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19-1472; Rev 0; 4/99
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
General Description
The MAX4370 is a circuit-breaker IC designed to offer protection in hot-swap applications using Maxim's DualSpeed/BiLevelTM detection. This controller, designed to reside either on the backplane or on the removable card, is used to protect a system from startup damage when a card or board is inserted into a rack with the main system power supply turned on. The card's discharged filter capacitors provide a low impedance that can momentarily cause the main power supply to collapse. The MAX4370 prevents this start-up condition by providing inrush current regulation during a programmable start-up period, allowing the system to stabilize safely. In addition, two on-chip comparators provide DualSpeed/BiLevel short-circuit protection and overcurrent protection during normal operation. The MAX4370 provides protection for a +3V to +12V single supply. An internal charge pump generates the controlled gate drive for an external N-channel MOSFET power switch. The MAX4370 latches the switch off after a fault condition until an external reset signal clears the device. Other features include a status pin to indicate a fault condition, an adjustable overcurrent response time, and a power-on reset comparator. The MAX4370 is specified for the extended-industrial temperature range (-40C to +85C) and is available in an 8-pin SO package.
Features
o DualSpeed/BiLevel Protection During Normal Operation o Inrush Current Regulated at Start-Up o Resides Either on the Backplane or on the Removable Card o Programmable Start-Up Period and Response Time o Allows Safe Board Insertion and Removal from Live Backplane o Protection for +3V to +12V Single Supplies o Latched Off After Fault Condition o Status Output Pin o Internal Charge Pump Generates Gate Drive for External N-Channel MOSFET
MAX4370
Ordering Information
PART MAX4370ESA TEMP. RANGE -40C to +85C PIN-PACKAGE 8 SO
Pin Configuration appears at end of data sheet.
Applications
Hot Board Insertion Solid-State Circuit Breaker
Typical Operating Circuit
BACKPLANE VCC REMOVABLE CARD RSENSE M1 N VOUT CBOARD VIN STAT GND ON CSPD STAT VSEN GATE
MAX4370
ON CSPD GND CTIM CTIM
DualSpeed/BiLevel is a trademark of Maxim Integrated Products. Patent Pending
________________________________________________________________ Maxim Integrated Products 1
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769.
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
ABSOLUTE MAXIMUM RATINGS
VIN to GND...........................................................................+15V STAT to GND ..........................................................-0.3V to +14V GATE to GND ..............................................-0.3V to (VIN + 8.5V) ON to GND (Note 1) ................................................. -1V to +14V CSPD to GND .............-0.3V to the lower of (VIN + 0.3V) or +12V VSEN, CTIM to GND ....................................-0.3V to (VIN + 0.3V) Current into ON...................................................................2mA Current into Any Other Pin................................................50mA Continuous Power Dissipation (TA = +70C) SO (derate 5.9mW/C above +70C) ........................... 471mW Operating Temperature Range ...........................-40C to +85C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10sec) ............................ +300C
Note 1: ON can be pulled below ground. Limiting the current to 2mA ensures that this pin is never lower than about -0.8V.
Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VIN = +2.7V to +13.2V, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25C.) (Note 2) PARAMETER POWER SUPPLIES Input Voltage Range Supply Current CURRENT CONTROL Slow Comparator Threshold Slow Comparator Response Time Fast Comparator Threshold Fast Comparator Response Time VSEN Input Bias Current MOSFET DRIVER Start-Up Period (Note 3) Gate Charge Current Turn-Off Time tSTART IGATE tOFF 100nF on CTIM CTIM = floating VGATE = VIN (Note 4) Time from current overload to VGATE < 0.1V, CGATE = 1000pF to GND (triggered by the fast comparator during normal operation) During start-up (current regulation provided by fast comparator) Gate Discharge Current IGATE,DIS During turn-off, triggered by a fault in normal operation or ON falling edge Measured with respect to VIN; voltage at which internal clamp circuitry is triggered IGATE = 8.5A, measured above VIN VIN 5V VIN 2.7V 5 2.7 0.1 75 21 31 5.5 100 60 41 ms s A s VSC,TH tCSPD VFC,TH tFCD IB,VSEN VIN - VSEN CSPD = floating 100nF on CSPD to GND VIN - VSEN 10mV overdrive, from overload condition to GATE discharging VSEN = VIN TA = +25C TA = TMIN to TMAX 45 43.5 10 10 180 20 20 200 460 0.2 10 50 55 56 40 40 220 mV s ms mV ns A VIN IQ ON = VIN 2.7 0.6 13.2 1 V mA SYMBOL CONDITIONS MIN TYP MAX UNITS
