MIC2582/MIC2583 Micrel MIC2582/MIC2583 Single Channel Hot Swap Controllers Final General Description Features The MIC2582 and MIC2583 are single channel positive voltage hot swap controllers designed to allow the safe insertion of boards into live system backplanes. The MIC2582 and MIC2583 are available in 8-pin SOIC and 16-pin QSOP packages, respectively. Using a few external components and by controlling the gate drive of an external N-Channel MOSFET device, the MIC2582/83 provide inrush current limiting and output voltage slew rate control in harsh, critical power supply environments. Additionally, a circuit breaker function will latch the output MOSFET off if the current limit threshold is exceeded for a determined period. The MIC2583R option includes an auto-restart function upon detecting an overcurrent condition. • MIC2582: Pin-for-pin functional equivalent to the LTC1422 • 2.3V to 13.2V supply voltage operation • Surge voltage protection up to 20V • Current regulation limits inrush current regardless of load capacitance • Programmable inrush current limiting • Electronic circuit breaker • Dual-level overcurrent fault sensing eliminates false tripping • Fast response to short circuit conditions (<1µs) • Programmable output undervoltage detection • Undervoltage Lockout (UVLO) protection • Auto-restart function (MIC2583R) • Power-On Reset and Power-Good status outputs (Power-Good for the MIC2583 and MIC2583R only) • /FAULT status output (MIC2583 and MIC2583R) Applications • • • • • RAID systems Base stations PC board hot swap insertion and removal Hot swap CompactPCI cards Network switches Typical Application BACKPLANE PCB EDGE CONNECTOR CONNECTOR RSENSE 0.006Ω 2% 2 1 Long Pin VIN 12V R1 3.3Ω **D1 (18V) 3 C1 1µF Q1 Si7892DP (PowerPAK™ SO-8) 16 CLOAD 500µF 15 VCC SENSE GATE Short Pin 3 R2 76.8kΩ 1% R3 R4 9.76kΩ 47kΩ 1% /FAULT Signal Medium (or Short) Pin C2 0.01µF DIS 2 /POR 1 FB CPOR GND CFILTER 7, 8 C3 0.1µF 5 C4 8200pF R5 93.1kΩ 1% VIN VIN 13 PWRGD FAULT 4 Long Pin 14 ON MIC2583/83R 11 VOUT 12V@6A 4 R7 47kΩ R8 47kΩ Power-Good Output Power-On Reset Output DOWNSTREAM CONTROLLER EN /RESET 12 R6 12.4kΩ 1% GND *Undervoltage (Input) = 10.5V *Undervoltage (Output) & Power-Good (Output) = 11.0V *START-UP Delay = 12ms */POR Delay = 50ms *Circuit Breaker Response Time = 1.5ms **D1 is BZX84C18 *(See Functional Description and Applications Information) Figure 1. MIC2583/83R Typical Application Circuit Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com April 2003 1 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Ordering Information Part Number Fast Circuit Breaker Threshold MIC2582-JBM 100mV Latched off 8-pin SOIC MIC2583-xBQS x = J, 100mV Latched off 16-pin QSOP Auto-retry 16-pin QSOP Circuit Breaker Package x = K, 150mV x = L, 200mV x = M, Off MIC2583R-xBQS x = J, 100mV x = K*, 150mV x = L*, 200mV x = M*, Off * Contact factory for availability. Pin Configuration /POR 1 PWRGD 2 /POR 1 8 VCC ON 2 ON 3 7 SENSE CPOR 3 CPOR 4 6 GATE GND 4 CFILTER 5 5 FB NC 6 8-Pin SOIC (M) 16 VCC 15 SENSE 14 GATE 13 DIS 12 FB 11 /FAULT GND 7 10 NC GND 8 9 NC 16-Pin QSOP (QS) MIC2582/MIC2583 2 April 2003 MIC2582/MIC2583 Micrel Pin Description Pin Name 8-pin SOIC 16-pin QSOP /POR 1 1 Power-On Reset Output: Open drain N-channel device, Active Low. This pin remains asserted during start-up until a time period (tPOR) after the FB pin voltage rises above the power-good threshold (VFB). The timing capacitor CPOR determines tPOR. When the output voltage monitored at the FB pin falls below VFB, /POR is asserted for a minimum of one timing cycle (tPOR). The /POR pin requires a pull-up resistor (10kΩ minimum) to VCC. ON 2 3 ON Input: Active High. The ON pin, an input to a Schmitt-triggered comparator used to enable/disable the controller, is compared to a 1.24V reference with 50mV of hysteresis. When a logic high is applied to the ON pin (VON > 1.24V), a start-up sequence begins when the GATE pin starts ramping up towards its final operating voltage. When the ON pin receives a logic low signal (VON < 1.19V), the GATE pin is grounded and /FAULT remains high if VCC is above the UVLO threshold. ON must be low for 20µs in order to initiate a start-up sequence. Additionally, toggling the ON pin LOW to HIGH resets the circuit breaker. CPOR 3 4 Power-On Reset Timer: A capacitor connected between this pin and ground sets the supply contact start-up delay (tSTART) and the power-on reset interval (tPOR). When VCC rises above the UVLO threshold, the capacitor connected to CPOR begins to charge. When the voltage at CPOR crosses 0.3V, the start-up threshold (VSTART), a start cycle is initiated if ON is asserted while capacitor CPOR is immediately discharged to ground. When the voltage at FB rises above VFB, capacitor CPOR begins to charge again. When the voltage at CPOR rises above the power-on reset delay threshold (VTH), the timer resets by pulling CPOR to ground, and /POR is de-asserted. If CPOR = 0, then tSTART defaults to 20µs. GND 4 7,8 Ground connection: Tie to analog ground. FB 5 12 Power-Good Threshold Input (Undervoltage Detect): This input is internally compared to a 1.24V reference with 30mV of hysteresis. An external resistive divider may be used to set the voltage at this pin. If this input momentarily goes below 1.24V, then /POR is activated for one timing cycle, tPOR, indicating an output undervoltage condition. The /POR signal deasserts one timing cycle after the FB pin exceeds the power-good threshold by 30mV. A 5µs filter on this pin prevents glitches from inadvertently activating this signal. GATE 6 14 Gate Drive Output: Connects to the gate of an external N-channel MOSFET. An internal clamp ensures that no more than 9V is applied between the GATE pin and the source of the external MOSFET. The GATE pin is immediately brought low when either the circuit breaker trips or an undervoltage lockout condition occurs. SENSE 7 15 Circuit Breaker Sense Input: A resistor between this pin and VCC sets the current limit threshold. Whenever the voltage across the sense resistor exceeds the slow trip current limit threshold (VTRIPSLOW), the GATE voltage is adjusted to ensure a constant load current. If VTRIPSLOW (50mV) is exceeded for longer than time period tOCSLOW, then the circuit breaker is tripped and the GATE pin is immediately pulled low. If the voltage across the sense resistor exceeds the fast trip circuit breaker threshold, VTRIPFAST, at any point due to fast, high amplitude power supply faults, then the GATE pin is immediately brought low without delay. To disable the circuit breaker, the SENSE and VCC pins can be tied together. The default VTRIPFAST for either device is 100mV. Other