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MIC2588-1BM Datasheet(PDF) 11 Page - Micrel Semiconductor |
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MIC2588-1BM Datasheet(HTML) 11 Page - Micrel Semiconductor |
11 / 14 page December 2003 11 M9999-122303 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 V DRAIN drops below V PGTH, and will pull down to VDRAIN when V DRAIN is above VPGTH. For the MIC2588-2 and the MIC2594-2, /PWRGD will pull down to the potential of the V DRAIN pin when VDRAIN drops below VPGTH, and will be high impedance when V DRAIN 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 compo- nents against faults such as short circuits. The current limit threshold is set via an external resistor, R SENSE, connected between the V EE and SENSE pins. An internal 400µs timer limits the length of time (t FLT) 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 R SENSE exceeds 50mV, two things happen: 1. A constant-current regulation loop is engaged de- signed to hold the voltage across R SENSE equal to 50mV. This protects both the load and the MIC2588 circuit from excessively high currents. This loop will engage in less than 1 µs from the time at which the overvoltage condition on R SENSE occurs. 2. The internal 400 µs 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 V EE 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. Sense Resistor Selection The sense resistor is nominally valued at: R (nom) V (typ) I (nom) SENSE TRIP HOT_SWAP = where V TRIP(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. 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: I (min) 40mV 1.03 R (nom) 38.8mV R (nom) HOT_SWAP SENSE SENSE = × () = 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 R SENSE 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 direc- tion. Here, the worst-case maximum is found using a V TRIP(max) of 60mV and a sense resistor, 3% low in value: I (max) 60mV 0.97 R (nom) 61.9mV R (nom) HOT_SWAP SENSE SENSE = × () = 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, R SENSE should be set to 38.8mV 4A 9.7m =Ω , and the nearest 1% standard value is 9.76m Ω. At the other tolerance extremes, I HOT_SWAP(max) for the circuit in question is then simply I (max) 61.9mV 9.76m 6.3A HOT_SWAP = Ω = 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)(R SENSE(nom)) = 9.47mΩ. Thus, the sense resistor’s maximum power dissipation is: P MAX = (6.3A) 2 × (9.47mΩ) = 0.376W A 0.5 Ω 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 V UVL 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 V UVH threshold or OV drops below its V OVL threshold. The UV and OV circuit’s threshold trip points are programmed using the resistor divider |
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