MIC2588/MIC2594 Micrel MIC2588/MIC2594 Single-Channel, Negative High-Voltage Hot Swap Power Controllers General Description Features The MIC2588 and the MIC2594 are single-channel, negative-voltage hot swap controllers designed to address the need for safe insertion and removal of circuit boards into “live” high-voltage system backplanes, while using very few external components. The MIC2588 and the MIC2594 are each available in an 8-pin SOIC package and work in conjunction with an external N-Channel MOSFET for which the gate drive is controlled to provide inrush current limiting and output voltage slew-rate control. Overcurrent fault protection is also provided for which the overcurrent threshold is programmable. During an output overload condition, a constantcurrent regulation loop is engaged to ensure that the system power supply maintains regulation. If a fault condition exceeds a built-in 400µs nuisance-trip delay, the MIC2588 and the MIC2594 will latch the circuit breaker’s output off and will remain in the off state until reset by cycling either the UV/OFF pin or the power to the IC. A master Power-Good signal is provided to indicate that the output voltage of the soft-start circuit is within its valid output range. This signal can be used to enable one or more DC-DC converter modules. All support documentation can be found on Micrel’s web site at www.micrel.com. • MIC2588: Pin-for-pin functional equivalent to the LT1640/LT1640A/LT4250 • Provides safe insertion and removal from live –48V (nominal) backplanes • Operates from –19V to –80V • Electronic circuit breaker function • Built-in 400µs “nuisance-trip” delay (tFLT) • Regulated maximum output current into faults • Programmable inrush current limiting • Fast response to short circuit conditions (< 1µs) • Programmable undervoltage and overvoltage lockouts (MIC2588-xBM) • Programmable UVLO hysteresis (MIC2594-xBM) • Fault reporting: Active-HIGH (-1BM) and Active-LOW (-2BM) Power-Good signal output Applications • Central office switching • –48V power distribution • Distributed power systems Typical Application –48V RETURN (Long Pin) –48V RETURN (Short Pin) MIC2588-2BM R1 698kΩ 1% VDD IN+ 3 R2 11.8kΩ 1% UV /PWRGD OV DRAIN 1 SENSE 4 OUT+ +5VOUT /ON/OFF IN– 2 VEE R3 12.4kΩ 1% DC-DC Converter 8 7 OUT– 5V RETURN GATE 5 6 CFDBK RFDBK CGATE R4 M1 –48V INPUT (Long Pin) 0.1µF 100µF RSENSE Input Overvoltage = 71.2V Input Undervoltage = 36.5V (See "Functional Description" for more detail) Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com December 2003 1 M9999-122303 MIC2588/MIC2594 Micrel Ordering Information Part Number PWRGD Polarity Lockout Functions Circuit Breaker Function Package MIC2588-1BM Active-High Undervoltage and Overvoltage Latched Off 8-pin SOIC MIC2588-2BM Active-Low Undervoltage and Overvoltage Latched Off 8-pin SOIC MIC2594-1BM Active-High Programmable UVLO Hysteresis Latched Off 8-pin SOIC MIC2594-2BM Active-Low Programmable UVLO Hysteresis Latched Off 8-pin SOIC Pin Configuration PWRGD 1 8 VDD /PWRGD 1 OV 2 7 DRAIN OV 2 7 DRAIN UV 3 6 GATE UV 3 6 GATE VEE 4 5 SENSE VEE 4 8-Pin SOIC (M) MIC2588-1BM PWRGD 1 5 SENSE 8-Pin SOIC (M) MIC2588-2BM 8 VDD /PWRGD 1 8 VDD ON 2 7 DRAIN ON 2 7 DRAIN OFF 3 6 GATE OFF 3 6 GATE VEE 4 5 SENSE VEE 4 5 SENSE 8-Pin SOIC (M) MIC2594-1BM M9999-122303 8 VDD 8-Pin SOIC (M) MIC2594-2BM 2 December 2003 MIC2588/MIC2594 Micrel Pin Description ○ ○ ○ Pin Number Pin Name Pin Function 1 PWRGD /PWRGD Power-Good Output: Open-drain. Asserted when the voltage on the DRAIN pin (VDRAIN) is within VPGTH of VEE, indicating that the output voltage is within proper specifications. ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ 1 ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ MIC2588-1 and MIC2594-1: PWRGD will be high-impedance when VDRAIN is less than VPGTH, and will pull-down to VDRAIN when VDRAIN is greater than VPGTH. Asserted State: Open-Drain. ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ MIC2588-2 and MIC2594-2: /PWRGD will pull-down to VDRAIN when VDRAIN is less than VPGTH, and will be high impedance when VDRAIN is greater than VPGTH. Asserted State: Active-Low. OV Threshold 2 ○ ○ MIC25XX-2 /PWRGD Active-Low 1 ○ ○ MIC25XX-1 PWRGD Active-High MIC2588: Overvoltage Threshold Input. When the voltage at the OV pin is greater than the VOVH threshold, the GATE pin is immediately pulled low by an internal 100µA current pull-down. ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ 2 ON Turn-On Threshold MIC2594: Turn-On Threshold. At initial system power-up or after the device has been shut off by the OFF pin, the voltage on the ON pin must exceed the VONH threshold in order for the MIC2594 to be enabled. 3 UV Threshold MIC2588: Undervoltage Threshold Input. When the voltage at the UV pin is less than the VUVL threshold, the GATE pin is immediately pulled low by an internal 100µA current pull-down. The UV pin is also used to cycle the device off and on to reset the circuit breaker. Taken together, the OV and UV pins form a window comparator which defines the limits of VEE within which the load may safely be powered. ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ 3 OFF Turn-Off Threshold 4 VEE 5 SENSE Circuit Breaker Sense Input: The current-limit threshold is set by connecting a resistor between this pin and VEE. When the current-limit threshold of IR = 50mV is exceeded for an internal delay tFLT (400µs), the circuit breaker is tripped and the GATE pin is immediately pulled low. Toggling UV or OV will reset the circuit breaker. To disable the circuit breaker, externally connect SENSE and VEE together. 6 GATE Gate Drive Output: Connect to the gate of an external N-Channel MOSFET. 7 DRAIN Drain Sense Input: Connect to the drain of an external N-Channel MOSFET. 8 VDD December 2003 MIC2594: Turn-Off Threshold. When the voltage at the OFF pin is less than the VOFFL threshold, the GATE pin is immediately pulled low by an internal 100µA current pull-down. The OFF pin is also used to cycle the device off and on to reset the circuit breaker. Taken together, the ON and OFF pins provide programmable hysteresis for the turn-on command voltage. Negative Supply Voltage Input. Positive Supply Input. 3 M9999-122303 MIC2588/MIC2594 Micrel Absolute Maximum Ratings(1) Operating Ratings(2) (All voltages are referred to VEE) Supply Voltage (VDD – VEE) ......................... –0.3V to 100V DRAIN, PWRGD pins ................................... –0.3V to 100V GATE pin ..................................................... –0.3V to 12.5V SENSE, OV, UV, ON, OFF pins ....................... –0.3V to 6V ESD Ratings(3) Human Body Model ................................................... 2kV Soldering Vapor Phase .......................... (60 sec.) +220°C +5 ±0°C Infrared ................................... (15 sec.) +235°C +5 ±0°C Supply Voltage (VDD – VEE) .......................... +19V to +80V Ambient Temperature Range (TA) ............... –40°C to 85°C Junction Temperature (TJ) ........................................ 125°C Package Thermal Resistance SOIC (θJA) ......................................................... 152°C/W DC Electrical Characteristics(4) VDD = 48V, VEE = 0V, TA = 25°C, unless otherwise noted. Bold indicates specifications apply over the full operating temperature range of –40°C to +85°C. Symbol Parameter Condition Min VDD – VEE Supply Voltage IDD Supply Current VTRIP Circuit Breaker Trip Voltage VTRIP = VSENSE – VEE IGATEON GATE Pin Pull-up Current IGATEOFF Typ 19 Max Units 80 3 5 mA 40 50 60 mV VGATE = VEE to 8V 19V ≤ (VDD – VEE) ≤ 80V 30 45 60 µA GATE Pin Sink Current (VSENSE – VEE) = 100mV VGATE = 2V 100 230 VGATE GATE Drive Voltage, (VGATE – VEE) 15V ≤ (VDD – VEE) ≤ 80V 9 10 ISENSE SENSE Pin Current