4 - 50 - 25 0 75 50 25 TEMPERATURE (C) 100 125 10.5 VCC = 12V
VGATE - VCC (V) VGATE -VCC (V)
GATE DRIVE (VGATE - VCC) (V)
VCC = 18V
VCC = 10.8V
4
UW
100
4254 G04
4254 G07
FB Pin Threshold Voltage (Lowto-High) vs Temperature
4.02 4.01 4.00 3.99 3.98 3.97
FB Pin Threshold Voltage (Highto-Low) vs Temperature
125
50 25 75 0 TEMPERATURE (C)
100
125
3.96 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
4254 G05
4254 G06
GATE Pin Pull-Up Current vs Temperature
80 75 70 65 60 55 50 45
GATE Pin Pull-Down Current vs Temperature
-10
40 - 50 - 25
0
75 50 25 TEMPERATURE (C)
100
125
4254 G08
4254 G09
VGATE - VCC vs Temperature
13.0 12.5 12.0 VCC = 24V, 36V 11.5 VCC = 20V 11.0 14 12 10 8 6 4 2
GATE Drive vs VCC
TA = 25C
10.0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
0 10.8
20.8 VCC (V)
30.8
36
4254 G12
4254 G10
4254 G11
4254f
LT4254 TYPICAL PERFOR A CE CHARACTERISTICS
TIMER Pin Pull-Up Current vs Temperature
-75
TIMER PULL-DOWN CURRENT (A)
TIMER PULL-UP CURRENT (A) 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 - 50 - 25 0 75 50 25 TEMPERATURE (C) 100 125
ITIMER DOWN (A)
-85 -95 -105 -115 -125 -135 -145 -50 -25
50 25 0 75 TEMPERATURE (C)
Timer Shutdown Threshold vs Temperature
TIMER SHUTDOWN THRESHOLD VOLTAGE (V)
TA = 25C
UV THRESHOLD VOLTAGE HIGH-TO-LOW (V)
5.0
4.8
ITIMER UP (A)
4.6
4.4
4.2
4.0 -50 -25
50 25 0 75 TEMPERATURE (C)
UV Threshold Voltage (Low-toHigh) vs Temperature
UV THRESHOLD VOLTAGE LOW-TO-HIGH (V)
4.01 4.00 3.99 3.98 3.97 3.96 -50 -25
450
OV THRESHOLD VOLTAGE (LOW-TO-HIGH) (V)
4.02
UV HYSTERESIS (mV)
50 25 75 0 TEMPERATURE (C)
UW
100
4254 G13
TIMER Pin Pull-Down Current vs Temperature
3.4 3.2 3.0 2.8 2.6 2.4 2.2
TIMER Pin Pull-Down Current vs VCC
TA = 25C
125
2.0 10.8
15.8
20.8
25.8
30.8
36
4254 G15
VCC (V)
4254 G14
ITIMER Up vs VCC
-110 3.66 3.65 3.64 3.63 3.62 3.61 3.60 3.59
UV Threshold Voltage (High-toLow) vs Temperature
-112
-114
-116
-118
100
125
-120 10.8
15.8
25.8 20.8 VCC (V)
30.8
36
4254 G17
3.58 - 50 - 25
0
75 50 25 TEMPERATURE (C)
100
125
4254 G16
4254 G18
UV Hysteresis vs Temperature
500 4.02 4.01 4.00 3.99 3.98 3.97
OV Threshold Voltage (Low-toHigh) vs Temperature
400 350 300 250 200 -50 -25
100
125
50 25 75 0 TEMPERATURE (C)
100
125
3.96 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
4254 G19
4254 G20
4257 G21
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5
LT4254 TYPICAL PERFOR A CE CHARACTERISTICS
OV Threshold Voltage (High-toLow) vs Temperature
OV THRESHOLD VOLTAGE (HIGH-TO-LOW) (V)
3.70
OV HYSTERESIS (mV)
3.65
OPEN VOL (V)
3.60
3.55
3.50 -50 -25
75 0 25 50 TEMPERATURE (C)
OPEN Pin Threshold Voltage vs Temperature
5.0 6 5 4.5 PWRGD VOL (V) 4 3 2 1 3.0 -50 -25 0 75 0 25 50 TEMPERATURE (C) 100 125
OPEN THRESHOLD VOLTAGE (mV)
4.0
3.5
PI FU CTIO S
VCC: Input Supply Voltage. The positive supply input ranges from 10.8V to 36V for normal operation. ICC is typically 1.9mA. An internal circuit disables the LT4254 for inputs less than 9.8V (typ). GND: Device Ground. This pin must be tied to a ground plane for best performance. FB: Power Good Comparator Input. FB monitors the output voltage through an external resistive divider. When the voltage on the FB pin is lower than the high-to-low threshold of 4V, the PWRGD pin is pulled low and released when the FB pin is pulled above the 4.45V low-to-high threshold. The FB pin also affects foldback current limit (see Figure 7 and related discussion). To disable PWRGD monitoring, connect FB to the output voltage and float the PWRGD pin. TIMER: Timing Input. An external timing capacitor from TIMER to GND programs the maximum time the part is allowed to remain in current limit. When the part goes into current limit, a 120A pull-up current source starts to charge the timing capacitor. When the voltage on the TIMER pin
