19-1472; Rev 0; 4/99 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection Features The MAX4370 is a circuit-breaker IC designed to offer protection in hot-swap applications using Maxim’s DualSpeed/BiLevel™ 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. ♦ DualSpeed/BiLevel 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. ♦ Status Output Pin The MAX4370 is specified for the extended-industrial temperature range (-40°C to +85°C) and is available in an 8-pin SO package. ♦ Inrush Current Regulated at Start-Up ♦ Resides Either on the Backplane or on the Removable Card ♦ Programmable Start-Up Period and Response Time ♦ Allows Safe Board Insertion and Removal from Live Backplane ♦ Protection for +3V to +12V Single Supplies ♦ Latched Off After Fault Condition ♦ Internal Charge Pump Generates Gate Drive for External N-Channel MOSFET Ordering Information PART MAX4370ESA TEMP. RANGE PIN-PACKAGE -40°C to +85°C 8 SO Pin Configuration appears at end of data sheet. Applications Hot Board Insertion Solid-State Circuit Breaker Typical Operating Circuit BACKPLANE REMOVABLE CARD M1 N RSENSE VCC VOUT CBOARD VIN STAT GND VSEN GATE STAT MAX4370 ON ON CSPD CSPD CTIM GND 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. MAX4370† General Description MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 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 = +70°C) SO (derate 5.9mW/°C above +70°C) ........................... 471mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10sec) ............................ +300°C 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 = -40°C to +85°C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25°C.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS POWER SUPPLIES Input Voltage Range VIN Supply Current IQ ON = VIN 2.7 Slow Comparator Threshold VSC,TH VIN - VSEN Slow Comparator Response Time tCSPD Fast Comparator Threshold VFC,TH Fast Comparator Response Time tFCD 13.2 V 0.6 1 mA 50 55 CURRENT CONTROL VSEN Input Bias Current IB,VSEN TA = +25°C 45 TA = TMIN to TMAX 43.5 56 mV CSPD = floating 10 20 40 µs 100nF on CSPD to GND 10 20 40 ms VIN - VSEN 180 200 220 mV 10mV overdrive, from overload condition to GATE discharging 460 VSEN = VIN 0.2 10 µA 31 41 ms ns MOSFET DRIVER Start-Up Period (Note 3) tSTART Gate Charge Current IGATE Turn-Off Time Gate Discharge Current tOFF IGATE,DIS 100nF on CTIM 21 CTIM = floating 5.5 µs VGATE = VIN (Note 4) 100 µA Time from current overload to VGATE < 0.1V, CGATE = 1000pF to GND (triggered by the fast comparator during normal operation) 60 µs During start-up (current regulation provided by fast comparator) 80 µA During turn-off, triggered by a fault in normal operation or ON falling edge Maximum Gate Voltage Measured with respect to VIN; voltage at which internal clamp circuitry is triggered Minimum Gate Drive Voltage IGATE = 8.5µA, measured above VIN Gate Overvoltage Threshold Start-up is initiated only if VGATE is less than this voltage 2 VIN ≥ 5V VIN ≥ 2.7V 75 225 550 6.7 7.5 5 2.7 0.1 _______________________________________________________________________________________ V V V Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection MAX4370 ELECTRICAL CHARACTERISTICS (continued) (VIN = +2.7V to +13.2V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at VIN = +5V and TA = +25°C.