MIC2584/MIC2585 Dual-Channel Hot Swap Controller/Sequencer - Micrel - #19

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March 2005 19 MIC2584/2585 MIC2584/2585 Micrel Fast Output Discharge for Capacitive Loads In many applications where a switch controller is turned off by either removing the PCB from the backplane or the ON pin is reset, capacitive loading will cause the output to retain voltage unless a ‘bleed’ (low impedance) path is in place in order to discharge the capacitance. The MIC2585 is equipped with an internal MOSFET that allows the discharging of any load capacitance to ground through a 50Ù to 170Ù path. The discharge feature is configured by wiring the DIS pin to the output (source) of the external MOSFET and is enabled if the TRK pin is below 0.3V after the controller has been disabled by a logic low signal received at the ON pin of Figure 1. See the "Typical Application" circuit of Figure 1. A series resistor is required from DIS to VOUT so that the maximum current of 25mA for the DIS pin is not exceeded. Output Turn-Off Sequencing - No Tracking There are many applications where it is necessary or desirable for the supply rails to sequence during turn-on and turnoff, as is the case with some microprocessor requirements. The MIC2585 can be configured to allow one output to shut off first, followed by the other output. Figure 8 illustrates an example circuit that sequences OUT1 and OUT2 in a first on– last off application. During start-up, capacitor CDLY allows for VOUT1 to turn on followed by VOUT2 20ms later. Once the ON pin receives a low signal by removing the PCB from the backplane, or by an external processor signal, DIS1 and DIS2 will assert low. The external crowbar circuit connected from the DIS2 pin will immediately bring VOUT2 to ground while VOUT1 will discharge to ground through the 750Ù (680Ù external, 70Ù internal) series path. MIC2585 ON /FAULT PG GATE1 OUT1 GATE2 OUT2 VOUT1 VIN1 VIN2 VIN3 VIN4 EN VOUT2 MIC2585 ON /FAULT GATE1 OUT1 GATE2 OUT2 VOUT3 VOUT4 VOUT1/VOUT2 Short Circuit on VOUT1 System Timing VOUT3/VOUT4 ON PG /FAULT Figure 7. Supporting More Than Two Supplies