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6
UW
100
4254 G22
OV Hysteresis vs Temperature
500 450 400 350 300 250 200 -50 -25
2 10
OPEN Output Voltage vs ILOAD
TA = 25C
8
6
4
125
0
50 25 75 0 TEMPERATURE (C)
100
125
0
2
4
6 8 ILOAD (mA)
10
12
4254 G24
4254 G23
PWRGD Output Voltage vs ILOAD
TA = 25C
0
2
4
6 8 ILOAD (mA)
10
12
4254 G26
4254 G25
U
U
U
LT4254
PI FU CTIO S
reaches 4.65V (typ), the GATE pin is pulled low; the TIMER pull-up current will be turned off and the capacitor is discharged by a 3A pull-down current. When the TIMER pin falls below 0.65V (typ), the GATE pin turns on again if the RETRY pin is high (if the RETRY pin is low, the UV pin must be pulsed low to reset the internal fault latch before the GATE pin will turn on). If the RETRY pin is grounded and the UV pin is not cycled low, the GATE pin remains latched off and the TIMER pin will be discharged near ground. The UV pin must be cycled low after the TIMER pin has discharged below 0.65V (typ) to reset the part. If the RETRY pin is floating or connected to a voltage above its 1.2V threshold, the LT4254 automatically restarts after a current fault. Under an output short-circuit condition, the LT4254 cycles on and off with a 3% on-time duty cycle. RETRY: Current Fault Retry. RETRY commands the operational mode of the current limit. If the RETRY pin is floating, the LT4254 automatically restarts after a current fault. If it is connected to a voltage below 0.4V, the part latches off after a current fault (which requires that the UV pin be cycled low in order to start normal operation again). GATE: High Side Gate Drive for the External N-Channel MOSFET. An internal charge pump guarantees at least 10V of gate drive for VCC supply voltages above 20V and 4.5V gate drive for VCC supply voltages between 10.8V and 20V. The rising slope of the voltage on GATE is set by an external capacitor connected from the GATE pin to GND and an internal 35A pull-up current source from the charge pump output. If the current limit is reached, the GATE pin voltage is adjusted to maintain a constant voltage across the sense resistor while the timing capacitor starts to charge. If the TIMER pin voltage ever exceeds 4.65V, the GATE pin is pulled low. The GATE pin is also pulled to GND whenever the UV pin is pulled low, or the VCC supply voltage drops below the externally programmed undervoltage threshold or above the overvoltage threshold. The GATE pin is clamped internally to a maximum voltage of 11V (typ) above VCC under normal operating conditions. Driving this pin beyond the clamp voltage may damage the part. A zener diode is needed between the gate and source of the external MOSFET to protect its gate oxide under instantaneous short-circuit conditions. See Applications Information. SENSE: Current Limit Sense. A sense resistor is placed in the supply path between VCC and SENSE. The current limit circuit regulates the voltage across the sense resistor (VCC - SENSE) to 50mV while in current limit when FB is 2V or higher. If