fast trip thresholds are available: 150mV, 200mV, or OFF (VTRIPFAST disabled). Please contact factory for availability of other options. VCC 8 16 Positive Supply Input: 2.3V to 13.2V. The GATE pin is held low by an internal undervoltage lockout circuit until VCC exceeds a threshold of 2.2V. If VCC exceeds 13.2V, an internal shunt regulator protects the chip from transient voltages up to 20V at the VCC and SENSE pins. April 2003 Pin Function 3 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Pin Name 8-pin SOIC 16-pin QSOP PWRGD N/A 2 Power-Good Output: Open drain N-channel device, Active High. When the voltage at the FB pin is lower than 1.24V, PWRGD output is held low. When the voltage at the FB pin exceeds 1.24V, then PWRGD is asserted immediately. The PWRGD pin requires a pull-up resistor (10kΩ minimum) to VCC. CFILTER N/A 5 Current Limit Response Timer: A capacitor connected to this pin defines the period of time (tOCSLOW) in which an overcurrent event must last to signal a fault condition and trip the circuit breaker. If no capacitor is connected, then tOCSLOW defaults to 5µs. /FAULT N/A 11 Circuit Breaker Fault Status Output: Open drain N-channel device, Active Low. The /FAULT pin is asserted when the circuit breaker trips due to an overcurrent condition or when an undervoltage lockout condition exists. The /FAULT pin requires a pull-up resistor (10kΩ minimum) to VCC. DIS N/A 13 Discharge Output: When the MIC2583/83R is turned off, a 500Ω internal resistor at this output allows the discharging of any load capacitance to ground. NC N/A 6,9,10 MIC2582/MIC2583 Pin Function No internal connection. 4 April 2003 MIC2582/MIC2583 Micrel Absolute Maximum Ratings (Note 1) Operating Ratings (Note 2) All voltages are referred to GND Supply Voltage (VCC) .................................... –0.3V to 20V /POR, /FAULT, PWRGD pins ....................... –0.3V to 15V SENSE pin ........................................... –0.3V to VCC+0.3V ON pin .................................................. –0.3V to VCC+0.3V GATE pin ...................................................... –0.3V to 20V FB input pins ................................................... –0.3V to 6V Junction Temperature .............................................. 125°C ESD Rating ........................................................................ Human body model ............................................... 2kV Machine model .................................................... 100V Electrical Characteristics (Note 3) Supply Voltage (VCC) ................................... 2.3V to 13.2V Thermal Resistance (Rθ(J-A)) 8-pin SOIC ........................................................ 163°C/W 16-pin QSOP .................................................... 112°C/W Operating Temperature Range ................. –40°C to +85°C VCC = 5.0V, TA = 25°C unless otherwise noted. Bold values indicate –40°C ≤ TA ≤ +85°C. Symbol Parameter Condition Min VCC Supply Voltage ICC Supply Current VON = 2V VTRIP Circuit Breaker Trip Voltage (Current Limit Threshold) VTRIP = VCC – VSENSE External Gate Drive VTRIPSLOW IGATEOFF GATE Pin Pull-Up Current GATE Pin Sink Current ITIMER ICPOR VTH VUV Power-On-Reset Timer Current V 1.5 2.5 mA 50 59 mV 100 mV 100 150 200 110 170 225 mV mV mV 7 8 9 V 3.5 4.8 6.5 V Start Cycle, VGATE = 0V, VCC =13.2V –30 17 –8 µA VCC = 2.3V –26 17 –8 µA VGATE – VCC VCC > 3V VGATE >1V VCC = 13.2V, Note 4 100 mA VCC = 2.3V, Note 4 50 mA Turn off 110 µA VCC – VSENSE > VTRIPSLOW (timer on) –8.5 –6.5 –4.5 µA VCC – VSENSE < VTRIPSLOW (timer off) timer on 4.5 6.5 8.5 µA –3.5 2.5 –1.5 µA timer off 0.5 1.3 mA VCPOR rising Timer (CFILTER) Threshold VCFILTER rising (MIC2583/83R only) 1.19 1.245 1.30 V Undervoltage Lockout Threshold VCC rising 2.1 2.2 2.3 V VCC falling 1.90 2.05 2.20 V Undervoltage Lockout Hysteresis VON ON Pin Threshold Voltage 150 mV ON rising 1.19 1.24 1.29 V ON falling 1.14 1.19 1.24 V VONHYS ON Pin Hysteresis ION ON Pin Input Current VON = VCC VSTART Start-Up Delay Timer Threshold VCPOR rising 0.26 VAUTO Auto-Restart Threshold Voltage (MIC2583R only) upper threshold Auto-Restart Current (MIC2583R only) April 2003 13.2 POR Delay and Overcurrent VUVHYS IAUTO Units 85 130 175 /FAULT = 0 (MIC2583/83R only) Current Limit/Overcurrent Timer (CFILTER) Current (MIC2583/83R) 42 VTRIPFAST (MIC2582) VCC = 2.3V IGATE Max 2.3 VTRIPFAST x = J (MIC2583/83R) x = K x=L VGS Typ 50 mV –0.5 µA 0.31 0.36 V 1.19 1.24 1.30 V lower threshold 0.26 0.31 0.36 V charge current 10 13 16 µA 1.4 2 µA discharge current 5 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Symbol Parameter Condition Min Typ Max Unit VFB Power-Good Threshold Voltage FB rising 1.19 1.24 1.29 V FB falling 1.15 1.20 1.25 V VFBHYS FB Hysteresis 40 VOL /POR, /FAULT, PWRGD Output Voltage (/FAULT, PWRGD MIC2583/83R only) RDIS Output Discharge Resistance (MIC2583/83R only) IOUT = 1mA 500 mV 0.4 V 1000 Ω AC Parameters (Note 4) tOCFAST Fast Overcurrent SENSE to GATE Low Trip Time VCC = 5V VCC – VSENSE = 100mV CGATE = 10nF Figure 2 1 µs tOCSLOW Slow Overcurrent SENSE to GATE Low Trip Time VCC = 5V, VCC – VSENSE = 50mV CFILTER = 0 Figure 2 5 µs tONDLY ON Delay Filter 20 µs tFBDLY FB Delay Filter 20 µs Note 1. Exceeding the absolute maximum rating may damage the device. Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Specification for packaged product only. Note 4. Not a tested parameter, guaranteed by design. Timing Diagrams VTRIPFAST 50mV (VCC – VSENSE) tOCFAST tOCSLOW GATE 1V 1V Figure 2. Current Limit Response 1.2V FB tPOR 1.5V /POR 1.5V /PWRGD Figure 3. Power-On Reset Response VUVLO VCC tSTART 1V VGATE Figure 4. Power-On Start-Up Delay Timing MIC2582/MIC2583 6 April 2003 MIC2582/MIC2583 Micrel Typical Characteristics 1.260 1.250 VCC = 13.2 1.240 1.230 VCC = 2.3 1.220 1.210 VCC = 5.0 1.200 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 1.290 1.280 1.270 1.260 1.250 1.240 1.230 1.220 1.210 110 100 VCC = 2.3V 90 80 VCC = 5.0V 70 60 1.300 1.280 1.260 1.240 VCC = 2.3V 1.220 1.200 1.180 1.160 VCC = 13.2V VCC = 5.0V 1.140 1.120 1.100 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) AUTO-RESTART THRESHOLD (V) Power-Good Threshold vs. Temperature (Decreasing) -25 -20 -10 VCC = 2.3V -5.0 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) VCC = 13.2V 1.190 1.275 Power-Good Threshold vs. Temperature (Increasing) VCC = 13.2V 1.250 1.225 1.200 VCC = 2.3V VCC = 5.0V 1.175 1.150 1.125 Auto-Restart Threshold Voltage vs. Temperature (Upper) MIC2583R 0.500 0.450 0.400 VCC = 13.2V 0.350 0.300 VCC = 2.3V 0.250 VCC = 5.0V 0.200 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 1.400 1.350 1.300 UVLO+ 2.20 2.10 2.00 1.90 UVLO– 1.80 1.70 1.60 