80 A 225 6.7 550 7.5 V V V
Maximum Gate Voltage Minimum Gate Drive Voltage Gate Overvoltage Threshold
Start-up is initiated only if VGATE is less than this voltage
2
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
ELECTRICAL CHARACTERISTICS (continued)
(VIN = +2.7V to +13.2V, TA = -40C to +85C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25C.) (Note 2) PARAMETER ON COMPARATOR Threshold Voltage Hysteresis Power-Supply Rejection Ratio Propagation Delay Input Voltage Range Input Bias Current ON Pulse Width Low DIGITAL OUTPUT (STAT) Output Leakage Current Output Voltage Low VOL VIN UNDERVOLTAGE LOCKOUT Threshold Hysteresis UVLO to Start-Up Delay VUVLO VUVLO,HYST tD,UVLO Time which input voltage must exceed undervoltage lockout before start-up is initiated 100 Start-up is initiated when this threshold is reached at VIN 2.25 100 150 200 2.67 V mV ms VSTAT +13.2V ISINK = 1mA 1 0.4 A V VTH,ON VHYST PSRR tD,COMP VON IB,ON tRESTART To restart after a fault 20 2.7V VIN 13.2V 10mV overdrive Input can be driven to the absolute maximum limit without false output inversion -0.1 0.001 VIN = 5V, rising threshold 0.575 0.6 3 0.1 10 13.2 1 1 0.625 V mV mV/V s V A s SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX4370
3
Note 2: All devices are 100% tested at TA = +25C. All temperature limits are guaranteed by design. Note 3: The start-up period (tSTART) is the time during which the slow comparator is ignored and the device acts as a current limiter by regulating the sense current with the fast comparator. It is measured from ON rising above 0.6V to STAT rising. Note 4: The current available at GATE is a function of VGATE (see Typical Operating Characteristics.)
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Typical Operating Characteristics
(Circuit of Figure 7, VIN = 5V, RSENSE = 100m, M1 = FDS6670A, CBOARD = 470F, CGATE = 0, RS = 0, TA = +25C, unless otherwise noted.)
SLOW COMPARATOR THRESHOLD vs. INPUT VOLTAGE
MAX4370 toc02
SUPPLY CURRENT vs. INPUT VOLTAGE
MAX4370 toc01
SUPPLY CURRENT vs. TEMPERATURE
1.0 0.9 0.8 SUPPLY CURRENT (mA) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 VIN = 3V VIN = 12V VIN = 5V ON = VIN 51.0 50.8 50.6 50.4 VSC, TH (mV) 50.2 50.0 49.8 49.6 49.4 49.2 49.0 -15 10 35 60 85 0
0.9 0.8 SUPPLY CURRENT (mA) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 2 4 6 8 10 12 ON = GND ON = VIN IGATE = 10A
TA = +25C TA = -40C
TA = +85C
14
0 -40
2
4
6
8
10
12
14
VIN (V)
TEMPERATURE (C)
VIN (V)
SLOW COMPARATOR RESPONSE TIME vs. INPUT VOLTAGE
24 23 22 tCSPD (s) 21 20 19 18
MAX4370 toc05
FAST COMPARATOR THRESHOLD vs. INPUT VOLTAGE
MAX4370 toc06
FAST COMPARATOR RESPONSE TIME vs. OVERDRIVE VOLTAGE
MAX4370 toc07
24 23 22 21 20 19 18
210 108
700 600 500 tFCD (ns) 400 VIN = 5V 300 200 100 VIN = 3V VIN = 12V
CSPD = 110nF TIME IN ms
206 204 tCSPD (ms) VFC, TH (mV) 202 200 198 196 194 192 190 TA = +85C TA = -40C TA = +25C
CSPD = 0 TIME IN s
0
2
4
6
8
10
12
14
0
2
4
VIN (V)
6 8 VIN (V)
10
12
14
0 1 10 VOD (mV) 100 1000
FAST COMPARATOR RESPONSE TIME vs. TEMPERATURE
490 480 470 tFCD (ns) 460 450 440 430 420 410 400 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) VOD = 10mV 250 0 2 VIN = 3V VIN = 12V 310 VIN = 5V
MAX4370 toc08
START-UP TIME vs. INPUT VOLTAGE
350 CTIM = 100nF TIME IN ms
MAX4370 toc09
500
35
330
33 tSTART (ms)
tSTART (s)
31 CTIM = 1nF TIME IN s
290
29
270
27
25 4 6 8 10 12 14 VIN (V)
4
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MAX4370 toc03
1.0
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
Typical Operating Characteristics (continued)
(Circuit of Figure 7, VIN = 5V, RSENSE = 100m, M1 = FDS6670A, CBOARD = 470F, CGATE = 0, RS = 0, TA = +25C, unless otherwise noted.)