VSENSE = 50mV VUVH UV Pin High Threshold Voltage Low-to-High Transition 1.213 1.243 1.272 V VUVL UV Pin Low Threshold Voltage High-to-Low Transition 1.198 1.223 1.247 V VUVHYS UV Pin Hysteresis VOVH OV Pin High Threshold Voltage Low-to-High Transition 1.198 1.223 1.247 V VOVL OV Pin Low Threshold Voltage High-to-Low Transition 1.165 1.203 1.232 V VOVHYS OV Pin Hysteresis VONH ANSI ON Pin High Threshold Voltage Low-to-High Transition 1.198 1.223 1.247 V VOFFH ANSI OFF Pin Low Threshold Voltage High-to-Low Transition 1.198 1.223 1.247 V ICNTRL Input Bias Current (OV, UV, ON, OFF Pins) VUV = 1.25V 0.5 µA VPGTH Power-Good Threshold High-to-Low Transition (VDRAIN – VEE) 1.40 V VOLPG PWRGD Output Voltage VOLPG – VDRAIN (relative to voltage at the DRAIN pin) 0mA ≤ IPG(LOW) ≤ 1mA ILKG(PG) mA 11 µA 0.2 20 mV 20 1.1 1.26 V mV MIC25XX-1 (VDRAIN – VEE) < VPGTH –0.25 0.8 V MIC25XX-2 (VDRAIN – VEE) > VPGTH –0.25 0.8 V PWRGD Output Leakage Current VPWRGD = VDD = 80V 1 µA Notes: 1. Exceeding the “Absolute Maximum Ratings” may damage the devices. 2. The devices are not guaranteed to function outside the specified operating conditions. 3. Devices are ESD sensitive. Handling precautions recommended. Human body model: 1.5kΩ in series with 100pF. Machine model: 200pF, no series resistance. 4. Specification for packaged product only. M9999-122303 4 December 2003 MIC2588/MIC2594 Micrel AC Electrical Characteristics(5) Symbol Parameter Condition Min Typ Max Units tFLT Built-in Overcurrent Nuisance Trip Time Delay (Figure 1) Note 6 tOCSENSE Overcurrent Sense to GATE Low (Figure 2) VSENSE – VEE = 100mV tOVPHL OV to GATE Low (Figure 3) Note 6 1 µs tOVPLH OV to GATE High (Figure 3) Note 6 1 µs tUVPHL UV to GATE Low (Figure 4) Note 6 1 µs tUVPLH UV to GATE High (Figure 4) Note 6 µs 400 3.5 µs 1 µs on PWRGD = 50pF(6) 1 µs tPGL(1) DRAIN High to PWRGD Output Low (-1 Version parts only) RPULLUP = 100kΩ, CLOAD tPGL(2) DRAIN Low to /PWRGD Output Low (-2 Version parts only) RPULLUP = 100kΩ, CLOAD on /PWRGD = 50pF(6) 1 µs tPGH(1) DRAIN Low to PWRGD Output High (-1 Version parts only) RPULLUP = 100kΩ, CLOAD on PWRGD = 50pF(6) 2 µs tPGH(2) DRAIN High to /PWRGD Output High (-2 Version parts only) RPULLUP = 100kΩ, CLOAD on /PWRGD = 50pF(6) 2 µs Notes: 5. Specification for packaged product only. 6. Not 100% production tested. Parameters are guaranteed by design. Test Circuit [Section under construction] December 2003 5 M9999-122303 MIC2588/MIC2594 Micrel Typical Characteristics [Section under construction] MICx xxx vs. xxx 8 8 8 7 6 7 6 7 6 5 4 1 0 XXX (X) 10 9 3 2 5 4 3 2 0 2 4 6 XXX (X) 8 1 0 10 0 2 8 1 0 10 10 9 8 7 8 7 8 7 6 5 6 5 6 5 4 3 XXX (X) 10 9 0 4 3 2 1 0 2 4 6 XXX (X) 8 0 10 0 2 4 6 XXX (X) 8 0 10 8 7 8 7 6 5 6 5 6 5 XXX (X) 8 7 XXX (X) 10 9 2 1 4 3 2 1 4 6 XXX (X) 8 0 10 0 2 4 6 XXX (X) 8 0 10 8 7 6 5 6 5 6 5 XXX (X) 8 7 XXX (X) 8 7 4 3 2 1 M9999-122303 2 4 6 XXX (X) 8 10 0 4 6 XXX (X) 8 10 2 4 6 XXX (X) 8 10 8 10 MICx xxx vs. xxx 10 9 0 0 MICx xxx vs. xxx 10 9 0 2 2 1 MICx xxx vs. xxx 2 1 10 4 3 10 9 4 3 8 MICx xxx vs. xxx 10 9 2 0 MICx xxx vs. xxx 4 3 4 6 XXX (X) 4 3 10 9 0 2 2 1 MICx xxx vs. xxx 0 0 MICx xxx vs. xxx 10 9 XXX (X) XXX (X) 4 6 XXX (X) MICx xxx vs. xxx 2 1 XXX (X) 5 4 3 2 MICx xxx vs. xxx XXX (X) MICx xxx vs. xxx 10 9 XXX (X) XXX (X) MICx xxx vs. xxx 10 9 4 3 2 1 0 2 4 6 XXX (X) 6 8 10 0 0 2 4 6 XXX (X) December 2003 MIC2588/MIC2594 Micrel Timing Diagrams OVERCURRENT EVENT t ≥ tFLT t < tFLT ILIMIT ILOAD 0A Load current is regulated at ILIMIT = 50mV/RSENSE Output OFF (at VDD) VDRAIN (at VEE) (at VEE) VGATE (VEE +10V) Reduction in VDRAIN to support ILIMIT = 50mV/RSENSE (at VEE) Figure 1. Overcurrent Response 100mV VSENSE - VEE tOCSENSE VGATE 1V Figure 2. SENSE to GATE LOW Timing Response 1.223V 1.203V VOV tOVPLH tOVPHL VGATE 1V 1V Figure 3. Overvoltage Response December 2003 7 M9999-122303 MIC2588/MIC2594 Micrel VUV 