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP MAX 0.575 0.6 0.625 UNITS ON COMPARATOR Threshold Voltage VTH,ON Hysteresis VHYST Power-Supply Rejection Ratio PSRR Propagation Delay tD,COMP Input Voltage Range VON Input Bias Current IB,ON ON Pulse Width Low tRESTART VIN = 5V, rising threshold 3 2.7V ≤ VIN ≤ 13.2V 0.1 10mV overdrive 10 Input can be driven to the absolute maximum limit without false output inversion -0.1 0.001 To restart after a fault V mV 1 mV/V µs 13.2 V 1 µA 20 µs DIGITAL OUTPUT (STAT) VSTAT ≤ +13.2V Output Leakage Current Output Voltage Low VOL ISINK = 1mA 1 µA 0.4 V 2.67 V VIN UNDERVOLTAGE LOCKOUT Threshold VUVLO Hysteresis VUVLO,HYST UVLO to Start-Up Delay tD,UVLO Start-up is initiated when this threshold is reached at VIN 2.25 100 Time which input voltage must exceed undervoltage lockout before start-up is initiated 100 150 mV 200 ms Note 2: All devices are 100% tested at TA = +25°C. 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.) _______________________________________________________________________________________ 3 Typical Operating Characteristics (Circuit of Figure 7, VIN = 5V, RSENSE = 100mΩ, M1 = FDS6670A, CBOARD = 470µF, CGATE = 0, RS = 0, TA = +25°C, unless otherwise noted.) 0.8 0.5 0.4 ON = GND 0.7 50.8 50.4 0.6 0.5 0.4 VIN = 3V 0.3 50.0 49.8 0.2 49.4 0.1 49.2 2 4 6 8 10 12 49.0 0 -40 14 -15 10 SLOW COMPARATOR RESPONSE TIME vs. INPUT VOLTAGE 24 60 23 22 21 21 20 20 CSPD = 0 TIME IN µs 19 0 2 4 6 8 12 14 202 TA = +25°C 198 TA = +85°C 196 VIN = 12V VIN = 5V 300 200 100 190 18 0 2 4 6 8 VIN (V) 10 12 490 480 VIN = 3V 0 14 1 10 100 VOD (mV) START-UP TIME vs. INPUT VOLTAGE MAX4370 toc09 350 MAX4370 toc08 500 CTIM = 100nF TIME IN ms 330 35 33 VIN = 12V VIN = 5V tSTART (µs) tFCD (ns) 400 194 FAST COMPARATOR RESPONSE TIME vs. TEMPERATURE 460 450 440 VIN = 3V 430 420 31 310 CTIM = 1nF TIME IN µs 290 29 27 270 410 VOD = 10mV 400 -40 -20 0 20 14 500 200 VIN (V) 470 12 600 TA = -40°C 204 19 10 10 700 192 18 8 FAST COMPARATOR RESPONSE TIME vs. OVERDRIVE VOLTAGE tFCD (ns) 22 6 FAST COMPARATOR THRESHOLD vs. INPUT VOLTAGE 206 tCSPD (ms) VFC, TH (mV) CSPD = 110nF TIME IN ms 4 VIN (V) 108 23 2 TEMPERATURE (°C) 210 24 0 85 MAX4370 toc06 MAX4370 toc05 35 40 60 TEMPERATURE (°C) 80 100 25 250 0 2 4 6 8 10 12 14 VIN (V) _______________________________________________________________________________________ tSTART (ms) 0 TA = +85°C 49.6 0.1 VIN (V) 4 50.2 0.2 0 TA = +25°C TA = -40°C 50.6 MAX4370 toc07 0.6 0.3 VIN = 12V VIN = 5V VSC, TH (mV) SUPPLY CURRENT (mA) 0.7 ON = VIN 0.9 SUPPLY CURRENT (mA) ON = VIN IGATE = 10µA 51.0 MAX4370 toc02 0.9 0.8 1.0 MAX4370 toc01 1.0 SLOW COMPARATOR THRESHOLD vs. INPUT VOLTAGE SUPPLY CURRENT vs. TEMPERATURE MAX4370 toc03 SUPPLY CURRENT vs. INPUT VOLTAGE tCSPD (µs) MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 1000 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection GATE CHARGE CURRENT vs. TEMPERATURE 100 IGATE = 10µA 20 TA = +85°C IGATE (µA) VIN = 5.0V VIN = 3.0V 60 VGATE (V) 100 80 IGATE (µA) VGATE = 0 VIN = 12V 125 25 MAX4370 toc11 VIN = 12V GATE VOLTAGE vs. INPUT VOLTAGE 150 MAX4370 toc10 120 VIN = 5V 75 TA = +25°C 15 TA = -40°C 10 40 50 20 25 MAX4370 toc12 GATE CHARGE CURRENT vs. GATE VOLTAGE MAX4370 Typical Operating Characteristics (continued) (Circuit of Figure 7, VIN = 5V, RSENSE = 100mΩ, M1 = FDS6670A, CBOARD = 470µF, CGATE = 0, RS = 0, TA = +25°C, unless otherwise noted.) VIN = 3V 0 0 2 4 6 8 35 60 0 85 2 4 6 8 10 12 VIN (V) GATE DISCHARGE CURRENT vs. GATE VOLTAGE GATE DISCHARGE CURRENT vs. TEMPERATURE START-UP TIME (CBOARD = 470µF) MAX4370 toc13 350 VIN = 3V 250 250 IGATE (µA) 300 200 VIN = 5V 150 VIN = 12V 50 0 4 6 8 10 12 14 16 18 20 VOUT (2V/div) VIN = 5V & 12V VGATE = VIN TRIGGERED BY A FAULT OR BY ON FALLING 50 0 -40 -15 10 35 60 85 500µs/div CBOARD = 470µF, RSENSE = 