MIC2582/MIC2583 Micrel MIC2582/MIC2583 Single Channel Hot Swap Controller Preliminary Information 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. Using few external components, the parts act in conjunction with an external N-Channel MOSFET device, for which the gate drive is controlled to provide inrush current-limiting and output voltage slew rate control. The MIC2582/3 are rated for operation from supply voltages from 2.3V to 13.2V, but can withstand transient voltages as high as 20V without damage. Overcurrent fault protection is provided along with a programmable over-current threshold. Active current regulation during start-up ensures that inrush current never exceeds the programmed threshold. The MIC2582/3 provide a circuit breaker function that latches the output MOSFET off if the current limit threshold is exceeded for a programmed period of time. Dual-level fault detection allows very fast response to short-circuit faults while preventing lower level overcurrent transients from causing "nuisance tripping" of the circuit breaker. A /FAULT signal is provided indicating overcurrent or undervoltage fault conditions. * Provides safe insertion and removal from live backplanes * 2.3V to 13V supply voltage operation * Surge voltage protection up to 20V * Undervoltage Lockout protection * Programmable inrush current-limiting * Current regulation limits inrush current regardless of load capacitance * Dual-level overcurrent fault sensing eliminates false tripping * Fast response to short circuit conditions (< 1 s) * Power-On Reset and Power-Good status outputs (MIC2583) * Programmable output undervoltage detection * Electronic circuit breaker * /FAULT status output * Auto-restart function (MIC2583R) Applications * * * * * RAID systems Base stations PC Board Hot-Swap insertion and removal Hot-Swap CompactPCI Cards Network Switches Typical Application VIN 3.3V RSENSE VOUT VIN 12V RSENSE VOUT 0.1F 0.1F Logic Level Input VCC SENSE GATE ON R1 CL 100F Logic Level Input FB VCC SENSE R2 MIC2583 /PWRGD Open-Drain Output /POR CPOR GND Logic VCC 0.1F MIC2582 Typical Application Circuit R1 10k /POR R1 10k Power Good Power Reset DIS /FAULT GND Logic VCC FB R3 12.4k Open-Drain Output Logic VCC GATE ON R2 12.4k MIC2582 CL 100F 0.1F 0.1F CPOR CFILTER 0.1F CFILTER MIC2583 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 2001 1 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Ordering Information Part Number Circuit Breaker Threshold Circuit Breaker Package MIC2582-xBM x = J, 50mV Latched off 8 pin SOIC Latched off 16 pin QSOP Auto-retry 16 pin QSOP x = K, 100mV x = L, 150mV x = M, 200mV x = N, Off MIC2582/83-xBQS x = J, 50mV x = K, 100mV x = L, 150mV x = M, 200mV x = N, Off MIC2583R-xBQS x = J, 50mV x = K, 100mV x = L, 150mV x = M, 200mV x = N, Off Pin Configuration /POR 1 16 VCC /PWRGD 2 /POR 1 8 VCC ON 2 ON 3 7 SENSE CPOR 3 14 GATE CPOR 4 6 GATE GND 4 15 SENSE 13 DIS CFILTER 5 5 FB 12 FB NC 6 8-Pin SOIC 11 /FAULT GND 7 10 NC GND 8 9 NC 16-Pin QSOP MIC2582/MIC2583 2 April 2001 MIC2582/MIC2583 Micrel Pin Description Pin Name SOIC QSOP /FAULT NA 11 Circuit Breaker Fault Output. Open-Drain, Active-Low. Asserted when the circuit breaker is tripped. ON 2 3 ON input. With this pin high, the device is enabled, a start-up sequence is initiated, and the GATE pin starts ramping up towards its final operating voltage. The ON input is compared to a VTH reference which has 25mV of hysteresis. Toggling ON is also used to reset the circuit breaker. ON must be low for 20s in order to initiate a start-up sequence. This pin should not be taken higher than VCC + 0.3V, or 7.5V, whichever is less. /POR 1 1 Power-On Reset Output. Open-Drain. Asserted during start-up until a time period tPOR after the Power-Good threshold is reached. Also asserted during undervoltage lockout fault conditions. (CPOR determines time