1.223V 1.243V tUVPHL tUVPLH VGATE 1V 1V Figure 4. Undervoltage Response MIC2588/94-1 VDRAIN VPGTH VPGTH VEE tPGL1 tPGH1 PWRGD not asserted PWRGD asserted - High Impedance VPWRGD — VDRAIN = 0V PWRGD not asserted VPWRGD — VDRAIN = 0V PWRGD VEE MIC2588/94-2 VDRAIN VPGTH VPGTH VEE tPGL2 tPGH2 /PWRGD VEE Figure 5. DRAIN to Power-Good Response M9999-122303 8 December 2003 MIC2588/MIC2594 Micrel Functional Diagram VDD1 Internal VDD and Reference Generator VDD VDD1 45µA VREF1 GATE SENSE + – VEE 100µA Current Limit State 50mV VEE VDD1 VEE PWRGD Nuisance Trip Filter (400µs) VEE /PWRGD Logic + Circuit Breaker – UV + EN VTH(UV/OV) VEE – OV + – Internal PG + 6V Clamp DRAIN VPGTH For Power Good circuitry only denotes -2 option MIC2588 Block Diagram December 2003 9 M9999-122303 MIC2588/MIC2594 Micrel CGATE and RFDBK prevent turn-on and hot swap current surges which would otherwise be caused by (CFDBK + CD-G(M1)) coupling turn-on transients from the drain to the gate of M1. An appropriate value for CGATE may be determined using the formula for a capacitive voltage divider: Maximum voltage on CGATE at turn-on must be less than VTHRESHOLD of M1: 1. For a standard 10V enhancement N-Channel MOSFET, VTHRESHOLD is about 4.25V. 2. Choose 3.5V as a safe maximum voltage to safely avoid turn-on transients. VG-S(M1) × [CGATE + (CFDBK + CD-G(M1))] = [(VDD – VEE(min)) × (CFDBK + CD-G(M1))] Functional Description Hot Swap Insertion When circuit boards are inserted into systems carrying live supply voltages (“hot swapped”), high inrush currents often result due to the charging of bulk capacitance that resides across the circuit board’s supply pins. These current spikes can cause the system’s supply voltages to temporarily go out of regulation, causing data loss or system lock-up. In more extreme cases, the transients occurring during a hot swap event may cause permanent damage to connectors or onboard components. The MIC2588 and the MIC2594 are designed to address these issues by limiting the magnitude of the transient current during hot swap events. This is achieved by controlling the rate at which power is applied to the circuit board (di/dt and dv/dt management). In addition, to inrush current control, the MIC2588 and the MIC2594 incorporate input voltage supervisory functions and current limiting, thereby providing robust protection for both the system and the circuit board. Start-Up Cycle When the input voltage to the IC is between the overvoltage and undervoltage thresholds (MIC2588) or is greater than VON (MIC2594), a start cycle is initiated. At this time, the GATE pin of the IC applies a constant charging current (IGATEON) to the gate of the external MOSFET (M1). CFDBK creates a Miller integrator out of the MOSFET circuit, which limits the slew-rate of the voltage at the drain of M1. The drain voltage rate-of-change (dv/dt) of M1 is: ( dv M1DRAIN dt VG-S(M1) × CGATE = [(VDD – VEE(min)) – VG-S(M1)] × (CFDBK + CD-G(M1)) ( C FDBK where IGATE(+) = Gate Charging Current = IGATEON; I GATE(–) ≅ –I GATE(+) , due to the extremely high transconductance values of power MOSFETs; and IGATE(–) = CFDBK × dv(M1DRAIN ) ( CLOAD × dv M1DRAIN (2) CFDBK = ) 150µF × 60µA = 5.3nF 1.7A where the nearest standard 5% value is 5.6nF. Substituting 5.6nF into Equation 2 from above yields: CFDBK = dt I ICHARGE = CLOAD × – GATEON C FDBK | ICHARGE | = VG-S(M1) 150µF × 45µA = 3.97nF 1.7A Good engineering practice suggests the use of the worstcase parameter values for IGATEON from the “DC Electrical Characteristics” section: dt Relating the above to the maximum transient current into the load capacitance to be charged upon hot swap or power-up involves a simple extension of the same formula: ICHARGE = (VDD – VEE (min)) – VG-S(M1) While the value for RFDBK is not critical, it should be chosen to allow a maximum of several milliamperes to flow in the gate-drain