100mΩ, CTIM = 10nF, CGATE = 0 VGATE (V) TEMPERATURE (°C) START-UP TIME (CBOARD = 0) START-UP TIME (EXTERNAL CGATE = 22nF, CBOARD = 470µF) MAX4370-16 TURN-OFF TIME (CBOARD = 470µF) MAX4370-17 ON VGATE (2V/div) VGATE (2V/div) 200 100 TRIGGERED BY A FAULT OR BY ON FALLING ON ILOAD (1A/div) 150 100 14 MAX4370-15 400 300 2 -10 TEMPERATURE (°C) VIN = 3V 0 -15 VGATE (V) 400 350 0 -40 10 12 14 16 18 20 MAX4370 toc14 0 IGATE (µA) 5 MAX4370-18 ON ON ILOAD (1A/div) VGATE (2V/div) ILOAD (1A/div) 0A VOUT (2V/div) VOUT (2V/div) VOUT (2V/div) VGATE (2V/div) 0V 100µs/div CBOARD = 0, RSENSE = 100mΩ, CTIM = 10nF, CGATE = 0 1ms/div CBOARD = 470µF, RSENSE = 100mΩ, CGATE = 22nF, CTIM = 10nF, RS = 0 50µs/div CBOARD = 470µF, RSENSE = 100mΩ, CGATE = 0 _______________________________________________________________________________________ 5 Typical Operating Characteristics (continued) (Circuit of Figure 7, VIN = 5V, RSENSE = 100mΩ, M1 = FDS6670A, CBOARD = 470µF, CGATE = 0, RS = 0, TA = +25°C, unless otherwise noted.) MAX4370-20 VGATE ILOAD (1A/div) (2V/div) 0V 0V VIN = 5V 0.1 VIN = 3V 0.1 1 10 100 0.1 RISING 0.601 FALLING 0.599 0.597 0 2 4 6 8 10 12 0.6075 VIN = 5V 0.6025 } } 0.6000 VIN = 3V 0.5975 -40 -15 10 35 TEMPERATURE (°C) UVLO DELAY vs. TEMPERATURE MAX4370 toc26 MAX4370 toc25 160 155 UVLO DELAY (ms) RISING 2.50 2.45 FALLING 2.40 VIN = 5V & 12V VIN = 3V 150 145 2.35 2.30 140 -40 -15 10 35 TEMPERATURE (°C) 6 60 1000 VIN = 12V 0.6050 14 UVLO THRESHOLD VOLTAGE vs. TEMPERATURE 2.55 100 0.6100 VIN (V) 2.60 10 0.5950 0.595 1000 1 MAX4370 toc24 0.603 CGATE (nF) UVLO THRESHOLD (V) VIN = 3V TO VGATE = 6V ON COMPARATOR THRESHOLD vs. TEMPERATURE ON COMPARATOR THRESHOLD (V) 1 0.001 0.01 0.1 CGATE (nF) MAX4370 toc23 ON COMPARATOR THRESHOLDD (V) VIN = 12V 0.01 VIN = 5V TO VGATE = 10V 0.001 0.01 0.605 MAX4370 toc22 DISCHARGE TO VGATE=0.1V 1 ON COMPARATOR THRESHOLD vs. INPUT VOLTAGE TIME TO DISCHARGE GATE vs. CGATE NO EXTERNAL MOSFET VIN = 12V TO VGATE = 17V 10 0.01 200µs/div CBOARD = 470µF, RSENSE = 100mΩ, CGATE = 22nF, RS = 0 50µs/div CBOARD = 0, RSENSE = 100mΩ, CGATE = 0, RS = 0 100 FALLING RISING VOUT (2V/div) 10 TIME TO CHARGE GATE (ms) 0A VOUT (2V/div) 100 NO EXTERNAL MOSFET ILOAD (1A/div) 0A 1000 1000 ON ON MAX4370 toc21 MAX4370-19 VGATE (2V/div) TIME TO CHARGE GATE vs. CGATE TURN-OFF TIME (EXTERNAL CGATE = 22nF, CBOARD = 470µF) TURN-OFF TIME (CBOARD = 0) TIME TO DISCHARGE GATE (ms) MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 85 -40 -15 10 35 60 TEMPERATURE (°C) _______________________________________________________________________________________ 85 60 85 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection PIN NAME FUNCTION 1 VIN 2 VSEN Current-Sense Resistor Voltage Input. RSENSE is connected from VIN to VSEN. 3 GATE Gate Drive Output. Connect to gate of external N-channel MOSFET. 4 GND Ground 5 CSPD Slow Comparator Speed Setting. Leave floating or connect the timing capacitor from CSPD to GND. See Slow Comparator Response Time section. 6 CTIM Start-Up Timer Setting. Leave floating or connect the timing capacitor from CTIM to GND. See Start-Up Timing Capacitor section. 7 STAT Status Output—open drain. High indicates start-up completed with no fault. See Table 1. 8 ON Supply Voltage Input. Connect to 2.7V to 13.2V. 