GATE CHARGE CURRENT vs. GATE VOLTAGE
MAX4370 toc10
MAX4370
GATE CHARGE CURRENT vs. TEMPERATURE
VGATE = 0 125 100 IGATE (A) VIN = 12V
MAX4370 toc11
GATE VOLTAGE vs. INPUT VOLTAGE
IGATE = 10A 20 TA = +85C TA = +25C
MAX4370 toc12
120 100 80 IGATE (A) VIN = 3.0V 60 40 20 0 0 2 4 6 8
150
25
VIN = 12V
VIN = 5.0V
75 50
VIN = 5V
VGATE (V)
15
10 VIN = 3V 25 0 5
TA = -40C
0 -40 -15 -10 35 60 85 0 2 4 6 8 10 12 14 TEMPERATURE (C) VIN (V)
10 12 14 16 18 20
VGATE (V)
GATE DISCHARGE CURRENT vs. GATE VOLTAGE
MAX4370 toc13
GATE DISCHARGE CURRENT vs. TEMPERATURE
MAX4370 toc14
START-UP TIME (CBOARD = 470F)
MAX4370-15
400 350 300 IGATE (A) VIN = 3V
400 350 VIN = 3V 300 IGATE (A) 250 200 150 100 VIN = 5V & 12V VGATE = VIN TRIGGERED BY A FAULT OR BY ON FALLING -40 -15 10 35 60
ON VGATE (2V/div)
250 200 150 100 50 0 0 2 4 6 8 10 12 14 16 18 20 VGATE (V) TRIGGERED BY A FAULT OR BY ON FALLING VIN = 5V VIN = 12V
ILOAD (1A/div)
VOUT (2V/div)
50 0
85
TEMPERATURE (C)
500s/div CBOARD = 470F, RSENSE = 100m, CTIM = 10nF, CGATE = 0
START-UP TIME (CBOARD = 0)
MAX4370-16
START-UP TIME (EXTERNAL CGATE = 22nF, CBOARD = 470F)
MAX4370-17
TURN-OFF TIME (CBOARD = 470F)
MAX4370-18
ON VGATE (2V/div)
ON VGATE (2V/div)
ON
ILOAD (1A/div)
ILOAD (1A/div) 0A
VOUT (2V/div)
VOUT (2V/div)
VOUT (2V/div) 0V VGATE (2V/div) 50s/div CBOARD = 470F, RSENSE = 100m, CGATE = 0
100s/div CBOARD = 0, RSENSE = 100m, CTIM = 10nF, CGATE = 0
1ms/div CBOARD = 470F, RSENSE = 100m, CGATE = 22nF, CTIM = 10nF, RS = 0
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5
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Typical Operating Characteristics (continued)
(Circuit of Figure 7, VIN = 5V, RSENSE = 100m, M1 = FDS6670A, CBOARD = 470F, CGATE = 0, RS = 0, TA = +25C, unless otherwise noted.)
TURN-OFF TIME (CBOARD = 0)
MAX4370-19
TURN-OFF TIME (EXTERNAL CGATE = 22nF, CBOARD = 470F)
MAX4370-20
TIME TO CHARGE GATE vs. CGATE
NO EXTERNAL MOSFET TIME TO CHARGE GATE (ms) 100 10 1 0.1 0.01 0.001 0.01 VIN = 3V TO VGATE = 6V VIN = 12V TO VGATE = 17V
MAX4370 toc21
1000 ON ILOAD (1A/div) 0A
ON VGATE (2V/div) VGATE ILOAD (1A/div) (2V/div) 0A VOUT (2V/div) VOUT (2V/div)
VIN = 5V TO VGATE = 10V
0V
0V
50s/div CBOARD = 0, RSENSE = 100m, CGATE = 0, RS = 0
200s/div CBOARD = 470F, RSENSE = 100m, CGATE = 22nF, RS = 0
0.1
1
10
100
1000
CGATE (nF)
TIME TO DISCHARGE GATE vs. CGATE
NO EXTERNAL MOSFET DISCHARGE TO VGATE=0.1V
MAX4370 toc22
ON COMPARATOR THRESHOLD vs. INPUT VOLTAGE
ON COMPARATOR THRESHOLDD (V)
MAX4370 toc23
ON COMPARATOR THRESHOLD vs. TEMPERATURE
MAX4370 toc24
1000 TIME TO DISCHARGE GATE (ms) 100 10 1
0.605
0.6100 ON COMPARATOR THRESHOLD (V) 0.6075 0.6050 0.6025 0.6000 VIN = 3V 0.5975 0.5950 VIN = 12V VIN = 5V
0.603
RISING
VIN = 12V
0.601 FALLING 0.599
0.1 0.01 VIN = 3V
0.597
} }
10 35 60 85 TEMPERATURE (C)
0.001 0.01
0.1
1
10
100
1000
0.595 0 2 4 6 8 10 12 14 VIN (V)
-40
-15
CGATE (nF)
UVLO THRESHOLD VOLTAGE vs. TEMPERATURE
MAX4370 toc25
UVLO DELAY vs. TEMPERATURE
MAX4370 toc26
2.60 2.55 UVLO THRESHOLD (V) RISING 2.50 2.45 FALLING 2.40 2.35 2.30 -40 -15 10 35 60
160
155 UVLO DELAY (ms)
VIN = 5V & 12V VIN = 3V
150
145
140 85 -40 -15 10 35 60 85 TEMPERATURE (C) TEMPERATURE (C)
6
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FALLING RISING
VIN = 5V