1.50 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 7 VCC = 13.2V 1.250 1.200 V = 2.3V CC VCC = 5.0V 1.150 1.100 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) Gate Voltage vs. Temperature UVLO Threshold vs. Temperature UVLO THRESHOLD (V) -5.5 1.200 Auto-Restart Threshold Voltage vs. Temperature (Lower) MIC2583R -7.5 VCC = 5.0V 1.210 1.100 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 2.40 2.30 -6.0 VCC = 2.3V 0 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) 2.50 -6.5 VCC = 2.3V -5 -8.0 -7.0 VCC = 5.0V -15 Current-Limit Timer Current vs. Temperature VCC = 13.2V VCC = 13.2V PWRGD THRESHOLD (V) VCC = 13.2V VCC = 5.0V 1.220 1.300 -30 130 120 1.230 1.180 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) AUTO-RESTART THRESHOLD (V) 150 140 GATE CURRENT-ON (µA) GATE CURRENT-OFF (µA) VCC = 5.0 IGATE(ON) vs. Temperature 50 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) PWRGD THRESHOLD (V) VCC = 2.3 1.200 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) IGATE(OFF) vs. Temperature TIMER CURRENT (µA) VCC = 13.2 ON PIN THRESHOLD (V) 1.280 1.270 April 2003 1.240 1.300 20 18 GATE VOLTAGE (V) 1.300 1.290 ON Pin Threshold vs. Temperature (Lower Threshold) ON Pin Threshold vs. Temperature (Upper Threshold) ON PIN THRESHOLD (V) VOLTAGE THRESHOLD (V) Voltage Threshold (VTH) vs. Temperature VCC = 12.0V 16 14 12 10 VCC = 5.0V 8 6 4 VCC = 2.3V 2 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) MIC2582/MIC2583 MIC2582/MIC2583 Micrel Circuit Breaker Fast (VTRIP) vs. Temperature 120 VCC = 2.3V 50 49 VCC = 13.2V 48 47 46 45 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) CURRENT (µA) 20 18 16 Gate Current vs. Gate Voltage @ –40°C VCC = 13.2V 50 40 30 20 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) VCC = 5.0V 0 2 4 6 8 10 12 14 16 18 20 VOLTAGE (V) MIC2582/MIC2583 12 VCC = 5.0V 8 6 2 0 VCC = 2.3V 2.5 2.0 VCC = 13.2V 1.5 VCC = 5.0V Gate Current vs. Gate Voltage @ 85°C 14 VCC = 13.2V 4 VCC = 2.3V 3.0 16 16 10 3.5 1.0 -40 -20 0 20 40 60 80 100 TEMPERATURE (°C) Gate Current vs. Gate Voltage @ 25°C 14 10 8 2 0 90 80 VCC = 5.0V 70 60 18 VCC = 13.2V 14 12 6 4 VCC = 2.3V 110 100 Power-On Reset Timer Current vs. Temperature 4.0 VCC = 2.3V 0 2 4 6 8 10 12 14 16 18 20 VOLTAGE (V) 8 CURRENT (µA) VCC = 5.0V 52 51 FAST THRESHOLD (mV) 54 53 CURRENT (µA) SLOW THRESHOLD (mV) 55 POR TIMER CURRENT (µA) Circuit Breaker Slow (VTRIP) vs. Temperature 12 VCC = 13.2V 10 8 VCC = 2.3V 6 4 2 0 VCC = 5.0V 0 2 4 6 8 10 12 14 16 18 20 VOLTAGE (V) April 2003 MIC2582/MIC2583 Micrel Test Circuit RSENSE 0.025Ω IIN 1 + 3 100kΩ VIN IRF7413 or equivalent 2 IOUT + 4 CLOAD CIN VCC SENSE GATE ON – CGATE DUT – VOUT RLOAD R1 FB 12.4kΩ 1% Figure 5. Applications Test Circuit (not all pins shown for simplicity) April 2003 9 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Functional Characteristics (See Figure 5, Applications Test Circuit) Turn Off - VOUT = 12V VOUT PWRGD 5V/div 5V/div PWRGD 5V/div VOUT 5V/div ON 5V/div ON 5V/div Turn On - VOUT = 12V IIN 500mA/div IIN 500mA/div CIN = 4.7µF CLOAD = 100µF CGATE = 47nF RLOAD = 12Ω R1 = 100kΩ TIME (10ms/div.) Turn On - VOUT = 5V CIN = 4.7µF CLOAD = 100µF CGATE = 47nF RLOAD = 12Ω R1 = 100kΩ TIME (1ms/div.) IIN VOUT 500mA/div 2V/div PWRGD 2V/div PWRGD VOUT 5V/div 2V/div ON 2V/div ON 2V/div Turn Off - VOUT = 5V IIN 500mA/div CIN = 4.7µF CLOAD = 100µF CGATE = 47nF RLOAD = 5Ω R1 = 33kΩ TIME (5ms/div.) Turn On (CGATE = 0) - VOUT = 5V (MIC2583) TIME (1ms/div.) VOUT 2V/div VOUT 2V/div FAULT 5V/div GATE 5V/div ON 5V/div ON 5V/div Inrush Current Response - VOUT = 5V IIN 500mA/div IOUT 500mA/div CIN = 4.7µF CGATE = 0 CLOAD = 10µF RLOAD = 5Ω R1 = 33kΩ TIME (250µs/div.) MIC2582/MIC2583 CIN = 4.7µF CLOAD = 100µF CGATE = 47nF RLOAD = 5Ω R1 = 33kΩ CIN = 0.1µF CLOAD = 100µF CGATE = 10nF RLOAD = 5Ω R1 = 33kΩ TIME (2.5ms/div.) 10 April 2003 MIC2582/MIC2583 Micrel Functional Characteristics (See Figure 5, Applications Test Circuit) CFILTER ON 1V/div 5V/div Turn On Into Short Circuit - VOUT = 5V GATE 2V/div 1.85A CIN = 4.7µF CGATE = 0 CLOAD = 100µF CFILTER = 100nF RLOAD = 6Ω ILIM = 1.7A R1 = 100kΩ IIN 500mA/div IIN 500mA/div FAULT CFILTER ON 10V/div 1V/div 5V/div Turn On Into Heavy Load - VOUT = 12V TIME (20ms/div.) CGATE = CLOAD = 0 CFILTER = 100nF CIN = 4.7µF ILIM = 1.7A R1 = 33kΩ TIME (2.5ms/div.) GATE 5V/div FAULT 5V/div Shutdown by Short Circuit - VOUT = 5V (MIC2583) IOUT 500mA/div CGATE = 0 CIN = 4.7µF CLOAD = 10µF RLOAD = 5Ω ILIM = 3.3A R1 = 33kΩ TIME (100µs/div.) April 2003 11 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Functional Diagram MIC2583/83R SENSE VCC 15(7) 500Ω + – 6.5µA GATE 21V 9V 50mV VCC 14(6) Charge Pump + – 16(8) UVLO 2.2V 100mV 13 Circuit Breaker Trips or UVLO DIS ITIMER 11 /FAULT 5 CFILTER 6.5µA VREF + – Logic 1(1) /POR GND FB 7,8(4) 12(5) VREF + – 2 Glitch Filter PWRGD VCC 2.5µA CPOR 4 (3) ICPOR 0.3V + – VREF + – Glitch Filter + – 3(2) ON VREF 1.24V Reference Pin numbers for MIC2582 are in parenthesis ( ) where applicable MIC2582/MIC2583 12 April 2003 MIC2582/MIC2583 Micrel supply is already present (i.e., not a “hot swapping” condition) and the MIC2582/83 device is enabled by applying a logic high signal at the ON pin, the GATE output begins ramping immediately as the first CPOR timing cycle is bypassed. Active current regulation is employed to limit the inrush current transient response during start-up by regulating the load current at the programmed current limit value (See Current Limiting and DualLevel Circuit Breaker section). The following equation is used to determine the nominal current limit value: Functional Description Hot Swap Insertion When circuit boards are inserted into live system backplanes and supply voltages, high inrush currents can result due to the charging of bulk capacitance that resides across the supply pins of the circuit board. This inrush current, although transient in nature, may be high enough to cause permanent damage to on board components or may cause the system’s supply voltages to go out of regulation during the transient period which may result in system failures. The MIC2582 and MIC2583 act as a controller for external N-Channel MOSFET devices in which the gate drive is controlled to provide inrush current limiting and output voltage slew rate control during hot plug insertions. V 50mV ILIM = TRIPSLOW = RSENSE RSENSE (2) where VTRIPSLOW is the current limit slow trip threshold found in the electrical table and RSENSE is the selected value that will set the desired current limit. There are two basic start-up modes for the MIC2582/83: 1) Start-up dominated by load capacitance and 2) start-up dominated by total gate capacitance. The magnitude of the inrush