circuit of M1 during turn-on. While the final value for RFDBK is determined empirically, initial values between RFDBK = 15kΩ to 27kΩ for systems with a maximum value of 75V for (VDD – VEE(min)) are appropriate. Resistor R4, in series with the MOSFETs gate, minimizes the potential for parasitic high frequency oscillations from occurring in M1. While the exact value of R4 is not critical, commonly used values for R4 range from 10Ω to 33Ω. For example, let us assume a hot swap controller is required to maintain the inrush current into a 150µF load capacitance at 1.7A maximum, and that this circuit may operate from supply voltages as high as (VDD – VEE) = 75V. The MOSFET to be used with the MIC2588/94 is an IRF540NS 100V D2PAK device which has a typical (CD-G) of 250pF. Calculating a value for CFBDK using Equation 1 yields: ) = IGATE(–) = – IGATEON C FDBK ) CGATE = CFDBK + CD − G(Q1) × CGATE = (5.6nF + 250pF) × CLOAD × IGATEON (75V – 3.5V) = 0.12µF 3.5V Finally, choosing R4 = 10Ω and RFDBK = 20kΩ will yield a suitable, initial design for prototyping. CFDBK Transposing: CFDBK = M9999-122303 CLOAD × IGATEON | ICHARGE | (1) 10 December 2003 MIC2588/MIC2594 Micrel Power-Good (PWRGD or /PWRGD) Output For the MIC2588-1 and the MIC2594-1, the Power-Good output signal (PWRGD) will be high impedance when VDRAIN drops below VPGTH, and will pull down to VDRAIN when VDRAIN is above VPGTH. For the MIC2588-2 and the MIC2594-2, /PWRGD will pull down to the potential of the VDRAIN pin when VDRAIN drops below VPGTH, and will be high impedance when VDRAIN is above VPGTH. Hence, the -1 parts have an active-high PWRGD signal and the -2 parts have an active-low /PWRGD output. Either PWRGD or /PWRGD may be used as an enable signal for one or more subsequent DC/DC converter modules or for other system uses as desired. When used as an enable signal, the time necessary for the PWRGD (or /PWRGD) signal to pull-up (when in high impedance state) will depend upon the load (RC) that is present on this output. Circuit Breaker Function The MIC2588 and the MIC2594 employ an electronic circuit breaker that protects the MOSFET and other system components against faults such as short circuits. The current limit threshold is set via an external resistor, RSENSE, connected between the VEE and SENSE pins. An internal 400µs timer limits the length of time (tFLT) for which the circuit can draw current in excess of its programmed threshold before the circuit breaker is tripped. This short delay prevents nuisance tripping of the circuit breaker due to system transients while providing rapid protection against large-scale transient faults. Whenever the voltage across RSENSE exceeds 50mV, two things happen: 1. A constant-current regulation loop is engaged designed to hold the voltage across RSENSE equal to 50mV. This protects both the load and the MIC2588 circuit from excessively high currents. This loop will engage in less than 1µs from the time at which the overvoltage condition on RSENSE occurs. 2. The internal 400µs timer is started. If the 400µs timeout period is exceeded, the circuit breaker trips and the GATE pin is immediately pulled low by an internal current pull-down. This operation turns off the MOSFET quickly and disconnects the input from the load. Current Sensing As mentioned before, the MIC2588 and the MIC2594 employ an external low-value resistor in series with the source of the external MOSFET to measure the current flowing into the load. The VEE connection to the IC from the negative supply is also one input to the part’s internal current sensing circuits and the SENSE input is the other input. To accommodate worst-case tolerances in the sense resistor (for a ±1% initial tolerance, allow ±3% tolerance for variations over time and temperature) and circuit breaker threshold voltages, a slightly more detailed calculation must be used to