period tPOR) CPOR 3 4 Power-On-Reset Timer. A capacitor connected between this pin and ground sets the Power-On-Reset interval, tPOR, and start-up delay tDELAY. When VCC rises above the UVLO, the capacitor connected to CPOR begins to charge. When the voltage at CPOR crosses 0.3V, a start cycle is initiated if ON is asserted. CPOR is reset to zero volts. When the voltage at FB rises above VFBTH, CPOR begins to charge again. When the voltage at CPOR rises above VPORDLY, the timer resets, pulls CPOR to ground, and /POR is de-asserted. If CPOR = 0, then tDELAY is set to 20s. SENSE 7 15 Circuit Breaker Sense Input. A resistor between this pin and VCC sets the current-limit threshold. When the current-limit threshold of 50mV is exceeded for tOCSLOW (internally set at 5s for the MIC2582), the circuit breaker is tripped and the GATE pin is immediately pulled low. If the voltage across the sense resistor exceeds VTHSLOW due to fast high amplitude faults such as short-circuits, then the GATE pin is immediately brought low (no delay). VTHSLOW is available for values of 50mV, 100mV, 200mV, or Off. Whenever the voltage across the sense resistor exceeds the current-limit threshold, the GATE voltage is adjusted to ensure a constant load current. To disable the circuit breaker, the SENSE and VCC pins can be tied together. GATE 6 14 Gate Drive Output. Connects to the gate of an N-Channel MOSFET. An internal clamp ensures that no more than 8V is applied between the GATE and Source. When the circuit breaker trips or when undervoltage lockout occurs the GATE pin is immediately brought low. FB 5 12 Power-Good Threshold Input. This input is internally compared to a 1.24V reference which has 30mV of hysteresis. Whenever this input goes below 1.24V, /POR is asserted. A 5s filter on this pin prevents negative-going glitches from causing nuisance activation of this pin. If this input momentarily goes below 1.24V, then /POR is activated for one timing cycle, tPOR. VCC 8 16 Positive Supply Input. 2.3V - 13.2V. The GATE pin is held low by an internal undervoltage lockout circuit until VCC exceeds a threshold of 2.1V. If VCC exceeds 13.2V, an internal shunt regulator protects the chip from transient voltages up to 20V at the VCC and SENSE pins. CFILTER NA 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 to this pin, then tOCSLOW is set to 5s. During auto-retry, the MIC2583R exhibits a preset duty cycle of 10%. The value of CFILTER will set the interval between auto-retry attempts. See Applications section. April 2001 Pin Function 3 MIC2582/MIC2583 MIC2582/MIC2583 Micrel Pin Name SOIC QSOP /PWRGD NA 2 Power-Good Output. Open Drain. When the voltage at the FB pin is lower than 1.24V, /PWRGD output is held low. When the voltage at the FB pin is higher than 1.24V, then /PWRGD is de-asserted. A pull-up resistor connected to this pin and to VCC will pull the output up to VCC. DIS NA 13 Discharge output. When the MIC2582/MIC2583 is turned off, a 500 resistor connected to this output allows the discharging of external load capacitance to ground. GND 4 7,8 Ground connection NC NA 6,9,10 MIC2582/MIC2583 Pin Function No Connection 4 April 2001 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 OR 8V, whichever is lower GATE pin ........................................................ -0.3V to 20V FB input pins .................................................... -0.3V to 6V Junction Temperature ............................................... 125C ESD Rating, Note 3 Supply Voltage (VCC) .................................... 2.3V to 13.2V Thermal Resistance R(J-A) (8-pin SOIC) .............. 163C/W Thermal Resistance R(J-A) (16-pin SOIC) .......... 