current delivered to the load will determine the dominant mode. If the inrush current is greater than the programmed current limit (ILIM), then load capacitance is dominant. Otherwise, gate capacitance is dominant. The expected inrush current may be calculated using the following equation: Power Supply VCC is the supply input to the MIC2582/83 controller with a voltage range of 2.3V to 13.2V. The VCC input can withstand transient spikes up to 20V. In order to ensure stability of the supply voltage, a minimum 0.47µF capacitor from VCC to ground is recommended. Alternatively, a low pass filter, shown in the typical application circuit (see Figure 1), can be used to eliminate high frequency oscillations as well as help suppress transient spikes. Also, due to the existence of an undetermined amount of parasitic inductance in the absence of bulk capacitance along the supply path, placing a Zener diode at the VCC of the controller to ground in order to provide external supply transient protection is strongly recommended for relatively high current applications (≥3A). See Figure 1. Start-Up Cycle where IGATE is the GATE pin pull-up current, CLOAD is the load capacitance, and CGATE is the total GATE capacitance (CISS of the external MOSFET and any external capacitor connected from the MIC2582/83 GATE pin to ground). Supply Contact Delay Load Capacitance Dominated Start-Up During a hot insert of a PC board into a backplane or when the supply (VCC) is powered up, as the voltage at the ON pin rises above its threshold (1.24V typical), the MIC2582/83 first checks that both supply voltages are above their respective UVLO thresholds. If so, the device is enabled and an internal 2.5µA current source begins charging capacitor CPOR to 0.3V to initiate a start-up sequence. Once the start-up delay (tSTART) elapses, the CPOR pin is pulled immediately to ground and a 17µA current source begins charging the GATE output to drive the external MOSFET that switches VIN to VOUT. The programmed contact start-up delay is calculated using the following equation: In this case, the load capacitance (CLOAD) is large enough to cause the inrush current to exceed the programmed current limit but is less than the fast-trip threshold (or the fast-trip threshold is disabled, ‘M’ option). During start-up under this condition, the load current is regulated at the programmed current limit value (ILIM) and held constant until the output voltage rises to its final value. The output slew rate and equivalent GATE voltage slew rate is computed by the following equation: t START = CPOR × VSTART ICPOR INRUSH ≅ IGATE × Output Voltage Slew Rate, dVOUT /dt = (3) ILIM CLOAD (4) where ILIM is the programmed current limit value. Consequently, the value of CFILTER must be selected to ensure that the overcurrent response time, tOCSLOW, exceeds the time needed for the output to reach its final value. For example, given a MOSFET with an input capacitance CISS = CGATE = 4700pF, CLOAD is 2200µF, and ILIM is set to 6A with a 12V input, then the load capacitance dominates as determined by the calculated INRUSH > ILIM. Therefore, the output voltage slew rate determined from Equation 4 is: ≅ 0.12 × CPOR (µF) (1) where the start-up delay timer threshold (VSTART) is 0.3V, and the Power-On Reset timer current (ICPOR) is 2.5µA. See Table 2 for some typical supply contact start-up delays using several standard value capacitors. As the GATE voltage continues ramping toward its final value (VCC + VGS) at a defined slew rate (See Load Capacitance/Gate Capacitance Dominated Startup sections), a second CPOR timing cycle begins if: 1)/FAULT is high and 2)CFILTER is low (i.e., not an overvoltage, undervoltage lockout, or overcurrent state). This second timing cycle (tPOR) begins when the voltage at the FB pin exceeds its threshold (VFB). This condition indicates that the output voltage is valid. See Figure 3 in the Timing Diagrams. When the power April 2003 CLOAD C ≅ 17µA × LOAD CGATE CGATE Output Voltage Slew Rate, dVOUT /dt = 6A V = 2.73 2200µF ms and the resulting tOCSLOW needed to achieve a 12V output is approximately 4.5ms. (See Power-On Reset and Overcurrent Timer Delays section to calculate tOCSLOW) 13 MIC2582/MIC2583 MIC2582/MIC2583 Micrel GATE Capacitance Dominated Start-Up charges capacitor CPOR. Once the CPOR pin voltage reaches 1.24V, the time period tPOR elapses as the CPOR pin is pulled to ground and the /POR pin goes HIGH. If the voltage at FB drops below VFB for more than 10µs, the /POR pin resets for at least one timing cycle defined by tPOR (See Applications Information for an example). Power-On Reset and Overcurrent Timer Delays The Power-On Reset delay, tPOR, is the time period for the /POR pin to go HIGH once the voltage at the FB pin exceeds the power-good threshold (VFB). A capacitor connected to CPOR sets the interval and is determined by using Equation 1 with VTH substituted for VSTART. The resulting equation becomes: In this case, the value of the load capacitance relative to the GATE capacitance is small enough such that the load current during start-up never exceeds the current limit threshold as determined by Equation 3. The minimum value of CGATE that will ensure that the current limit is never exceeded is given by the equation below: I CGATE (min) = GATE × CLOAD ILIM (5) where CGATE is the summation of the MOSFET input capacitance (CISS) and the value of the external capacitor connected to the GATE pin of the MIC2582/83 to ground. Once CGATE is determined, use the following equation to determine the output slew rate for gate capacitance dominated start-up. dVOUT /dt = IGATE tPOR = CPOR × Table 1 depicts the output slew rate for various values of CGATE. IGATE = 17µA CGATE dVOUT/dt 0.001µF 17V/ms 0.01µF 1.7V/ms 0.1µF 0.17V/ms 1µF 0.017V/ms Table 1. Output Slew Rate Selection for GATE Capacitance Dominated Start-Up Current Limiting and Dual-Level Circuit Breaker Many applications will require that the inrush and steady state supply current be limited at a specific value in order to protect critical components within the system. Connecting a sense resistor between the VCC and SENSE pins sets the nominal current limit value of the MIC2582/83 and the current limit is calculated using Equation 2. The MIC2582/83 also features a dual-level circuit breaker triggered via 50mV and 100mV current limit thresholds sensed across the VCC and SENSE pins. The first level of the circuit breaker functions as follows. For the MIC2583/83R, once the voltage sensed across these two pins exceeds 50mV, the overcurrent timer, its duration set by capacitor CFILTER, starts to ramp the voltage at CFILTER using a 6.5µA constant current