determine the minimum and maximum hot swap load currents. As the MIC2588/94’s minimum current limit threshold voltage is 40mV, the minimum hot swap load current is determined where the sense resistor is 3% high: IHOT_SWAP (min) = IHOT_SWAP (max) = ) ( 60mV 61.9mV = 0.97 × RSENSE (nom) RSENSE (nom) ) In this case, the application circuit must be sturdy enough to operate over a ~1.6-to-1 range in hot swap load currents. For example, if an MIC2594 circuit must pass a minimum hot swap load current of 4A without nuisance trips, RSENSE 38.8mV = 9.7mΩ , and the nearest 1% 4A standard value is 9.76mΩ. At the other tolerance extremes, IHOT_SWAP(max) for the circuit in question is then simply should be set to 61.9mV = 6.3A 9.76mΩ With a knowledge of the application circuit’s maximum hot swap load current, the power dissipation rating of the sense resistor can be determined using P = I2 × R. Here, the I is IHOT_SWAP(max) = 6.3A and the R is RSENSE(min) = (0.97)(RSENSE(nom)) = 9.47mΩ. Thus, the sense resistor’s maximum power dissipation is: PMAX = (6.3A)2 × (9.47mΩ) = 0.376W A 0.5Ω sense resistor is a good choice in this application. Undervoltage/Overvoltage Detection—MIC2588 The MIC2588 has “UV” and “OV” input pins. These pins can be used to detect input supply rail undervoltage and overvoltage conditions. Undervoltage lockout prevents energizing the load until the supply input is stable and within tolerance. In a similar fashion, overvoltage turn-off prevents damage to sensitive circuit components should the input voltage exceed normal operational limits. Each of these pins is internally connected to an analog comparator with 20mV of hysteresis. When the UV pin falls below its VUVL threshold or the OV pin is above its VOVH threshold, the GATE pin is immediately pulled low. The GATE pin will be held low until UV exceeds its VUVH threshold or OV drops below its VOVL threshold. The UV and OV circuit’s threshold trip points are programmed using the resistor divider IHOT_SWAP (max) = The sense resistor is nominally valued at: VTRIP (typ) IHOT_SWAP (nom) where VTRIP(typ) is the nominal circuit breaker threshold voltage (= 50mV) and IHOT_SWAP(nom) is the nominal hot swap load current level to trip the internal circuit breaker in the application. December 2003 ( Keep in mind that the minimum hot swap load current should be greater than the application circuit’s upper steady-state load current boundary. Once the lower value of RSENSE has been calculated, it is good practice to check the maximum hot swap load current (IHOT_SWAP(max)) which the circuit may let pass in the case of tolerance build-up in the opposite direction. Here, the worst-case maximum is found using a VTRIP(max) of 60mV and a sense resistor, 3% low in value: Sense Resistor Selection RSENSE (nom) = 40mV 38.8mV = R 1.03 × RSENSE (nom) SENSE (nom) 11 M9999-122303 MIC2588/MIC2594 Micrel R1, R2, and R3 as shown in the “Typical Application.” The equations to set the trip points are shown below. For the following example, the circuit’s UV threshold is set to VUV = 37V and the OV threshold is placed at VOV = 72V, values commonly used in Central Office power distribution applications. VUV = VUVL (typ) × VOV = VOVH (typ) × analog comparator with 20mV of hysteresis. The MIC2594 holds the output off until the voltage at the ON pin exceeds its VONH threshold value given in the “Electrical Characteristics” table. Once the output has been enabled by the ON pin, it will remain on until the voltage at the OFF pin falls below its VOFFL threshold value, or the part turns off due to a fault. Should either event occur, the GATE pin is immediately pulled low and will remain low until the ON pin once again exceeds its VONH threshold. The circuit’s turn-on and turn-off points are set using the resistor divider R1, R2, and R3 as shown in the “Typical Application.” The equations to