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 20µs, then high to restart after a fault. CSPD CSPD VIN 6µA MAX4370 6µA 2.45V VSC, TH 50mV SLOW COMPARATOR RSENSE INPUT UVLO VFC, TH 200mV FAST COMPARATOR 150ms DELAY ON VIN RISING VSEN GATE DRIVE DISCHARGE STAT LOGIC CONTROL VIN CHARGE PUMP ENABLE 0.1V 4µA M1 GATE N GATE OVLO CTIM ON ON COMPARATOR CTIM 0.6V VOUT GND Figure 1. Functional Diagram _______________________________________________________________________________________ 7 MAX4370 Pin Description MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 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 VTH VGATE VOUT CBOARD = LARGE IFAST, SET 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 100µA 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 80µA 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. 8 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 (20µs 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. _______________________________________________________________________________________ Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection VGATE ~VIN Fast Comparator VOUT ~VTH 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 200µA current. The slow comparator threshold is set at 50mV and has a default delay of 20µs (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 20µs (or the external programmed value) for the device to trip. When the current is above the threshold, CSPD is charged with a 6µA current source; when the current is below the threshold, CSPD is discharged with a 6µA 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 80µA 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 200µA 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 60µs. 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 20µs (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 20µs (min) to restart. _______________________________________________________________________________________ 9 MAX4370 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. STAT MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 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 ON VIN OV 1.2V CTIM 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. OV FAULT CONDITION OR ON FALLING EDGE VIN STAT OV NO FAULT CONDITIONS PRESENT 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. Figure 4. Status Output (STAT) Timing Diagram VIN RSENSE 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. MAX4370 VSEN VGD 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. GATE GATE DRIVE CHARGE PUMP Gate Overvoltage Protection M1 N VGS VOUT D1 CBOARD Figure 5. External Gate-Source Protection Table 1. Status Output Truth Table PART IN START-UP ON PIN OVERCURRENT CONDITION ON VIN PART IN LATCHED-OFF MODE DUE TO OVERCURRENT CONDITION STAT PIN (STATUS) Yes X X X Low No Low X X Low No High Yes X Low No High No Yes Low No High No No High X = Don’t care 10 ______________________________________________________________________________________ Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 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 5.5µs. This is also the minimum value (not controlled and dependent on stray 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) Component Selection 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. 1 0.1 0.01 0.001 0.01 0.1 1 10 100 1000 CAPACITANCE (nF) Figure 6. Start-Up Period vs. CTIM Table 3. Current Levels vs. RSENSE RSENSE (mΩ) OVERLOAD THRESHOLD SET BY SLOW COMPARATOR (A) FAULT CURRENT THRESHOLD SET BY FAST COMPARATOR (A) 10 5 20 50 1 4 100 0.5 2 Table 2. Component Manufacturers COMPONENT Sense Resistors MOSFETs MANUFACTURER PHONE INTERNET Dale-Vishay 402-564-3131 www.vishay.com IRC 704-264-8861 www.irctt.co Fairchild 888-522-5372 www.fairchildsemi.com International Rectifier 310-322-3331 www.irf.com Motorola 602-244-3576 www.mot-sps.com/ppd/ ______________________________________________________________________________________ 11 MAX4370 __________Applications Information MAX4370 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. 