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME VIN VSEN GATE GND CSPD CTIM STAT ON Supply Voltage Input. Connect to 2.7V to 13.2V. Current-Sense Resistor Voltage Input. RSENSE is connected from VIN to VSEN. Gate Drive Output. Connect to gate of external N-channel MOSFET. Ground Slow Comparator Speed Setting. Leave floating or connect the timing capacitor from CSPD to GND. See Slow Comparator Response Time section. Start-Up Timer Setting. Leave floating or connect the timing capacitor from CTIM to GND. See Start-Up Timing Capacitor section. Status Output--open drain. High indicates start-up completed with no fault. See Table 1. ON Comparator Input. Connect high for normal operation; connect low to force the MOSFET off. Comparator threshold VTH,ON = 0.6V allows for precise control over shutdown feature. Pulse ON low for at least 20s, then high to restart after a fault. FUNCTION
MAX4370
CSPD CSPD VIN 6A
MAX4370
2.45V SLOW COMPARATOR 6A
VSC, TH 50mV RSENSE
INPUT UVLO 150ms DELAY ON VIN RISING STAT
VFC, TH 200mV FAST COMPARATOR
VSEN GATE DRIVE CHARGE PUMP 0.1V 4A GATE OVLO ON ON COMPARATOR 0.6V VOUT GND GATE
DISCHARGE LOGIC CONTROL VIN ENABLE
M1 N
CTIM CTIM
Figure 1. Functional Diagram
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Detailed Description
The MAX4370 is a circuit-breaker IC designed for hotswap applications where a card or board is to be inserted into a rack with the main system power supply turned on. Normally, when a card is plugged into a live backplane, the card is discharged filter capacitors provide a low impedance, which can momentarily cause the main power supply to collapse. The MAX4370 is designed to reside either in the backplane or in the removable card to provide inrush-current limiting and short-circuit protection. This is achieved using a charge pump as gate drive for an external N-channel MOSFET, an external current-sense resistor, and two on-chip comparators. Figure 1 shows the device's functional diagram. The slow comparator response time and the start-up timer can be adjusted with external capacitors. The timing components are optional; without them the part is set to its nominal values, as shown in the Electrical Characteristics.
ON STAT tSTART VGATE
~VIN
VOUT VGATE VOUT IFAST, SET VTH
CBOARD = LARGE CBOARD = 0
ILOAD tON
Start-Up Period
CTIM sets the start-up period. This mode starts when the power is first applied to VIN if ON is connected to VIN, or at the rising edge of ON. In addition, the voltage at V IN must be above the undervoltage lockout for 150ms (see Undervoltage Lockout). During start-up, the slow comparator is disabled and current limiting is provided two different ways: 1) Slow ramping of the current to the load by controlling the external MOSFET gate voltage. 2) Limiting the current to the load by regulating the voltage across the external current-sense resistor. Unlike other circuit-breaker ICs, the MAX4370 hot-swap controller regulates the current to a preset level instead of completely turning off if an overcurrent occurs during start-up. In start-up mode, the gate drive current is limited to 100A and decreases with the increase of the gate voltage (see Typical Operating Characteristics). This allows the MAX4370 to slowly enhance the MOSFET. If the fast comparator detects an overcurrent, the gate voltage is momentarily discharged with a fixed 80A current until the load current through the sense resistor (R SENSE) decreases below its threshold point. This effectively regulates the turn-on current during start-up. Figure 2 shows the start-up waveforms. STAT goes high at the end of the start-up period if no fault condition is present.