FB drops below 2V, the regulated voltage across the sense resistor decreases linearly and stops at 15mV when FB is 0V. The OPEN output also uses SENSE to detect when the output current is less than (3.5mV)/R5. To defeat current limit, connect SENSE to VCC. PWRGD: Open Collector Output to GND. PWRGD is pulled low whenever the voltage on FB falls below the high-to-low threshold voltage. It goes into a high impedance state when the voltage on FB exceeds the low-to-high threshold voltage. An external pull-up resistor can pull PWRGD to a voltage higher or lower than VCC. To disable PWRGD, float this pin and connect FB to the output voltage. UV: Undervoltage Sense. UV is an input that enables the output voltage. When the UV pin is driven above 4V, the GATE pin starts charging and the output turns on. When the UV pin goes below 3.6V, the GATE pin discharges and the output shuts off. Pulsing the UV pin to ground after a current limit fault cycle (TIMER pin dischaged to below 0.65V typ) resets the fault latch (when RETRY pin is low, commanding latch off operation) and allows the part to turn back on. To disable UV sensing, connect the pin to VCC through a 10k resistor. OV: Overvoltage Sense. OV is an input that disables the output voltage. If OV ever goes above 4V, the GATE pin is discharged and the output shuts off. When OV goes below 3.6V, the GATE pin starts charging and the output turns back on. To disable OV sensing, connect pin to ground. OPEN: Open Circuit Detect Output. This pin is an open collector output that releases and is pulled high through an external resistor if the load current is less than (3.5mV/R5). If not used, leave this pin disconnected.
U
U
U
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7
LT4254
BLOCK DIAGRA W
VCC 16 SENSE 15
+OPEN3.5mV
-
VP VP GEN FB 10
CIRCUIT DETECT
4
OPEN
-
12mV ~ 50mV CURRENT LIMIT
+ +
FOLDBACK REF GEN 4V 2V
CHARGE PUMP AND GATE DRIVER
13 GATE
-
4V
+
5 PWRGD
-
RETRY UV OV VCC
7 1 2
- +
INTERNAL UV 4V
9.8V
-
UV LOGIC
0.65V
+
TIMER LOW
+
VP OV 120A
-
4V
- +
+
TIMER HIGH 4.65V
-
9
TIMER
3A 8 GND
4254 BD
4254f
8
LT4254
TEST CIRCUIT
PWRGD OPEN FB VCC SENSE GATE TIMER RETRY GND 3V 100pF 24V
+-
-+
3V
OV UV
+-
4254 TC
Figure 1
TI I G DIAGRA S
4V UV tPLHUV 2V GATE 2V
4254 F02
Figure 2. UV to GATE Timing
APPLICATIO S I FOR ATIO
Hot Circuit Insertion
When circuit boards are inserted into a live backplane, the supply bypass capacitors on the boards draw high peak currents from the backplane power bus as they charge. The transient currents can permanently damage the connector pins and glitch the system supply, causing other boards in the system to reset. The LT4254 is designed to turn on a board's supply voltage in a controlled manner, allowing the board to be safely inserted or removed from a live backplane. The
U
W
W
U
U
UW
3.6V
FB
4V
3.65V tPLHFB tPHLFB
tPHLUV
PWRGD
1V
1V
4254 F03
Figure 3. VOUT to PWRGD Timing
VCC - SENSE
50mV tPHLSENSE VCC
4254 F04
GATE
Figure 4. SENSE to GATE Timing
device also provides undervoltage and overvoltage as well as overcurrent protection while a power good output signal indicates when the output supply voltage is ready with a high output. Power-Up Sequence An external N-channel MOSFET pass transistor (Q1) is placed in the power path to control the power up of the supply voltage (Figure 5). Resistor R5 provides current detection and capacitor C1 controls the GATE slew rate.