112C/W Operating Temperature Range .................. -40C to +85C Electrical Characteristics VCC = 5.0V, TA = 25C unless otherwise noted. Bold values indicate -40C TA +85C. Symbol Parameter Condition Min VCC Supply voltage Temp = -20C to +85C Temp = -40C to +85C ICC Supply current VON = 2V VTRIP Circuit breaker trip voltage VTRIP = VCC - VSENSE VGS IGATEON IGATEOFF ITIMER IPOR VTH VUV External Gate Drive GATE pin pull-up current GATE pin sink current Current-limit timer current Power-On-Reset timer current Typ Max Units 2.3 13.2 V 2.7 13.2 V 2.5 mA 1.6 VTRIPSLOW 37 43 50 mV VTRIPFAST 85 91 105 mV VCC > 3V 5.5 6.3 8 V VCC = 2.3V 2.7 3.8 5 V Start cycle, VGATE = 0V VCC =13.2V 10 16 24 A VCC = 2.3V 9 14 19 A |VCC - VGATE| /FAULT = 0, VGATE>1V VCC = 13.2V 100 mA VCC = 2.3V 50 mA Turn off 110 A 4.5 6.5 8.5 A -8.5 -6.5 -4.5 A Timer On 12 14 16 A Timer Off 0.5 1.45 VCC - VSENSE > VTRIPSLOW VCC - VSENSE < VTRIPSLOW mA POR Delay and CFILTER timer VCPOR rising threshold VCFILTER rising 1.19 1.24 1.29 V Undervoltage Lockout threshold VCC rising 2.1 2.2 2.3 V VCC falling 1.9 2.05 2.2 V VUVHYS Undervoltage Lockout hysteresis VONTH ON pin threshold voltage 175 mV ON rising 1.19 1.23 1.29 V ON falling 1.16 1.21 1.26 V -0.5 A ION ON pin input current VONHYS ON pin hysteresis VDELAYTH Under-voltage start-up timer threshold VCPOR rising VAUTO Auto-restart threshold voltage Upper Threshold 1.24 V Lower Threshold 0.33 V April 2001 VON = VCC 25 .28 5 .33 mV .38 V MIC2582/MIC2583 MIC2582/MIC2583 Micrel Symbol Parameter Condition IAUTO Auto-restart current charge current 12 A discharge current 1.2 A VFBTH Power-Good threshold voltage VFBHYS FB hysteresis VOL /FAULT , /POR , /PWRGD output voltage RDIS Output discharge resistance Min Typ Max Unit FB rising 1.19 1.24 1.29 V FB falling 1.16 1.21 1.26 V 30 IOUT = 1mA 500 mV 0.4 V 1000 AC Parameters tOCFAST Fast overcurrent SENSE to GATE low trip time VCC = 5V VCC - VSENSE = 100mV CGATE = 10nF Figure 1 1 s tOCSLOW Slow overcurrent SENSE to GATE low trip time VCC = 5V CTIM = 0, VIN - VSENSE = 50mV Figure 1 5 s tONDLY ON delay filter 20 s tFBDLY FB delay filter 20 s 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. Devices are ESD sensitive. Handling precautions recommended. Timing Diagrams VTRIPFAST 50mV (VCC - VSENSE) tOCFAST tOCSLOW GATE 1V 1V Figure 1. Current -Limit Rsponse 1.2V FB tPOR 1.5V /POR 1.5V /PWRGD Figure 2. Power-on Reset Response VUVLO VCC tDELAY 1V VGATE Figure 3. Power-on Start-up Delay Timing MIC2582/MIC2583 6 April 2001 MIC2582/MIC2583 Micrel Functional Description dv /dt (load) = Functional Overview When ON is asserted high, the device is enabled and a startup sequence is initiated. The GATE begins to ramp up at a rate determined by CGATE. When the FB pin exceeds the internal 1.24V threshold reference, /PWRGD is de-asserted, signaling an available output. Internal current regulation ensures that the inrush current will not exceed the programmed threshold. A circuit breaker function will latch the output MOSFET off if the current limit threshold is exceeded for a period, tOCSLOW, determined by CFILTER (tOCSLOW = 5s with no CFILTER). ON Pin (Maximum Voltage) The ON pin for the MIC2582/3 has an absolute maximum voltage rating of (VCC + 0.3V) or 8V, whichever is less. At operating voltages of 2.3 to 7.5V, the ON pin may be connected directly to the VCC pin through a pull-up resistor (100k suggested). When operating at voltages between 7.5 and 13.2V with ON tied to VCC, it is necessary to include a voltage divider from the ON pin to ground to keep the ON pin from exceeding the absolute maximum rating of 8V. A 100k resistor is also recommended for the divider. Start-Up Cycle There are two basic modes of operation that may occur during start-up with the MIC2582/3. CG Table 1 depicts the output slew rate for various values of CG. IGATE = 16A CG dv/dt (load) 0.001F 16000V/s 0.01F 1600V/s 0.1F 1600V/s 1F 16V/s Table 1. Output Slew-Rate Selection for GATE Capacitance Dominated Start-Up Supply Bypass For local supply bypass, a 0.1F