Figure 2. Start-Up Waveforms
Normal Operation (DualSpeed/BiLevel)
In normal operation (after the start-up period has expired), protection is provided by turning off the external MOSFET when a fault condition is encountered. DualSpeed/BiLevel fault protection incorporates two comparators with different thresholds and response times to monitor the load current: 1) Slow Comparator. This comparator has an externally set response time (20s to seconds) and a fixed 50mV threshold voltage. The slow comparator ignores low-amplitude momentary current glitches. After an extended overcurrent condition, a fault is detected and the MOSFET gate is discharged. 2) Fast Comparator. This comparator has a fixed response time and a higher 200mV threshold voltage. The fast comparator turns off the MOSFET immediately after it detects a large amplitude event such as a short circuit. In each case, when a fault is encountered, the status pin (STAT) goes low and the MAX4370 stays latched off. Figure 3 shows the waveforms after a fault condition.
8
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
STAT
VGATE
50% (i.e., > 50% of the time the current is above the threshold level) will be considered a fault condition even if it is never higher than the threshold for more than the slow comparator's set response time. Once the fault condition is detected, the STAT pin goes low and the device goes into latched mode. The GATE voltage discharge rate depends on the gate capacitance and the external capacitance at GATE. ~VIN
MAX4370
Fast Comparator
~VTH
VOUT td
ILIM ILOAD tOFF
Figure 3. Response to a Fault Condition
Slow Comparator
The slow comparator is disabled at start-up while the external MOSFET is turning on. This allows the part to ignore the higher-than-normal inrush current charging the board capacitors (CBOARD) when a card is first plugged in. If the slow comparator detects an overload current while in normal operation (after start-up is completed), it turns off the external MOSFET by discharging the gate capacitance with a 200A current. The slow comparator threshold is set at 50mV and has a default delay of 20s (CSPD floating), allowing it to ignore power-supply glitches and noise. The response time can be lengthened with an external capacitor at CSPD (Figure 8). If the overcurrent condition is not continuous, the duration above the threshold minus the duration below it must be greater than 20s (or the external programmed value) for the device to trip. When the current is above the threshold, CSPD is charged with a 6A current source; when the current is below the threshold, CSPD is discharged with a 6A current source. A fault is detected when CSPD is charged to the trip point of 1.2V. A pulsing current with a duty cycle greater than
The fast comparator behaves differently according to the operating mode. During start-up, the fast comparator is part of a simple current regulator. When the sensed current is above the threshold (V FC,TH = 200mV), the gate is discharged with a 80A current source. When the sensed current drops below the threshold, the charge pump turns on again. The sensed current will rise and fall near the threshold due to the fast comparator and charge-pump propagation delay. The gate voltage will be roughly saw-tooth shaped, and the load current will present a 20% ripple. The ripple can be reduced by adding a capacitor from GATE to GND. Once CBOARD is completely charged, the load current drops to its normal operating levels. If the sensed current is still high after the start-up timer expires, the MOSFET gate is discharged completely. In normal operation (after start-up), the fast comparator is used as an emergency off switch. If the load current reaches the fast comparator threshold, the device immediately forces the MOSFET off completely by discharging the GATE with a 200A current. This can occur in the event of a serious current overload or a dead short. Given a 1000pF gate capacitance and 12V gate voltage, the MOSFET will be off in less than 60s. Any additional capacitance connected between GATE and GND to slow down the turn-on time also increases the turn-off time.
Latched Mode and Reset
The MOSFET driver of the MAX4370 stays latched off after a fault condition until it is reset by a negativegoing pulse on the ON pin. Pulse ON low for 20s (min), then high to restart after a fault. During start-up, a negative-going edge on ON will force the device to turn off the MOSFET and place the device in latched mode. Keep ON low for 20s (min) to restart.
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9
Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Status Output
The status output is an open-drain output that goes low when the part is: 1) in start-up 2) forced off (on = GND) 3) in an overcurrent condition, or 4) latched off. STAT is high only if the part is in normal mode and no faults are present (Table 1). Figure 4 shows the STAT timing diagram.