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 = 470µF 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 VOUT M1 VIN CBOARD RS* RPULL-UP VIN STAT 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. VSEN CTIM CSPD GND CTIM CSPD 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 CGATE MAX4370 ON Case B: Fast Turn-On (with current limit) GATE *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 MOSFET TURN-ON (tON) MOSFET TURN-OFF (tOFF) CGATE (nF) VIN = 3V VIN = 5V VIN = 12V VIN = 3V VIN = 5V VIN = 12V 0 220µs 160µs 190µs 70µs 130µs 145µs 22 2.3ms 2ms 3.2ms 540µs 1.1ms 1.95ms International Rectifier IRF7401 0 175µs 130µs 160µs 75µs 130µs 160µs 22 1.9ms 1.8ms 3.5ms 540µs 1.1ms 2ms Motorola MMSF5N03HD 0 101µs 74µs 73µs 33µs 67µs 85µs 22 2ms 1.8ms 3.2ms 470µs 1ms 1.95ms Electrical characteristics as specified by the manufacturer’s data sheet: FDS6670A: CISS = 3200pF, QT(MAX) = 50nC, RDS(ON) = 8.2mΩ IRF7401: CISS = 1600pF, QT(MAX) = 48nC, RDS(ON) = 22mΩ MMSF5N03HD: CISS = 1200pF, QT(MAX) = 21nC, RDS(ON) = 40mΩ 12 ______________________________________________________________________________________ Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection RESPONSE TIME (ms) 100 MAX4370 VREF 1000 tCSPD (ms) = 0.2 · CSPD (nF) R1 ON 10 LOGIC CONTROL 1 NTC R2 0.6V 0.1 0.01 0.01 0.1 1 10 100 R1 = R2 · (VREF / O.6 - 1) R2 = VALUE OF THE NTC RESISTOR AT THE LIMIT TEMPERATURE VREF = ANY REFERENCE VOLTAGE AVAILABLE OR VIN 1000 CSPD (nF) Figure 8. Slow Comparator Response Time vs. CSPD Figure 9. Temperature Monitoring and Protection 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 REMOVABLE CARD VCC 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 20µs (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). 10k VIN VSEN GATE MAX4370 ON Set the slow comparator response time to be longer than the normal operation load transients. 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. 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. The ON comparator input and the STAT output can be pulled to voltages up to 14V independently of VIN. ______________________________________________________________________________________ 13 MAX4370 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection 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 VIN VSEN CBOARD 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. 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 (rθJA (t) or rθJC(t)), which is determined by the start-up time and the retry duty cycle (d = tSTART / tRETRY). The following 14 Figure 11. Using the MAX4370 on the Backplane equation is used to calculate the required transient thermal resistance: RθJA(t) = (TJ,MAX - TA) / PD,MAX(t) where PDMAX(t) = VIN · IFAULT and the resulting RθJA = RθJA(t) / rθJA(t). RθJA 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. ______________________________________________________________________________________ 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 8 ON 7 STAT 3 6 CTIM GND 4 5 CSPD VSEN 2 MAX4370 GATE SO Chip Information TRANSISTOR COUNT: 1792 ______________________________________________________________________________________ 15 Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection SOICN.EPS MAX4370 Package Information 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 © 1999 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.