source. If the voltage at CFILTER reaches the overcurrent timer threshold (VTH) of 1.24V, then CFILTER immediately returns to ground as the circuit breaker trips and the GATE output is immediately shut down. The default overcurrent time period for the MIC2582/83 is 5µs. For the second level, if the voltage sensed across VCC and SENSE exceeds 100mV at any time, the circuit breaker trips and the GATE shuts down immediately, bypassing the overcurrent time period. To disable current limit and circuit breaker operation, tie the SENSE and VCC pins together and the CFILTER (MIC2583/83R) pin to ground. Output Undervoltage Detection The MIC2582/83 employ output undervoltage detection by monitoring the output voltage through a resistive divider connected at the FB pin. During turn on, while the voltage at the FB pin is below the threshold (VFB), the /POR pin is asserted low. Once the FB pin voltage crosses VFB, a 2.5µA current source MIC2582/MIC2583 ICPOR ≅ 0.5 × CPOR (µF) (7) where the Power-On Reset threshold (VTH) and timer current (ICPOR) are typically 1.24V and 2.5µA, respectively. For the MIC2583/83R, a capacitor connected to CFILTER is used to set the timer which activates the circuit breaker during overcurrent conditions. When the voltage across the sense resistor exceeds the slow trip current limit threshold of 50mV, the overcurrent timer begins to charge for a time period (tOCSLOW), determined by CFILTER. When no capacitor is connected to CFILTER and for the MIC2582, tOCSLOW defaults to 5µs. If tOCSLOW elapses, then the circuit breaker is activated and the GATE output is immediately pulled to ground. For the MIC2583/83R, the following equation is used to determine the overcurrent timer period, tOCSLOW. (6) CGATE VTH t OCSLOW = CFILTER × VTH ITIMER ≅ 0.19 × CFILTER (µF) (8) where VTH, the CFILTER timer threshold, is 1.24V and ITIMER, the overcurrent timer current, is 6.5µA. Tables 2 and 3 provide a quick reference for several timer calculations using select standard value capacitors. CPOR 0.01µF 0.02µF 0.033µF 0.05µF 0.1µF 0.33µF 0.47µF 1µF tSTART 1.2ms 2.4ms 4ms 6ms 12ms 40ms 56ms 120ms tPOR 5ms 10ms 16.5ms 25ms 50ms 165ms 235ms 500ms Table 2. Selected Power-On Reset and Start-Up Delays CFILTER 680pF 2200pF 4700pF 8200pF 0.033µF 0.1µF 0.22µF 0.47µF tOCSLOW 130µs 420µs 900µs 1.5ms 6ms 19ms 42ms 90ms Table 3. Selected Overcurrent Timer Delays 14 April 2003 MIC2582/MIC2583 Micrel Applications Information V OUT(Good) R5 = R6 – 1 VFB(MAX) Output Undervoltage Detection For output undervoltage detection, the first consideration is to establish the output voltage level that indicates “power is good.” For this example, the output value for which a 12V supply will signal “good” is 11V. Next, consider the tolerances of the input supply and FB threshold (VFB). For this example, the 12V supply varies ±5%, thus the resulting output voltage may be as low as 11.4V and as high as 12.6V. Additionally, the FB threshold has ±50mV tolerance and may be as low as 1.19V and as high as 1.29V. Thus, to determine the values of the resistive divider network (R5 and R6) at the FB pin, shown in the typical application circuit on page 1, use the following iterative design procedure. where VFB(MAX) = 1.29V, VOUT(Good) = 11V, and R6 is 12.4kΩ. Substituting these values into Equation 9.1 now yields R5 = 93.33kΩ. A standard 93.1kΩ ± 1% is selected. Now, consider the 11.4V minimum output voltage, the lower tolerance for R6 and higher tolerance for R5, 12.28kΩ and 94.03kΩ, respectively. With only 11.4V available, the voltage sensed at the FB pin exceeds VFB(MAX), thus the /POR and PWRGD (MIC2583/83R) signals will transition from LOW to HIGH, indicating “power is good” given the worse case tolerances of this example. PCB Connection Sense 1) Choose R6 so as to limit the current through the divider to approximately 100µA or less. VFB(MAX) There are several configuration options for the MIC2582/83’s ON pin to detect if the PCB has been fully seated in the backplane before initiating a start-up cycle. In the typical applications circuit, the MIC2582/83 is mounted on the PCB with a resistive divider network connected to the ON pin. R2 is connected to a short pin on the PCB edge connector. Until the connectors mate, the ON pin is held low which keeps the GATE output charge pump off. Once the connectors mate, the resistor network is pulled up to the input supply, 12V in this example, and the ON pin voltage exceeds its threshold (VON) of 1.24V and the MIC2582/83 initiates a start-up cycle. In Figure 6, the connection sense consisting of a discrete logic-level MOSFET and a few resistors allows for interrupt 1.29V ≅ 12.9kΩ . 100µA 100µA R6 is chosen as 12.4kΩ ± 1%. 2) Next, determine R5 using the output “good” voltage of 11V and the following equation: R6 ≅ ≅ (R5 + R6) VOUT(Good) = VFB R6 (9) Using some basic algebra and simplifying Equation 9 to isolate R5, yields: Backplane PCB Edge Connector Connector VIN 5V RSENSE 0.010Ω 1 5% 2 Long Pin 3 C1 1F (9.1) Q1 Si7860DP (PowerPAK“ SO-8) VOUT 5V@3A 4 CLOAD 220 F **R8 10Ω R5 20kΩ 16 R4 20kΩ VCC 3 R1 33kΩ R3 100Ω /ON_OFF 15 SENSE GATE C2 0.01 F R2 *Q2 33kΩ MIC2583 PCB Connection Sense Short Pin Medium or Short Pin GND Long Pin DIS FB 11 /FAULT 14 ON 13 12 VIN R9 20Ω /FAULT CPOR GND 4 7,8 /POR R6 27.4kΩ 1% R7 10.5kΩ 1% 1 Downstream Signal C3 0.05 F Undervoltage (Output) = 4.45V /POR Delay = 25ms START-UP Delay = 6ms *Q2 is TN0201T (SOT-23) **R8 is optional for noise filtering Additional pins omitted for clarity. Figure 6. PCB Connection Sense with ON/OFF Control April 2003 15 MIC2582/MIC2583 MIC2582/MIC2583 Micrel control from the processor or other signal controller to shut off the output of the MIC2582/83. R4 pulls the GATE of Q2 to VIN and the ON pin is held low until the connectors are fully mated. Once the connectors fully mate, a logic LOW at the /ON_OFF signal turns Q2 off and allows the ON pin to pull up above its threshold and initiate a start-up cycle. Applying a logic HIGH at the /ON_OFF signal will turn Q2 on and short the ON pin of the MIC2582/83 to ground which turns off the GATE output charge pump. 5V Switch with 3.3V Supply Generation The MIC2582/83 can be configured to switch a primary supply while generating a secondary regulated voltage rail. The circuit in Figure 8 enables the MIC2582 to switch a 5V supply while also providing a 3.3V low dropout regulated supply with only a few added external components. Upon enabling the MIC2582, the GATE output voltage increases and thus the 3.3V supply also begins to ramp. As the 3.3V output supply crosses 3.3V, the FB pin threshold is also exceeded which triggers the power-on reset comparator. The /POR pin goes