ON VIN OV
1.2V CTIM OV FAULT CONDITION OR ON FALLING EDGE NO FAULT CONDITIONS PRESENT
VIN STAT OV
Over/Undervoltage Lockouts
The undervoltage lockout prevents the MAX4370 from turning on the external MOSFET until the input voltage at VIN exceeds the lockout threshold (2.25V min) for at least 150ms. The undervoltage lockout protects the external MOSFET from insufficient gate drive voltage. The 150ms timeout ensures that the board is fully plugged into the backplane and that VIN is stable. Voltage transients at VIN with voltages below the UVLO will reset the device and initiate a start-up sequence. The device also features a gate overvoltage lockout that prevents the device from restarting after a fault condition if the discharge has not been completed. V GATE must be discharged to below 0.1V before restarting. Since the MAX4370 does not monitor the output voltage, a start-up sequence can be initiated while the board capacitance is still charged.
Figure 4. Status Output (STAT) Timing Diagram
VIN
RSENSE
MAX4370
VSEN
VGD GATE DRIVE CHARGE PUMP GATE M1 N VGS VOUT D1 CBOARD
Gate Overvoltage Protection
Newer-generation MOSFETs have an absolute maximum rating of 8V for the gate-to-source voltage (VGS). To protect these MOSFETs, the MAX4370 limits the gate-to-drain (V GD ) to +7.5V with an internal zener diode. No protection is provided for negative V GD. If GATE can be discharged to GND faster than the output voltage, an external small-signal protection diode (D1) can be used, as shown in Figure 5.
Figure 5. External Gate-Source Protection
Table 1. Status Output Truth Table
PART IN START-UP Yes No No No No ON PIN X Low High High High OVERCURRENT CONDITION ON VIN X X Yes No No PART IN LATCHED-OFF MODE DUE TO OVERCURRENT CONDITION X X X Yes No STAT PIN (STATUS) Low Low Low Low High
X = Don't care
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
__________Applications Information
Component Selection
N-Channel MOSFET Select the external N-channel MOSFET according to the application's current level. The MOSFET's RDS(ON) should be chosen low enough to have a minimum voltage drop at full load to limit the MOSFET power dissipation. High RDS(ON) can cause output ripple if the board has pulsing loads, or it can trigger an external undervoltage reset monitor at full load. Determine the device's power rating requirement to accommodate a short-circuit condition on the board during start-up (see MOSFET Thermal Considerations). MOSFETs can typically withstand single-shot pulses with higher dissipation than the specified package rating. Also, since part of the inrush current limiting is achieved by limiting the gate dV/dt, it is not necessary to use a MOSFET with low gate capacitance. Table 2 lists some recommended manufacturers and components. Sense Resistor The slow comparator threshold voltage is set at 50mV. Select a sense resistor that causes a 50mV voltage drop at a current level above the maximum normal operating current. Typically, set the overload current at 1.2 to 1.5 times the nominal load current. The fast comparator threshold is set at 200mV. This sets the fault current limit at four times the overload current limit. Choose the sense-resistor power rating to accommodate the overload current (Table 3): PSENSE = (IOVERLOAD)2 * RSENSE Start-Up Timing Capacitor (CTIM) The start-up period (tSTART) is determined by the capacitor connected at CTIM. This determines the maximum time allowed to completely turn on the MOSFET. The default value for tSTART is chosen by leaving CTIM floating and is approximately 5.5s. This is also the minimum value (not controlled and dependent on stray
capacitance). Longer timings are determined by the value of the capacitor, according to Figure 6, and can be determined as follows: tSTART (ms) = 0.31 * CTIM (nF) Set the t START timer to allow the MOSFET to be enhanced and the load capacitor to be completely charged. There are two methods of completing the start-up sequences. Case A describes a start-up sequence that does not use the current-limiting feature and slowly turns on the MOSFET by limiting the gate dV/dt. Case B uses the current-limiting feature and turns on the MOSFET as fast as possible while still preventing high inrush current.