establish the trip points are shown below. In the following example, the circuit’s ON threshold is set to VON = 40V and the circuit’s OFF threshold is VOFF = 35V. (R1+ R2 + R3) (R2 + R3) (R1+ R2 + R3) R3 Given VUV, VOV, and any one resistor value, the remaining two resistor values can be found. A suggested value for R3 is that which will provide approximately 100µA of current through the voltage divider chain at VDD = VUV. This yields the following as a starting point: R3 = VOVH (typ) 100µA VON = VONH (typ) × = 12.23kΩ VOFF = VOFFL (typ) × The closest standard 1% value for R3 = 12.4kΩ. Solving for R2 and R1 yields: R3 (R1+ R2 + R3) (R2 + R3) Given VOFF, VON, and any one resistor value, the remaining two resistor values can be readily found. A suggested value for R3 is that which will provide approximately 100µA of current through the voltage divider chain at VDD = VOFF. This yields the following as a starting point: V R2 = R3 × OV – 1 VUV 72V R2 = 12.4kΩ × – 1 37V R3 = VOFFL (typ) 100µA = 12.23kΩ The closest standard 1% value for R3 = 12.4kΩ. Then, solving for R2 and R1 yields: R2 = 11.729kΩ The closest standard 1% value for R2 = 11.8kΩ. Next, the value for R1 is calculated: V R2 = R3 × ON – 1 VOFF V – 1.223V R1 = R3 × OV – R2 1.223V 40V R2 = 12.4kΩ × – 1 35V 72V – 1.223V R1 = 12.4kΩ × – R2 1.223V R2 = 1.771kΩ The closest standard 1% value for R2 = 1.78kΩ. R1 = 705.808kΩ The closest standard 1% value for R1 = 698kΩ. Using standard 1% resistor values, the circuit’s nominal UV and OV thresholds are: VUV = 36.5V VOV = 71.2V Programmable UVLO Hysteresis—MIC2594 The MIC2594 has user-programmable hysteresis by means of the ON and OFF pins. This allows setting the part to turn on at a voltage V1, and not turn off until a second voltage V2, where V2 < V1. This can significantly simplify dealing with source impedances in the supply bus while at the same time increasing the amount of available operating time from a loosely regulated power supply (for example, a battery supply). Similarly to the MIC2588, each of these pins is internally connected to an M9999-122303 (R1+ R2 + R3) R1= R3 × (VON – 1.223V) – R2 R1= 12.4kΩ × 1.223V (40V – 1.223V) – R2 1.223V R1= 391.380kΩ The closest standard 1% value for R1 = 392kΩ. Using standard 1% resistor values, the circuit’s nominal ON and OFF thresholds are: VON = 40.1V VOFF = 35V 12 December 2003 MIC2588/MIC2594 Micrel this will damage the transistor. However, the actual avalanche voltage is unknown; all that can be guaranteed is that it will be greater than the VBD(DS) of the MOSFET. The drain of the transistor is connected to the DRAIN pin of the MIC2588/94, and the resulting transient does have enough voltage and energy and can damage this, or any, high-voltage hot swap controller. 2. If the load’s bypass capacitance (for example, the input filter capacitors for a set of DC-DC converter modules) are on a board from which the board with the MIC2589/MIC2595 and the MOSFET can be unplugged, the same type of inductive transient damage can occur to the MIC2588/MIC2594. Protecting the controller and the power MOSFET from damage against these large-scale transients can take the forms shown in Figure 7. It is not mandatory that these techniques are used—the application environment will dictate suitability. As protection against sudden on-card load dumps at the DRAIN pin of the controller, a 2.2µF or larger capacitor directly from DRAIN to VEE of the controller can be used to serve as a charge reservoir. Alternatively, a 68V, 1W, 5% Zener diode clamp can be installed in a similar fashion. Note that the clamp diode’s cathode is connected to the DRAIN pin as shown in Figure 7. To protect the hot swap controller from large-scale transients at the card input, a 100V clamp diode (an SMAT70A or equivalent) can be used. In either case, the lead lengths should be short and the layout compact to