4254f
9
LT4254
APPLICATIO S I FOR ATIO
VIN 24V (SHORT PIN)
R1 324k 1 UV
C3 0.1F
R2 40.2k R3 40.2k
2 4 10
OV OPEN TIMER
GND
C2 33nF
Figure 5. 2A, 24V Application
Resistor R7 compensates the current control loop while R6 prevents high frequency oscillations in Q1. When the power pins first make contact, transistor Q1 is held off. If the voltage on the VCC pin is between the externally programmed undervoltage and overvoltage thresholds, and the voltage on the TIMER pin is less than 4.65V (typ), transistor Q1 will be turned on (Figure 6). The voltage at the GATE pin rises with a slope equal to 35A/ C1 and the supply inrush current is set at: IINRUSH = CL * 35A/C1 If the voltage across the current sense resistor R5 reaches VSENSETRIP, the inrush current will be limited by the internal current limit circuitry. The voltage on the GATE pin is adjusted to maintain a constant voltage across the sense resistor and the TIMER pin begins to charge. When the FB pin voltage goes above the low-to-high VFB threshold, the PWRGD pin goes high. Short-Circuit Protection The LT4254 features a programmable foldback current limit with an electronic circuit breaker that protects against short circuits or excessive load currents. The current limit is set by placing a sense resistor (R5) between VCC and SENSE. To limit excessive power dissipation in the pass transistor and to reduce voltage spikes on the input supply during short-circuit conditions at the output, the current folds
10
U
R5 0.025 Q1 IRF530 D1 CMPZ5241B 11V 13 R6 10 R7 100 C1 10nF R9 40.2k PWRGD
4254 F05
W
U
U
16 VCC
15 SENSE GATE
CL R8 140k R4 27k
VOUT 24V 1.5A
LT4254 10 7 5
FB RETRY PWRGD GND 8
UV = 20V OV = 40V PWRGD = 18V
IOUT 500mA/DIV PWRGD 20V/DIV VOUT 20V/DIV GATE 20V/DIV 2.5ms/DIV
4254 F06
Figure 6. Start-Up Waveforms
back as a function of the output voltage, which is sensed internally on the FB pin. When the voltage at the FB pin is 0V, the current limit circuit drives the GATE pin to force a constant 12mV drop across the sense resistor. As the output at the FB pin increases, the voltage across the sense resistor increases until the FB pin reaches 2V, at which point the voltage across the sense resistor is held constant at 50mV (see Figure 7). The current limit threshold is calculated as: ILIMIT = 50mV/R5 where R5 is the sense resistor. For a 0.025 sense resistor, the current limit is set at 2000mA and folds back to 600mA when the output is shorted to ground. Thus, MSOFET dissipation under shortcircuit conditions is reduced from 36W to12W. See the
4254f
LT4254
APPLICATIO S I FOR ATIO
VCC - VSENSE
50mV
RESPONSE TIME (s)
12mV
0V
2V
FB
4254 F07
Figure 7. Current Limit Sense Voltage vs Feedback Pin Voltage
Layout Considerations section for important information about board layout to minimize current limit threshold error. The LT4254 also features a variable overcurrent response time. The time required for the part to regulate the GATE pin voltage is a function of the voltage across the sense resistor connected between the VCC pin and the SENSE pin. This helps to eliminate sensitivity to current spikes and transients that might otherwise unnecessarily trigger a current limit response and increase MOSFET dissipation. Figure 8 shows the response time as a function of the overdrive at the SENSE pin. TIMER The TIMER pin provides a method for programming the maximum time the part is allowed to operate in current limit. When the current limit circuitry is not active, the TIMER pin is pulled to GND by a 3A current source. When the current limit circuitry becomes active, a 120A pull-up current source is connected to the TIMER pin and the voltage will rise with a slope equal to 117A/CTIMER as long as the circuitry stays active. Once the desired maximum current limit time is known, the capacitor value is: C(nF) = 25 * t(ms) Whenever the TIMER pin reaches 4.65V (typ), the internal fault latch is set causing the GATE to be pulled low and the TIMER pin to be discharged to GND by the 3A current source. The part is not allowed to turn on again until the voltage at the TIMER pin falls below 0.65V (typ).