1000 100 10 tSTART (ms) 1 0.1 0.01 0.001 0.01
MAX4370
0.1
1
10
100
1000
CAPACITANCE (nF)
Figure 6. Start-Up Period vs. CTIM
Table 3. Current Levels vs. RSENSE
RSENSE (m) 10 50 100 OVERLOAD THRESHOLD SET BY SLOW COMPARATOR (A) 5 1 0.5 FAULT CURRENT THRESHOLD SET BY FAST COMPARATOR (A) 20 4 2
Table 2. Component Manufacturers
COMPONENT Sense Resistors MANUFACTURER Dale-Vishay IRC Fairchild MOSFETs International Rectifier Motorola PHONE 402-564-3131 704-264-8861 888-522-5372 310-322-3331 602-244-3576 INTERNET www.vishay.com www.irctt.co www.fairchildsemi.com www.irf.com www.mot-sps.com/ppd/ 11
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection
Case A: Slow Turn-On (without overcurrent) There are two ways to turn on the MOSFET without reaching the fast comparator current limit: 1) If the board capacitance (C BOARD ) is low, the inrush current is low. 2) If the capacitance at GATE is high, the MOSFET turns on slowly. In both cases, the turn-on (tON) is determined only by the charge required to enhance the MOSFET--effectively, the small gate-charging current limits the output voltage dv/dt. This time can be extended by connecting an external capacitor between GATE and GND, as shown in Figure 7. The turn-on time is dominated by the external gate capacitance if its value is considerably higher than MOSFET gate capacitance. Table 4 shows the timing required to enhance the recommended MOSFET with or without an external capacitor at GATE; Figures 2 and 3 show the related waveforms and timing diagrams (see Start-Up Time with C BOARD = 0 and Start-Up Time with External C GATE in the Typical Operating Characteristics). Remember that a high gate capacitance also increases the turn-off time. When using the MAX4370 without an external gate capacitor, RS is not necessary. RS prevents MOSFET source oscillations that can occur when CGATE is high while CBOARD is low. Case B: Fast Turn-On (with current limit) In applications where the board capacitor (CBOARD) at VOUT is high, the inrush current causes a voltage drop across R SENSE that exceeds the fast comparator threshold (VFC,TH = 200mV). In this case, the current
CSPD
charging CBOARD can be considered constant and the turn-on time is determined by: tON = CBOARD * VIN / IFAST,SET where the maximum load current IFAST,SET = VFC,TH / RSENSE. Figure 2 shows the waveforms and timing diagrams for a turn-on transient with current regulation (see Start-Up Time with C BOARD = 470F in the Typical Operating Characteristics). When operating under this condition, an external gate capacitor is not required. Adding an external capacitor at GATE reduces the regulated current ripple but increases the turn-off time by increasing the gate delay (td) (Figure 3).
RSENSE VIN M1 VOUT
MAX4370
CBOARD RS* RPULL-UP VIN STAT ON VSEN GATE CGATE
MAX4370
CSPD
CTIM GND CTIM
*OPTIONAL (SEE TEXT)
Figure 7. Operation with External Gate Capacitor
Table 4. MOSFET Turn-On Time (start-up without current limit)
(CBOARD = 0, turn-on with no load current, turn-off with 2A fault current)
DEVICE Fairchild FDS6670A International Rectifier IRF7401 Motorola MMSF5N03HD CGATE (nF) 0 22 0 22 0 22 MOSFET TURN-ON (tON) VIN = 3V 220s 2.3ms 175s 1.9ms 101s 2ms VIN = 5V 160s 2ms 130s 1.8ms 74s 1.8ms VIN = 12V 190s 3.2ms 160s 3.5ms 73s 3.2ms MOSFET TURN-OFF (tOFF) VIN = 3V 70s 540s 75s 540s 33s 470s VIN = 5V 130s 1.1ms 130s 1.1ms 67s 1ms VIN = 12V 145s 1.95ms 160s 2ms 85s 1.95ms
Electrical characteristics as specified by the manufacturer's data sheet: FDS6670A: CISS = 3200pF, QT(MAX) = 50nC, RDS(ON) = 8.2m IRF7401: CISS = 1600pF, QT(MAX) = 48nC, RDS(ON) = 22m MMSF5N03HD: CISS = 1200pF, QT(MAX) = 21nC, RDS(ON) = 40m
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
1000 tCSPD (ms) = 0.2 * CSPD (nF) VREF
100 RESPONSE TIME (ms)
R1 ON
10 LOGIC CONTROL 1 NTC R2 0.6V
0.1
0.01 0.01
0.1
1
10
100
1000
CSPD (nF)
R1 = R2 * (VREF / O.6 - 1) R2 = VALUE OF THE NTC RESISTOR AT THE LIMIT TEMPERATURE VREF = ANY REFERENCE VOLTAGE AVAILABLE OR VIN
Figure 8. Slow Comparator Response Time vs. CSPD
Figure 9. Temperature Monitoring and Protection
REMOVABLE CARD
The actual turn-on time is determined by the longer of the two timings of Case A and Case B. Set the start-up timer (tSTART) at 2 * tON or longer to guarantee enough time for the output voltage to settle; also take into consideration device parameter variation.
BACKPLANE VCC
10k
Slow Comparator Response Time (CSPD) The slow comparator threshold is set at 50mV, and its response time is determined by the external capacitor connected to CSPD (Figure 8). A minimum response time of 20s (typ) is achieved by leaving this pin floating. This time is determined internally and is not affected by stray capacitance at CSPD (up to 100pF).
Set the slow comparator response time to be longer than the normal operation load transients.