ceramic capacitor is recommended Voltage Divider at FB Pin There are two important points to note here. First, consideration should be taken to determine the desired output value for MIC2582/3 turn on. For example, if the application is operating at 12V, the desired turn on may be 11V. The two FB resistors are selected to divide 11V accordingly, with 1.24V set at the FB pin. Second, the ratio of these two resistors should be chosen to maintain relatively low power consumption while maintaining good accuracy in the face of input bias currents. A current of approximately 100A is recommended. The following example determines the resistor values for the output voltage divider at the FB pin. Load Capacitance Dominated Start-up In this case the load capacitance, CL, is large enough to cause the inrush current to exceed the programmed currentlimit but is less than the fast-trip threshold (or the fast-trip threshold is disabled, `N' option). When this occurs the output current is regulated and held constant until the output voltage rises to its final value. The output rise-time is computed by the following equation: R1 + R2 R1 + R2 VOUT( turn - on) = VFB = 1.24V R2 R2 12VOUT 5% (supply tolerance) gives 11.4V to 12.6V output range. Select 11V output turn-on. I Output Rise Time = dv /dt = LIMIT CL In this case the value of CFILTER must be selected to ensure the overcurrent response time, tOCSLOW exceeds this value to prevent the circuit-breaker from tripping. Choose R2 = VFB 1.24V = = 12.4k,1% 100A 100A Find R1 GATE Capacitance Dominated Start-up R2 VOUT = 1.24V R1 + R2 In this case the value of the load capacitance relative to the GATE capacitance is small enough such that during start-up the output current never exceeds the current-limit threshold. The minimum value of CG that will ensure that the currentlimit is never exceeded is given by the equation below: VOUT R2 = 1.24V(R1 + R2). Simplifying, R1 = I CG (Min.) = GATE x CL ILIMIT (11x 12.4k ) - (1.24 x 12.4k ) 1.24 R1 = 97.6k, also a standard 1% resistor value. Power-On-Reset and Overcurrent Filter Delays The Power-On-Reset delay, tPOR, is the period of time for /POR to de-assert after the Power-Good threshold is reached. The value of capacitor CPOR will determine tPOR. The following equation is used to calculate tPOR: Where CG is the summation of the MOSFET input capacitance (CISS) specification and the value of the capacitor connected to the GATE pin of the MOSFET, CGATE. CL is the load capacitance connected to the output and ILIMIT and IGATE are respectively the current limit and gate charge current specifications found in the Electrical Characteristics Table. Once CG is determined use the equation below to determine the output slew rate dv/dt: April 2001 IGATE tPOR = 7 CPOR x VTH IPOR MIC2582/MIC2583 MIC2582/MIC2583 Micrel where VTH is the typical power-on reset delay threshold and IPOR is the typical power-on reset timer current. Capacitor CFILTER is utilized as part of the MIC2582/83 duallevel overcurrent fault detection. Overcurrent conditions which last longer than time period tOCSLOW will trip the circuit breaker. The following equation is used to calculate tOCSLOW: t OCSLOW = CFILTER x VTH ITIMER where VTH is the CFILTER timer threshold and ITIMER is the typical current-limit timer current. MIC2582/MIC2583 8 April 2001 MIC2582/MIC2583 Micrel MOSFET Selection Selecting the proper external MOSFET for use with the MIC2582/3 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 15V. Therefore, the drain-source breakdown voltage of the MOSFET must be at least 16V. For ample safety margin and standard availability, the closest value will be 20V. The second breakdown voltage criterion that must be met is a bit subtler than simple drain-source breakdown voltage, but is not hard to meet. In MIC2582/3 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 