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12 10 8 6 4 2 0 50 100 150 VCC - VSENSE (mV) 200
4254 F08
W
U
U
Figure 8. Response Time to Overcurrent
The TIMER pin must never be pulled high by a low impedance because whenever the TIMER pin voltage rises above the upper threshold (typically 4.65V) the pin characteristics change from a high impedance current source to a low impedance. If the pin must be pulled high by a logic signal, then a resistor must be put in series with the TIMER pin to limit the current to approximately 100 microamperes. The resistance should be chosen as follows: RSERIES = (VLOGIC - 4.65V)/100A Whenever the GATE pin is commanded off by any fault condition, it is discharged with a high current, turning off the external MOSFET. The waveform in Figure 9 shows how the output latches off following a short-circuit. The drop across the sense resistor is held at 12mV as the timer ramps up. Since the output did not rise bringing FB above 2V and the current is still 12mV/R5, the circuit latches off. Automatic Restart If the RETRY pin is floating, then the functionality is as described in the previous section. When the voltage at the TIMER pin ramps back down to 0.65V (typ), the LT4254 turns on again. If the short-circuit condition at the output still exists, the cycle will repeat itself indefinitely. The duty cycle under short-circuit conditions is 3% which prevents Q1 from overheating. Latch Off Operation If the RETRY pin is grounded, the LT4254 will latch off after a current fault. After the part latches off, it may be
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11
LT4254
APPLICATIO S I FOR ATIO
Latch Off Operation
IOUT 500mA/DIV TIMER 5V/DIV VOUT 20V/DIV
GATE 20V/DIV 2.5ms/DIV
4254 F09
Figure 9. Latch Off Waveforms
commanded to start back up. This can be commanded by cycling the UV pin to ground and then back high (this command can only be accepted after the TIMER pin discharges below the 0.65V typ threshold, so as to prevent overheating transistor Q1). Therefore, using the RETRY pin only, the LT4254 will either latch off after an overcurrent fault condition or it will go into a hiccup mode. Undervoltage and Overvoltage Detection The LT4254 uses the UV and OV pins to monitor the VCC voltage and allow the user the greatest flexibility for setting the operational thresholds. The UV and OV pins are internally connected to an analog window comparator. Any time that the UV pin goes below 3.6V or the OV pin goes above 4V, the gate will be pulled low until the UV/OV pin voltages return to the normal operation voltage window (4V and 3.65V, respectively). Power Good Detection The LT4254 includes a comparator for monitoring the output voltage. The output voltage is sensed through the FB pin via an external resistor string. The comparator's output (PWRGD pin) is an open collector capable of operating from a pull-up as high as 36V. The PWRGD pin can be used to directly enable/disable a power module with an active high enable input. Figure 11 shows how to use the PWRGD pin to control an active low enable input power module. Signal inversion is accomplished by transistor Q2 and R10.
12
U
Automatic Restart Operation (Short-Ciruit Output)
IOUT 500mA/DIV VOUT 20V/DIV TIMER 5V/DIV GATE 1V/DIV 2.5ms/DIV
4254 F10
W
U
U
Figure 10. RETRY Waveforms
Open FET Detection The LT4254 can be used to detect the presence of an open FET. When the voltage across the sense resistor is less than 3.5mV, the open collector pull-down device is shut off allowing the OPEN pin to be externally pulled high. An open FET condition is signalled when the OPEN pin is high and the PWRGD pin is low (after the part has completed its start-up cycle). This open FET condition can be falsely signalled during start-up if the load is not activated until after PWRGD goes high. To avoid this false indication, the OPEN and PWRGD pins should not be polled for a period of time, t STARTUP, given by:
3 * VCC * C1 = tSTARTUP 35A
This can be accomplished either by a microcontroller (if available) or by placing an RC filter as shown in Figure 12. Once the OPEN voltage exceeds the monitoring logic threshold, VTHRESH, and PWRGD is low, an open FET condition is signalled. In order to prevent a false indication, the RC product should be set with the following equation:
RC > 3 * VCC * C1 VLOGIC 35A ln VLOGIC - VTHRESH
Another condition that can cause a false indication is if the LT4254 goes into current limit during start-up. This will cause tSTARTUP to be longer than calculated. Also, if the
4254f
LT4254
APPLICATIO S I FOR ATIO
VCC (SHORT PIN)
R1 324k 1 C3 0.1F R2 40.2k 2 R3 40.2k 4 10 GND C2 33nF OV UV
OPEN TIMER
Figure 11. Active Low Enable PWRGD Application
VLOGIC LT4254 4 INTERNAL OPEN COLLECTOR PULL-DOWN C
4254 F12
R OPEN
TO MONITORING LOGIC
Figure 12. Delay Circuit for OPEN FET Detection