VIN
VSEN GATE
MAX4370
ON
ON Comparator
The ON/OFF function of the MAX4370 is controlled by the ON comparator. This is a precision voltage comparator that can be used for temperature monitoring (Figure 9) or as an additional undervoltage lockout. The comparator threshold voltage is set at 0.6V with a 3mV typical hysteresis. The ON comparator initiates start-up when its input voltage (VON) rises above the threshold voltage, and turns off the MOSFET when the voltage falls below the threshold. The ON comparator is also used to reset the MAX4370 after a fault condition. The ON comparator input and the STAT output can be pulled to voltages up to 14V independently of VIN.
RESET
1M
Figure 10. Fail-Safe Connector
In some applications, it is useful to use connectors with staggered leads. In Figure 10, the ON pin forces the removable board to be powered up only when all connections are made.
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Using the MAX4370 on the Backplane
The MAX4370 can be used on the backplane to regulate current upon insertion of a removable card. This allows multiple cards with different input capacitance to be inserted into the same slot even if the card doesn't have on-board hot-swap protection. The MAX4370 current-limiting feature is active during the start-up period set by CTIM. The start-up period can be triggered if VIN is connected to ON through a trace on the card. Once tSTART has expired (timed out), the load capacitance has to be charged or a fault condition is detected. To ensure start-up with a fixed CTIM, t START has to be longer than the time required to charge the board capacitance. The maximum load capacitance is calculated as follows: CBOARD < tSTART * IFAST,SET / VIN
BACKPLANE REMOVABLE CARD WITH NO HOT-INSERTION PROTECTION
VOUT
VIN
CBOARD
VIN
VSEN
GATE
MAX4370
CTIM ON
Input Transients
The voltage at V IN must be above the UVLO during inrush and fault conditions. When a short condition occurs on the board, the fault current can be higher than the fast comparator current limit. The gate voltage is discharged immediately, but note that the MOSFET is not completely off until VGS < VTH. If the main system power supply collapses below UVLO, the MAX4370 will force the device to restart once the supply has recovered. The main system power supply must be able to deliver this fault current without excessive voltage drop. The MOSFET is turned off in a very short time; therefore, the resulting di/dt can be considerable. The backplane delivering the power to the external card must have a fairly low inductance to limit the voltage transients caused by the removal of a fault.
Figure 11. Using the MAX4370 on the Backplane
equation is used to calculate the required transient thermal resistance: RJA(t) = (TJ,MAX - TA) / PD,MAX(t) where PDMAX(t) = VIN * IFAULT and the resulting RJA = RJA(t) / rJA(t). RJA is the thermal resistance determined with a continuous load and by the layout or heatsink.
Layout Considerations
To take full advantage of the switch response time to an output fault condition, it is important to keep all traces as short as possible and to maximize the high-current trace dimensions to reduce the effect of undesirable parasitic inductance. Place the MAX4370 close to the card's connector. Use a ground plane to minimize its impedance and inductance. Minimize the current-sense resistor trace length (<10mm), and ensure accurate current sensing with Kelvin connections (Figure 12). When the output is short circuited, the voltage drop across the external MOSFET becomes large. Hence, the power dissipation across the switch increases, as does the die temperature. An efficient way to achieve good power dissipation on a surface-mount package is to lay out two copper pads directly under the package on both sides of the board. Connect the two pads to the ground plane through vias, and use enlarged copper mounting pads on the top side of the board.
MOSFET Thermal Considerations
During normal operation, the MOSFET dissipates little power; it is fully turned on and its RDS(ON) is minimal. The power dissipated in normal operation is P D = (ILOAD)2 * RDS(ON). A considerable amount of power is dissipated during the turn-on and turn-off transients. The design must take into consideration the worst-case scenario of a continuous short-circuit fault present on the board. Two cases must be considered: 1) The single turn-on with the device latched after a fault. 2) An external circuit forces a continuous automatic retry after the fault. MOSFET manufacturers typically include the package normalized transient thermal resistance (rJA (t) or rJC(t)), which is determined by the start-up time and the retry duty cycle (d = tSTART / tRETRY). The following
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
HIGH-CURRENT PATH
SENSE RESISTOR
MAX4370
Figure 12. Kelvin Connections for the Current-Sense Resistors
Pin Configuration
TOP VIEW
VIN 1 VSEN 2
8 7
ON STAT CTIM CSPD
MAX4370
GATE 3 6 5 GND 4
SO
Chip Information
TRANSISTOR COUNT: 1792
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Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370
Package Information
SOICN.EPS
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 (c) 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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