4. At the present Applications Information Sense Resistor Selection The MIC2582, MIC2583 and MIC2583R use a low-value sense resistor to measure the current flowing through the MOSFET switch (and therefore the load). This sense resistor is nominally valued at 43mV/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 steadystate load current, a slightly more detailed calculation must be used. The current limit threshold voltage (the "trip point") for the MIC2582/3 may be as low as 37mV, which would equate to a sense resistor value of 37mV/ILOAD(CONT). Carrying the numbers through for the case where the value of the sense resistor is 3% high, this yields R SENSE(MAX) = 37mV/(1.03)(ILOAD(CONT)) = 35.9mV/ILOAD(CONT). 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 build-up in the opposite direction. Here, the worst-case maximum is found using a 53mV trip voltage and a sense resistor that is 3% low in value. The resulting current is ILOAD(CONT, MAX) = 53mV/(0.97)(RSENSE(NOM)) = 51.5mV/(RSENSE(NOM)). As an example, if an output must carry a continuous 1.4A without nuisance trips occurring, RSENSE for that output should be 35.9mV/1.4A = 25.64m. The nearest standard value is 25.0m, so a 25.0m 1% resistor would be a good choice. At the other set of tolerance extremes, ILOAD(CONT, MAX) for the output in question is then simply 51.5mV/25.0m = 2.06A. 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 = (2.06A2)(24.25m) = 0.103W. In this example, a 0.25W sense resistor would work well. 1N5240B 10V VIN 2.3V to 13.2V VOUT RSENSE CIN RG 47 Logic Level Input VCC SENSE 0.1F R1 GATE FB ON MIC2582 R2 12.4k Open-Drain Output /POR GND CPOR 0.1F Figure 4. April 2001 9 MIC2582/MIC2583 MIC2582/MIC2583 Micrel time, most power MOSFETs with a 20V gate-source voltage rating have a 30V drain-source breakdown rating or higher. As a general tip, look to surface-mount devices with a drainsource rating of 30V as a starting point. Finally, the external gate drive of the MIC2582/3 requires a low-voltage logic level MOSFET when operating at voltages lower than 3V. There are 2.5V logic level MOSFETs available (See MOSFET and Sense Resistor Vendors for suggested manufacturers). MOSFET Steady-State Thermal Issues 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) * 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?) Now it gets easy. 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 7V of enhancement. This is conservative, but it works. 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 10m at a 25C TJ, and the actual junction temperature ends up at 110C, a good first cut at the operating value for RON would be: 10m[1+ (110 - 25)(0.005)(10m)] = 10m[1 + (85)(0.005)(10m)] 14.3m 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 above, the final junction temperature is found by adding TA(MAX) and TJ. Since this is not a closed-form equation, getting a close approximation may take one or two iterations, But it's not a hard calculation to perform, and tends to converge quickly. 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. As a practical issue, surface-mount MOSFETs are often less than MIC2582/MIC2583 ideally specified in this regard - it's become common for manufacturers to simply state that the thermal data for the part is specified with the MOSFET "Surface mounted on FR4 board, t 10seconds," or something equally uninformative. So 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 of copper 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, if available, works wonders. This is not the place for a dissertation on how to perform airflow calculations, but even a few LFM (linear feet per minute) of air will cool a MOSFET down dramatically. If you can position the MOSFET(s) in question near the inlet of a power supply's fan, or the outlet of a processor's cooling fan, that's always a good free ride. 3. Although it seems a rather unsatisfactory statement, 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 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. 