LT4254 stays in current limit long enough for the TIMER pin to fully charge up to its threshold, the LT4254 will either latch off (RETRY = 0) or go into the current limit hiccup mode (RETRY = floating). In either case, an open FET condition will be falsely signalled. If the LT4254 does go into current limit during start-up, C1 can be increased (see Power-Up Sequence). Supply Transient Protection The LT4254 is 100% tested and guaranteed to be safe from damage with supply voltages up to 44V. However, voltage transients above 44V may damage the part. During a short-circuit condition, the large change in currents flowing through the power supply traces can cause inductive voltage transients which could exceed 44V. To minimize the voltage transients, the power trace parasitic inductance should be minimized by using wider traces or heavier trace plating and a 0.1F bypass capacitor should be placed between VCC and GND. A surge
VGATE (V)
U
R5 100m Q1 IRF530 D1 CMPZ5241B 11V 13 R6 10 R7 100 C1 10nF R9 40.2k R8 140k PWRGD R4 27k Q2 UV = 20V OV = 40V PWRGD = 18V
4254 F11
W
U
U
VOUT CL VLOGIC R10 27k
16 VCC
15 SENSE GATE
LT4254 10 7 5
FB RETRY PWRGD GND 8
suppressor (Transorb) at the input can also prevent damage from voltage transients. GATE Pin A curve of gate drive vs VCC is shown in Figure 13. The GATE pin is clamped to a maximum voltage of 12V above the VCC voltage. This clamp is designed to withstand the internal charge pump current. An external zener diode should be used if the possibility exists for an instantaneous low resistance short on VOUT to occur. At a minimum input supply voltage of 12V, the minimum gate drive voltage is 4.5V. When the input supply voltage is higher than 20V, the gate drive voltage is at least 10V and a
12 11 10 9 8 7 6 5 4 10 20 VGATE = VGATE - VCC 30 VCC (V)
4254 F13
40
Figure 13. VGATE vs VCC
4254f
13
LT4254
APPLICATIO S I FOR ATIO
VCC (SHORT PIN)
R1 324k 1 C3 0.1F R2 40.2k 2 R3 40.2k 4 10 GND C2 33nF OV UV
OPEN TIMER
Figure 14. Negative Output Voltage Protection Diode Application
standard threshold MOSFET can be used. In applications from 12V to 15V range, a logic level MOSFET must be used. In some applications it may be possible for the VOUT pin to ring below ground (due to the parasitic trace inductance). Higher current applications, especially where the output load is physically far away from the LT4254 will be more susceptible to these transients. This is normal and the LT4254 has been designed to allow for some ringing below ground. However, if the application is such that VOUT can ring more than 1V below ground, damage may occur to the LT4254 and an external diode from ground (anode) to VOUT (cathode) will have to be added to the circuit as shown in Figure 14 (it is critical that the reverse breakdown voltage of the diode be higher than the highest expected VCC voltage). A capacitor placed from ground to VOUT directly at the LT4254 pins can help reduce the amount of ringing on VOUT but it may not be enough for some applications. During a fault condition, the LT4254 pulls down on the GATE pin with a switch capable of sinking about 55mA. Once the GATE voltage drops below the output voltage by a diode forward voltage, the external zener will forward bias and the output will also be discharged to GND. In addition to the GATE capacitance, the output capacitance will be discharged through the LT4254. In applications
14
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R5 0.033 Q1 IRF530 D1 CMPZ5241B 11V R6 10 R7 100 C1 10nF R9 40.2k R4 27k R8 140k VOUT CL 100F 16 VCC 15 SENSE GATE LT4254 13 FB RETRY PWRGD GND 8 10 7 5
4254 F14
W
U
U
UV = 20V OV = 40V PWRGD = 18V
that have very large output capacitors, this could cause damage to the LT4254. Therefore, the maximum output capacitance that can be used with the LT4254 is 1000F. In applications utilizing very large external N-channel MOSFETs, the possibility exists for the MOSFET to turn on when initially inserted into a live backplane (before the LT4254 becomes active and pulls down on GATE). This is due to the drain to gate capacitance forcing current into R7 and C1 when the drain voltage steps up from ground to VCC with an extremely fast rise time. To alleviate this situation, a Schottky diode should be put across R7 with the cathode connected to C1 as shown in Figure 16. Layout Considerations To achieve accurate current sensing, a Kelvin connection to the current sense resistor (R5 in typical application circuit) is recommended. The minimum trace width for 1oz copper foil is 0.02" per amp to make sure the trace stays at a reasonable temperature. 0.03" per amp or wider is recommended. Note that 1oz copper exhibits a sheet resistance of about 530/ . Small resistances can cause large errors in high current applications. Noise immunity will be improved significantly by locating resistor dividers close to the pins with short VCC and GND traces. A 0.1F decoupling capacitor from UV to GND is also required. Figure 15 shows a layout that meets these requirements.