HIGH, turning on transistor Q3 which lowers the voltage on the gate of MOSFET Q2. The result is a regulated 3.3V supply with the gate feedback loop of Q2 compensated by capacitor C3 and resistors R4 and R5. For MOSFET Q2, special consideration must be given to the power dissipation capability of the selected MOSFET as 1.5V to 2V will drop across the device during normal operation in this application. Therefore, the device is susceptible to overheating dependent upon the current requirements for the regulated output. In this example, the power dissipated by Q2 is approximately ≤1W. However, a substantial amount of power will be generated with higher current requirements and/or conditions. As a general guideline, expect the ambient temperature within the power supply box to exceed the maximum operating ambient temperature of the system environment by approximately 20°C. Given the MOSFET’s Rθ(J-A) and the expected power dissipated by the MOSFET, an approximation for the junction temperature at which the device will operate is obtained as follows: TJ = (PD x Rθ(J-A)) + TA' (10) Higher UVLO Setting Once a PCB is inserted into a backplane (power supply), the internal UVLO circuit of the MIC2582/83 holds the GATE output charge pump off until VCC exceeds 2.2V. If VCC falls below 2.1V, the UVLO circuit pulls the GATE output to ground and clears the overvoltage and/or current limit faults. For a higher UVLO threshold, the circuit in Figure 7 can be used to delay the output MOSFET from switching on until the desired input voltage is achieved. The circuit allows the charge pump R1 × 1.24V . The GATE to remain off until VIN exceeds 1 + R2 drive output will be shut down when VIN falls below R1 × 1.19V . In the example circuit (Figure 7), the rising 1+ R2 UVLO threshold is set at approximately 9.5V and the falling UVLO threshold is established as 9.1V. The circuit consists of an external resistor divider at the ON pin that keeps the GATE output charge pump off until the voltage at the ON pin exceeds its threshold (VON) and after the start-up timer elapses. where TA'=TA(MAX OPERATING) + 20°C. As a precaution, the implementation of additional copper heat sinking is highly recommended for the area under/around the MOSFET. For additional information on MOSFET thermal considerations, please see MOSFET Selection text and subsequent sections. Q1 IRF7822 (SO-8) RSENSE 0.010Ω 1 5% 2 VIN 12V D1 (18V) C1 1µF R1 332kΩ 1% 3 8 VCC CLOAD 220µF R3 10Ω 7 SENSE GATE 2 VOUT 12V@4A 4 R4 133kΩ 1% 6 C2 0.01µF ON MIC2582 R2 49.9kΩ 1% FB 5 R5 16.2kΩ 1% GND 4 Undervoltage Lockout Threshold (rising) = 9.5V Undervoltage Lockout Threshold (falling) = 9.1V Undervoltage (Output) = 11.4V Additional pins omitted for clarity. Figure 7. Higher UVLO Setting MIC2582/MIC2583 16 April 2003 MIC2582/MIC2583 Micrel Q2 Si4876DY (SO-8) Backplane PCB Edge Connector Connector Q1 Si4876DY (SO-8) Long Pin VIN 5V 1 D1 (9V) C1 0.47 F 3 2 RSENSE 0.010Ω 2% VCC 2 C5 330 F 3 R3 10Ω R2 10Ω 7 SENSE GATE 6 ON R4 1.2MΩ C2 0.022 F C3 4700pF R5 510kΩ VIN MIC2582 Open Circuit Short Pin VOUT [email protected] VOUT [email protected] 4 8 R1 47kΩ C6 100 F /POR CPOR FB R8 20kΩ 1 R9 750Ω Q3 PN2222 R6 20kΩ 1% C4 0.1 F 5 GND R7 11.8kΩ 1% 4 GND Long Pin Undervoltage (Output) = 3.3V All resistors 5% unless specified otherwise Figure 8. 5V Switch/3.3V LDO Application Auto-Restart - MIC2583R The MIC2583R provides an auto-restart function. Upon an overcurrent fault condition such as a short circuit, the MIC2583R initially shuts off the GATE output. The MIC2583R attempts to restart with a 12µA charge current at a preset 10% duty cycle until the fault condition is removed. The interval between auto-retry attempts is set by capacitor CFILTER. Sense Resistor Selection Once the value of RSENSE has been chosen in this manner, it is good practice to check the maximum ILOAD(CONT) which the circuit may let through in the case of tolerance buildup in the opposite direction. Here, the worst-case maximum current is found using a 59mV trip voltage and a sense resistor that is 3% low in value. The resulting equation is: ILOAD(CONT,MAX) = The MIC2582 and MIC2583 use a low-value sense resistor to measure the current flowing through the MOSFET switch (and therefore the load). This sense resistor is nominally set at 50mV/ILOAD(CONT). To accommodate worst-case tolerances for both the sense resistor (allow ±3% over time and temperature for a resistor with ±1% initial tolerance) and still supply the maximum required steady-state load current, a slightly more detailed calculation must be used. The current limit threshold voltage (i.e., the “trip point”) for the MIC2582/83 may be as low as 42mV, which would equate to a sense resistor value of 42mV/ILOAD(CONT). Carrying the numbers through for the case where the value of the sense resistor is 3% high yields: RSENSE(MAX) = April 2003 42mV (1.03)(ILOAD(CONT) ) = 59mV (0.97)(RSENSE(NOM) ) = 60.8mV RSENSE(NOM) (12) As an example, if an output must carry a continuous 2A without nuisance trips occurring, Equation 11 40.8mV = 20.4mΩ . The next lowest 2A standard value is 20mΩ. At the other set of tolerance extremes for the output in question, yields: RSENSE(MAX) = 60.8mV = 3.04A , approximately 3A. 20.0mΩ Knowing this final datum, we can determine the necessary wattage of the sense resistor using P = I2R, where I will be ILOAD(CONT, MAX), and R will be (0.97)(RSENSE(NOM)). These numbers yield the following: PMAX = (3A)2 (19.4mΩ) = 0.175W. In this example, a 1/4W sense resistor is sufficient. ILOAD(CONT,MAX) = 40.8mV ILOAD(CONT) (11) 17 MIC2582/MIC2583 MIC2582/MIC2583 Micrel MOSFET Selection is not hard to meet. In MIC2582/83 applications, the gate of the external MOSFET is driven up to approximately 19.5V by the internal output MOSFET (again, assuming 12V operation). At the same time, if the output of the external MOSFET (its source) is suddenly subjected to a short, the gate-source voltage will go to (19.5V – 0V) = 19.5V. This means that the external MOSFET must be chosen to have a gate-source breakdown voltage of 20V or more, which is an available standard maximum value. However, if operation is at or above 13V, the 20V gate-source maximum will likely be exceeded. As a result, an external Zener diode clamp should be used to prevent breakdown of the external MOSFET when operating at voltages above 8V. A Zener diode with 10V rating is recommended as shown in Figure 9. At the present time, most power MOSFETs with a 20V gate-source voltage rating have a 30V drain-source breakdown rating or higher. As a general tip, choose surface-mount devices with a drainsource rating of 30V as a starting point. Selecting the proper external MOSFET