4. Finally, you may end up noting the following: modern surface-mount MOSFETs are readily and inexpensively available with such low values of RON that the finer points mentioned above are often almost moot. MOSFET Transient Thermal Issues Having chosen a MOSFET that will withstand the imposed voltage stresses, and the worst-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. Here, nature and physics work in our favor: 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. This is very easily understood by all of us who have stood waiting for a pot of water to boil. In terms related directly to the specification and use of power MOSFETs, this is known as "transient thermal impedance," or Z(J-A). Almost all power MOSFET data sheets give a Transient Thermal Impedance Curve, which is a handy tool for making sure that you can safely get by with a less expensive MOSFET than you thought you might need. For example, take the following case: VIN = 12V, TFLT has been set to 100msec, ILOAD(CONT. MAX) is 1.4A, the slow-trip threshold is 43mV nominal, and the fast-trip threshold is 100mV. If the output is accidentally connected to an 6 load, the output current from the MOSFET will be regulated to 1.4A 10 April 2001 MIC2582/MIC2583 Micrel for 100ms (TFLT) before the part trips. During that time, the dissipation in the MOSFET is given by: P = E x I EMOSFET = [12V-(1.4A)(4)] = 6.4V PMOSFET = (6.4V x 1.4A) = 9W for 100msec. Wow! Looks like we need a really hefty MOSFET to withstand this sort of fault condition. Or do we? This is where the transient thermal impedance curves become very useful. Figure 5 shows the curve for the Vishay (Siliconix) Si4410DY, a commonly used SO-8 power MOSFET: Assume TA = 55C maximum, 1 square inch of copper at the drain leads, no airflow. The part has an RON of (0.0335(/2) = 17m at 25C. Assume it has been carrying just about 1.4A for some time. Then the starting (steady-state)TJ is: TJ TA + [17m + (55C-25C)(0.005)(17m)] x (1.4A2) x (50C/W) TJ ( 55C + (0.0383W)(50C/W) 56.9C 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 57C: TJ TA + [17m + (57C-25C)(0.005)(17m)] x (1.4A2) x (50C/W) TJ ( 55C + (0.0383W)(50(CW) 56.9C So our original approximation of 56.9C was very close to the correct value. We will use TJ = 57C, which is close enough for all practical purposes. Finally, add (9W)(50C/W)(0.08) = 36C to the steady-state TJ to get TJ(TRANSIENT MAX.) = 93C. This is a completely acceptable maximum junction temperature for this part. Reading this graph is not nearly as daunting as it may at first appear. 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 (a vanishingly small repetition rate). 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 Z(J-A) of this MOSFET to a highly infrequent event of this duration is only 8% of its continuous R(J-A). This particular part is specified as having an R(J-A) of 50C/ W for intervals of 10 seconds or less. So, some further math, just to get things ready for the finale: 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 5. Transient Thermal Impedance April 2001 11 MIC2582/MIC2583 MIC2582/MIC2583 Micrel A final illustration of the use of the transient thermal impedance curves: assume that we are using an MIC2583R, which will auto-retry at a 10% dusty cycle into the same fault, with a one second interval between retry attempts. This frequency of restarts will significantly increase the dissipation in the Si4410DY MOSFET. Will the MOSFET be able to handle the increased dissipation? We get the following: The same part is operating into a persistent fault, so it is cycling in a square-wave fashion (no steady-state load) with a duty cycle of 10% = 0.1. From the