prevent unwanted transients in the protection circuit. [Circuit drawing under construction] Applications Information 4-Wire Kelvin Sensing Because of the low value typically required for the sense resistor, special care must be used to measure accurately the voltage drop across it. Specifically, the measurement technique across each RSENSE must employ 4-wire Kelvin sensing. This is simply a means of making sure that any voltage drops in the power traces connecting to the resistors are not picked up by the signal conductors measuring the voltages across the sense resistors. Figure 6 illustrates how to implement 4-wire Kelvin sensing. As the figure shows, all the high current in the circuit (from VEE through RSENSE, and then to the source of the output MOSFET) flows directly through the power PCB traces and RSENSE. The voltage drop resulting across RSENSE is sampled in such a way that the high currents through the power traces will not introduce any parasitic voltage drops in the sense leads. It is recommended to connect the hot swap controller’s sense leads directly to the sense resistor’s metalized contact pads. RSENSE metalized contact pads Power Trace From VEE RSENSE Power Trace To MOSFET Source PCB Track Width: 0.03" per Ampere using 1oz Cu Signal Trace to MIC2588/94 VEE Pin Signal Trace to MIC2588/94 SENSE Pin Note: Each SENSE lead trace shall be balanced for best performance — equal length/equal aspect ratio. Figure 6. 4-Wire Kelvin Sense Connections for RSENSE Figure 7. Using Large-Scale Transient Protection Devices Around the MIC2588/94 Protection Against Voltage Transients In many telecom applications, it is very common for circuit boards to encounter large-scale supply-voltage transients in backplane environments. Because backplanes present a complex impedance environment, these transients can be as high as 2.5 times steady-state levels, or 120V in worst-case situations. In addition, a sudden load dump anywhere on the circuit card can generate a very high voltage spike at the drain of the output MOSFET which, in turn, will appear at the DRAIN pin of the MIC2588/MIC2594. In both cases, it is good engineering practice to include protective measures to avoid damaging sensitive ICs or the hot swap controller from these large-scale transients. Two typical scenarios in which largescale transients occur are described below: 1. An output current load dump with no bypass (charge bucket or bulk) capacitance to VEE. For example, if LLOAD = 5µH, VIN = 56V and tOFF = 0.7µs, the resulting peak short-circuit current prior to the MOSFET turning off would reach: Power buss inductance could easily result in localized highvoltage transients during a turn-off event. The potential for overstressing the part in such a case should be kept in check with a suitable input capacitor and/or transient clamping diode. Power MOSFET Selection [Section under construction] Power MOSFET Operating Voltage Requirements [Section under construction] Power MOSFET Steady-State Thermal Issues [Section under construction] Power MOSFET Transient Thermal Issues [Section under construction] (55V × 0.7µs) = 7.7A 5µH PCB Layout Considerations [Section under construction] If there is no other path for this current to take when the MOSFET turns off, it will avalanche the drainsource junction of the MOSFET. Since the total energy represented is small relative to the sturdiness of modern power MOSFETs, it’s unlikely that December 2003 Power MOSFET and Sense Resistor Vendors [Section under construction] 13 M9999-122303 MIC2588/MIC2594 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) 0.197 (5.0) 0.189 (4.8) 0°–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 SOIC (M) MICREL, INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is at Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2003 Micrel, Incorporated. M9999-122303 14 December 2003