4254f
LT4254
APPLICATIO S I FOR ATIO
VCC
VCC SHORT PIN
R2 R1
1 2 3 4 5 6 7 8
UV OV NC OPEN PWRGD NC RETRY GND
VIA
2ND LAYER METAL
GND
Figure 15. Recommended Component Placement
PACKAGE DESCRIPTIO
GN Package 16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.045 .005 .189 - .196* (4.801 - 4.978) 16 15 14 13 12 11 10 9
.254 MIN
.150 - .165 .229 - .244 (5.817 - 6.198) .150 - .157** (3.810 - 3.988)
.0165 .0015
.0250 TYP 1 .015 .004 x 45 (0.38 0.10) .053 - .068 (1.351 - 1.727) 23 4 56 7 8 .004 - .0098 (0.102 - 0.249)
RECOMMENDED SOLDER PAD LAYOUT
.007 - .0098 (0.178 - 0.249) .016 - .050 (0.406 - 1.270) NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE
0 - 8 TYP
*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
R3 R5 RSENSE VCC SENSE LT4254 NC GATE NC NC FB TIMER C2 R9 16 15 14 13 12 11 10 9 R8 D1 R4 PWRGD OPEN C1 R6 Q1 VOUT R7 GND
4254 F15
U
W
U
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.009 (0.229) REF
.008 - .012 (0.203 - 0.305)
.0250 (0.635) BSC
GN16 (SSOP) 0502
4254f
15
LT4254
TYPICAL APPLICATIO
VCC (SHORT PIN) 16 R1 324k 1 C3 0.1F R2 40.2k UV VCC 15 SENSE GATE 13 R6 10
GND
Figure 16. High dV/dT MOSFET Turn-On Protection Circuit
RELATED PARTS
PART NUMBER LT1641-1/LT1641-2 LTC4211 LTC4251 DESCRIPTION Positive 48V Hot Swap Controller in SO-8 - 48V Hot Swap Controller in SOT-23 COMMENTS 9V to 80V Operation, Active Current Limit, Autoretry/Latchoff Floating Supply from -15V, Active Current Limiting, Fast Circuit Breaker Floating Supply from -15V, Active Current Limiting, Power Good Output Floating Supply from -15V, Active Current Limiting, Enables Three DC/DC Converters Single Hot Swap Controller with Multifunction Current Control 2.5V to 16.5V, Active Inrush Limiting, Dual Level Cicuit Breaker
LTC4252-1/LTC4252-2 - 48V Hot Swap Controller in MSOP LTC4253 - 48V Hot Swap Controller and Supply Sequencer
16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
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R5 0.033 Q1 IRF530 D1 CMPZ5241B 11V R8 140k VOUT CL 100F LT4254 R7 100 C1 10nF R9 40.2k R4 27k 2 R3 40.2k 4 10 C2 33nF OV OPEN TIMER FB RETRY PWRGD GND 8 10 7 5
4254 F16
UV = 20V OV = 40V PWRGD = 18V
4254f LT/TP 0303 2K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2003
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