Transient Thermal Impedance Curves, reading up from the X-axis to the line showing a duty cycle of 0.10, we get R(J-A) = (0.16 x 50C/W) = 8C/W. This is a bit marginal, but will be acceptable under fault conditions of limited duration. And finally, checking the RMS power dissipation just to be complete: PRMS = [17m+(127C-25C)(0.005)(17m)](1.4A2) 0.1 = 0.016W, which will result in a negligible temperature rise. The Si4410DY is suitable for this application. MOSFET and Sense Resistor Vendors The simplest way to address the issues of MOSFET and Sense Resistor selection is to give the names and web addresses of several major vendors of suitable parts. At the same time, it's quite possible to mention by type number some of the more popular MOSFET and resistor types used in the industry, which will constitute a good starting point for most designs. Calculating the peak junction temperature: TJ(MAX) = [(9W)(8C/W) + 55C] = 127C. MOSFET Vendors Vishay (Siliconix) Fairchild Semiconductor Key MOSFET Type(s) Si4420DY (SO-8 package) Si4420DY (SO-8 package) Si3442DV (for VCC < 3V) (SO-8 package) IRF7413A (SO-8 package) Si4420DY (second source to Vishay) IRF7601 (for VCC < 3V) (SO-8 package) FDS6880A (SO-8 package) Resistor Vendors Vishay (Dale) Sense Resistors "WSL" Series IRC "OARS" Series "LR" Series (second source to "WSL") International Rectifier MIC2582/MIC2583 12 Contact Information www.siliconix.com (203) 452-5664 www.irf.com (310) 322-3331 www.fairchildsemi.com (207) 775-8100 Contact Information www.vishay.com/docswsl_30100.pdf (203) 452-5664 www.irctt.com/pdf_files/OARS.pdf www.irctt.com/pdf_files/LRC.pdf (828) 264-8861 April 2001 MIC2582/MIC2583 Micrel PCB Layout Considerations Because of the low values of the sense resistors, special care must be used to accurately measure the voltage drop across them. Specifically, the voltage across RSENSE must be measured using Kelvin sensing, which 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 traces measuring the voltages across the sense resistors themselves. If accuracy must be paid for, it's worth keeping. Figure 6 (below) illustrates how Kelvin sensing is performed, with the Kelvin connections between the VCC and SENSE pins as shown. These Kelvin connection traces do not need POWER TRACE FROM VCC significant width as only signal currents will be flowing through them. As can be seen, all the high current in the circuit 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 connecting to the resistor will not introduce any extraneous IR drops. Tempting though it may become in some layouts, do not tap the sense voltages off of the power traces until all of the current being measured will absolutely have made its way into the sense resistor. Finally, to minimize IR drops between the input and the load, the PCB power traces should be as short as possible, and should be wide enough to carry the maximum required current with 10C ~ 20C maximum temperature rise. POWER TRACE TO MOSFET DRAIN RSENSE Signal Trace to MIC2583 VCC Signal trace to MIC2583 SENSE Figure 6. Kelvin Sensing Connetions for RSENSE April 2001 13 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) 0.050 (1.27) TYP 0.064 (1.63) 0.045 (1.14) 0.197 (5.0) 0.189 (4.8) 0.020 (0.51) 0.013 (0.33) 45 0.0098 (0.249) 0.0040 (0.102) 0-8 0.010 (0.25) 0.007 (0.18) 0.050 (1.27) 0.016 (0.40) SEATING PLANE 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) 0.196 (4.98) 0.189 (4.80) SEATING 0.0688 (1.748) PLANE 0.0532 (1.351) 0.0098 (0.249) 0.0075 (0.190) 45 8 0 0.050 (1.27) 0.016 (0.40) 0.2284 (5.801) 0.2240 (5.690) 16-pin QSOP (QS) MIC2582/MIC2583 14 April 2001 MIC2582/MIC2583 April 2001 Micrel 15 MIC2582/MIC2583 MIC2582/MIC2583 Micrel MICREL INC. TEL 1849 FORTUNE DRIVE SAN JOSE, CA 95131 + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. (c) 2001 Micrel Incorporated MIC2582/MIC2583 16 April 2001