for use with the MIC2582/83 involves three straightforward tasks: • Choice of a MOSFET which meets minimum voltage requirements. • Selection of a device to handle the maximum continuous current (steady-state thermal issues). • Verify the selected part’s ability to withstand any peak currents (transient thermal issues). MOSFET Voltage Requirements The first voltage requirement for the MOSFET is easily stated: the drain-source breakdown voltage of the MOSFET must be greater than VIN(MAX). For instance, a 12V input may reasonably be expected to see high-frequency transients as high as 18V. Therefore, the drain-source breakdown voltage of the MOSFET must be at least 19V. For ample safety margin and standard availability, the closest value will be 20V. Finally, the external gate drive of the MIC2582/83 requires a low-voltage logic level MOSFET when operating at voltages lower than 3V. There are 2.5V logic level MOSFETs available. Please see Table 4 MOSFET and Sense Resistor Vendors for suggested manufacturers. The second breakdown voltage criterion that must be met is a bit subtler than simple drain-source breakdown voltage, but Q1 IRF7822 (SO-8) RSENSE 0.006Ω 1 5% 2 VIN 12V D1 (18V) 3 *D2 1N5240B 10V 4 CLOAD 220µF C1 1µF R1 33kΩ 8 VCC R3 10Ω 7 SENSE GATE 2 VOUT 12V@6A 6 R4 100kΩ 1% C2 0.01µF ON MIC2582 FB 5 VIN R2 33kΩ R6 47kΩ CPOR /POR GND 3 R5 13.3kΩ 1% 1 DOWNSTREAM SIGNAL 4 C3 0.05µF Undervoltage (Output) = 11.0V /POR Delay = 25ms START-UP Delay = 6ms *Recommended for MOSFETs with gate-source breakdown of 20V or less for catastrophic output short circuit protection. (IRF7822 VGS(MAX) = 12V) Figure 9. Zener Clamped MOSFET Gate MIC2582/MIC2583 18 April 2003 MIC2582/MIC2583 Micrel MOSFET Steady-State Thermal Issues current. The use of a thermocouple on the drain leads, or infrared pyrometer on the package, will then give a reasonable idea of the device’s junction temperature. The selection of a MOSFET to meet the maximum continuous current is a fairly straightforward exercise. First, arm yourself with the following data: • The value of ILOAD(CONT, MAX.) for the output in question (see Sense Resistor Selection). MOSFET Transient Thermal Issues Having chosen a MOSFET that will withstand the imposed voltage stresses, and the worse case continuous I2R power dissipation which it will see, it remains only to verify the MOSFET’s ability to handle short-term overload power dissipation without overheating. A MOSFET can handle a much higher pulsed power without damage than its continuous dissipation ratings would imply. The reason for this is that, like everything else, thermal devices (silicon die, lead frames, etc.) have thermal inertia. • The manufacturer’s data sheet for the candidate MOSFET. • The maximum ambient temperature in which the device will be required to operate. • Any knowledge you can get about the heat sinking available to the device (e.g., can heat be dissipated into the ground plane or power plane, if using a surface-mount part? Is any airflow available?). The data sheet will almost always give a value of on resistance given for the MOSFET at a gate-source voltage of 4.5V, and another value at a gate-source voltage of 10V. As a first approximation, add the two values together and divide by two to get the on-resistance of the part with 8V of enhancement. Call this value RON. Since a heavily enhanced MOSFET acts as an ohmic (resistive) device, almost all that’s required to determine steady-state power dissipation is to calculate I2R. The one addendum to this is that MOSFETs have a slight increase in RON with increasing die temperature. A good approximation for this value is 0.5% increase in RON per °C rise in junction temperature above the point at which RON was initially specified by the manufacturer. For instance, if the selected MOSFET has a calculated RON of 10mΩ at a TJ = 25°C, and the actual junction temperature ends up at 110°C, a good first cut at the operating value for RON would be: In terms related directly to the specification and use of power MOSFETs, this is known as “transient thermal impedance,” or Zθ(J-A). Almost all power MOSFET data sheets give a Transient Thermal Impedance Curve. For example, take the following case: VIN = 12V, tOCSLOW has been set to 100msec, ILOAD(CONT. MAX) is 2.5A, the slow-trip threshold is 50mV nominal, and the fast-trip threshold is 100mV. If the output is accidentally connected to a 3Ω load, the output current from the MOSFET will be regulated to 2.5A for 100ms (tOCSLOW) before the part trips. During that time, the dissipation in the MOSFET is given by: P = E x I EMOSFET = [12V-(2.5A)(3Ω)] = 4.5V PMOSFET = (4.5V x 2.5A) = 11.25W for 100msec. At first glance, it would appear that a really hefty MOSFET is required to withstand this sort of fault condition. This is where the transient thermal impedance curves become very useful. Figure 10 shows the curve for the Vishay (Siliconix) Si4410DY, a commonly used SO-8 power MOSFET. Taking the simplest case first, we’ll assume that once a fault event such as the one in question occurs, it will be a long time– 10 minutes or more– before the fault is isolated and the channel is reset. In such a case, we can approximate this as a “single pulse” event, that is to say, there’s no significant duty cycle. Then, reading up from the X-axis at the point where “Square Wave Pulse Duration” is equal to 0.1sec (=100msec), we see that the Zθ(J-A) of this MOSFET to a highly infrequent event of this duration is only 8% of its continuous Rθ(J-A). RON ≅ 10mΩ[1 + (110 - 25)(0.005)] ≅ 14.3mΩ The final step is to make sure that the heat sinking available to the MOSFET is capable of dissipating at least as much power (rated in °C/W) as that with which the MOSFET’s performance was specified by the manufacturer. Here are a few practical tips: 1. The heat from a surface-mount device such as an SO-8 MOSFET flows almost entirely out of the drain leads. If the drain leads can be soldered down to one square inch or more, the copper will act as the heat sink for the part. This copper must be on the same layer of the board as the MOSFET drain. 2. Airflow works. Even a few LFM (linear feet per minute) of air will cool a MOSFET down substantially. If you can, position the MOSFET(s) near the inlet of a power supply’s fan, or the outlet of a processor’s cooling fan. This particular part is specified as having an Rθ(J-A) of 50°C/W for intervals of 10 seconds or less. Thus: Assume TA = 55°C maximum, 1 square inch of copper at the drain leads, no airflow. Recalling from our previous approximation hint, the part has an RON of (0.0335/2) = 17mΩ at 25°C. Assume it has been carrying just about 2.5A for some time. When performing this calculation, be sure to use the highest anticipated ambient temperature (TA(MAX)) in which the MOSFET will be operating as the starting temperature, and find the operating junction temperature increase (∆TJ) from that point. Then, as shown next, the final junction temperature is found by adding TA(MAX) and ∆TJ. Since this is not a closedform equation, getting a close approximation may take one or 3. The best test of a surface-mount MOSFET for an application (assuming the above tips show it to be a likely fit) is an empirical one. Check the MOSFET's temperature in the actual layout of the expected final circuit, at full operating April 2003 19 MIC2582/MIC2583 MIC2582/MIC2583 Micrel two iterations, But it’s not a hard calculation to perform, and tends to converge quickly. So our original approximation of 61.1°C was very close to the correct value. We will use TJ = 61°C. Then the starting (steady-state)TJ is: Finally, add (11.25W)(50°C/W)(0.08) = 45°C to the steadystate TJ to get TJ(TRANSIENT MAX.) = 106°C. This is an acceptable maximum junction temperature for this part. PCB Layout Considerations TJ ≅ TA(MAX) + ∆TJ ≅ TA(MAX) + [RON + (TA(MAX) – TA)(0.005/°C)(RON)] x I2 x Rθ(J-A) Because of the low values of the sense resistors used with the MIC2582/83 controllers, special attention to the layout must be used in order for the device’s circuit breaker function to operate properly. Specifically, the use of a 4-wire Kelvin connection to accurately measure the voltage across RSENSE is highly recommended. Kelvin sensing is simply a means of making sure that any voltage drops in the power traces connecting to the resistors does not get picked up by the traces themselves. Additionally, these Kelvin connections should be isolated from all other signal traces to avoid introducing noise onto these sensitive nodes. Figure 11 illustrates a recommended, single layer layout for the RSENSE, TJ ≅ 55°C + [17mΩ + (55°C-25°C)(0.005)(17mΩ)] x (2.5A)2 x (50°C/W) TJ ≅ (55°C + (0.122W)(50°C/W) ≅ 61.1°C Iterate the calculation once to see if this value is within a few percent of the expected final value. For this iteration we will start with TJ equal to the already calculated value of 61.1°C: TJ ≅ TA + [17mΩ + (61.1°C-25°C)(0.005)(17mΩ)] x (2.5A)2 x (50°C/W) TJ ≅ ( 55°C + (0.125W)(50°C/W) ≅ 61.27°C Normalized Thermal Transient Impedance, Junction-to-Ambient 2 1 Normalized Effective Transient Thermal Impedance Duty Cycle = 0.5 0.2 Notes: 0.1 PDM 0.1 0.05 t1 t2 t1 1. Duty Cycle, D = t2 2. Per Unit Base = RthJA = 50° C/W 0.02 3. TJM – TA = PDMZthJA(t) Single Pulse 4. Surface Mounted 0.01 10–4 10–3 10–2 10–1 1 10 30 Square Wave Pulse Duration (sec) Figure 10. Transient Thermal Impedance Current Flow to the Load Current Flow to the Load *POWER MOSFET (SO-8) *SENSE RESISTOR (2512) W D G D S D S D S W **RGATE 5 VCC SENSE GATE FB GND 6 CPOR 7 ON 8 /POR MIC2582-JBM 93.1k 1% 1 2 3 4 Current Flow to the Load 12.4k 1% **CGATE **CPOR W DRAWING IS NOT TO SCALE *See Table 4 for part numbers and vendors. **Optional components. Trace width (W) guidelines given in "PCB Layout Recommendations" section of the datasheet. Figure 11. Recommended PCB Layout for Sense Resistor, Power MOSFET, and Feedback Network MIC2582/MIC2583 20 April 2003 MIC2582/MIC2583 Micrel Power MOSFET, timer(s), and feedback network connections. The feedback network resistor values are selected for a 12V application. Many hot swap applications will require load currents of several amperes. Therefore, the power (VCC and Return) trace widths (W) need to be wide enough to allow the current to flow while the rise in temperature for a given copper plate (e.g., 1oz. or 2oz.) is kept to a maximum of 10°C~25°C. Also, these traces should be as short as possible in order to minimize the IR drops between the input and the load. For a starting point, there are many trace width calculation tools available on the web such as the following link: Finally, the use of plated-through vias will be needed to make circuit connections to power and ground planes when utilizing multi-layer PC boards. MOSFET and Sense Resistor Vendors Device types and manufacturer contact information for power MOSFETs and sense resistors is provided in Table 4. Some of the recommended MOSFETs include a metal heat sink on the bottom side of the package. The recommended trace for the MOSFET Gate of Figure 11 must be redirected when using MOSFETs packaged in this style. Contact the device manufacturer for package information. http://www.aracnet.com/cgi-usr/gpatrick/trace.pl MOSFET Vendors Key MOSFET Type(s) Applications* Contact Information Vishay (Siliconix) Si4420DY (SO-8 package) Si4442DY (SO-8 package) Si4876DY (SO-8 package) Si7892DP (PowerPAK™ SO-8) IOUT ≤ 10A IOUT = 10-15A, VCC < 3V IOUT ≤ 5A, VCC ≤ 5V IOUT ≤15A www.siliconix.com (203) 452-5664 International Rectifier IRF7413 (SO-8 package) IRF7457 (SO-8 package) IRF7601 (SO-8 package) IOUT ≤ 10A IOUT = 10-15A IOUT ≤ 5A, VCC < 3V www.irf.com (310) 322-3331 Fairchild Semiconductor FDS6680A (SO-8 package) IOUT ≤ 10A www.fairchildsemi.com (207) 775-8100 Philips Hitachi PH3230 (SOT669-LFPAK) HAT2099H (LFPAK) IOUT ≥ 20A IOUT ≥ 20A www.philips.com www.halsp.hitachi.com (408) 433-1990 * These devices are not limited to these conditions in many cases, but these conditions are provided as a helpful reference for customer applications. Resistor Vendors Vishay (Dale) IRC April 2003 Sense Resistors “WSL” Series Contact Information www.vishay.com/docswsl_30100.pdf (203) 452-5664 “OARS” Series www.irctt.com/pdf_files/OARS.pdf “LR” Series www.irctt.com/pdf_files/LRC.pdf (second source to “WSL”) (828) 264-8861 Table 4. MOSFET and Sense Resistor Vendors 21 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Package Information 0.026 (0.65) MAX) PIN 1 0.157 (3.99) 0.150 (3.81) DIMENSIONS: INCHES (MM) 0.020 (0.51) 0.013 (0.33) 0.050 (1.27) TYP 0.064 (1.63) 0.045 (1.14) 45° 0.0098 (0.249) 0.0040 (0.102) 0°–8° 0.197 (5.0) 0.189 (4.8) SEATING PLANE 0.010 (0.25) 0.007 (0.18) 0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.228 (5.79) 8-Pin SOP (M) PIN 1 DIMENSIONS: INCHES (MM) 0.157 (3.99) 0.150 (3.81) 0.009 (0.2286) REF 0.025 (0.635) BSC 0.0098 (0.249) 0.0040 (0.102) 0.012 (0.30) 0.008 (0.20) 0.0098 (0.249) 0.0075 (0.190) 0.196 (4.98) 0.189 (4.80) SEATING 0.0688 (1.748) PLANE 0.0532 (1.351) 45¡ 8¡ 0¡ 0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.229 (5.82) Rev. 04 16-pin QSOP (QS) MICREL, INC. TEL 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com The information furnished by Micrel in this datasheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2003 Micrel, Incorporated. MIC2582/MIC2583 22 April 2003