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SMH4811AS Datasheet(PDF) 8 Page - Summit Microelectronics, Inc. |
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SMH4811AS Datasheet(HTML) 8 Page - Summit Microelectronics, Inc. |
8 / 16 page 8 SMH4811A 2044 6.1 2/8/01 SUMMIT MICROELECTRONICS, Inc. Preliminary charging the filter capacitance (normally required at the input of the DC/DC controller) too quickly may generate very high current. The VGATE output of the SMH4811A is current limited to IVGATE, allowing the slew rate to be easily modified using external passive components. The slew rate may be found by dividing IVGATE by the gate-to- drain capacitance placed on the external FET. A complete design example is given in the Applications Section. Load Control — Sequencing the Secondary Sup- plies Once power has been ramped to the DC/DC controllers, two conditions must be met before the PG# output can be enabled: the Drain Sense voltage must be below 2.5V, and the VGATE voltage must be greater than V DD –VGT. The Drain Sense input helps ensure that the power MOS- FET is not absorbing too much steady state power from operating at a high V DS. This sensor remains active at all times (except during the current regulation period). The VGATE sensor makes sure that the power MOSFET is operating well into its saturation region before allowing the load to be switched on. Once VGATE reaches V DD –VGT this sensor is latched. Once the external MOSFET is properly switched on the PG# output may be enabled (if ENPG is high). The PG# output has a 12V withstand capability, so high voltages must not be connected to this pin. A bipolar transistor or opto-isolator can be used to boost the withstand voltage to that of the host supply. Circuit Breaker Operation The SMH4811A provides a number of circuit breaker functions to protect against over current conditions. A sustained over-current event could damage the host sup- ply and/or the load circuitry. The board’s load current passes through a series resistor (R S) connected between the MOSFET source (which is tied to CBSENSE) and V SS. The breaker trips whenever the voltage drop across R S is greater than 50mV for more than t CBD (a factory program- mable filter delay ranging from 10µs to 500µs). Quick-TripTM Circuit Breaker Additionally, the SMH4811A provides a Quick-Trip feature that will cause the circuit breaker to trip immediately if the voltage drop across R S exceeds VQCB. The Quick-Trip level may be factory set to 60mV, 100mV (default), 200mV, or the feature may be disabled. Current Regulation The current regulation mode is an optional feature that provides a means to regulate current through the MOS- FET for a programmable period of time. If enabled the device will start the internal timer when the voltage at CBSENSE exceeds 50mV. Also, it attempts to limit the voltage at CBSENSE to 60mV by regulating the VGATE output. The circuit breaker will trip if the over-current condition remains after the time-out. However, if CB- SENSE drops below 50mV before the timer ends, the timer is reset and VGATE resumes normal operation. If the Quick-Trip level is exceeded then the device will bypass the current regulation timer and shut down imme- diately. The Current Regulation feature is disabled in the default configuration. Non-Volatile Fault Latch The SMH4811A also provides an optional nonvolatile fault latch (NVFL) circuit breaker feature. The nonvolatile fault latch essentially provides a programmable fuse on the circuit breaker. When enabled the nonvolatile fault latch will be set whenever the circuit breaker trips. Once set, it cannot be reset by cycling power. NOTE: THE DEVICE REMAINS PERMANENTLY DISABLED UNTIL IT IS REPROGRAMMED AT THE FACTORY . As long as the NVFL is set the FAULT# output will be driven active. The Non-Volatile Fault Latch feature is disabled in the default configuration. Resetting FAULT# When the circuit breaker trips the VGATE output is turned off and FAULT# is driven low. In the default condition the breaker resets automatically after a time of t CYC. In the latched condition cycling power to the board or toggling the EN/TS input will also reset the circuit breaker. If the over current condition still exists after the MOSFET switches back on, the circuit breaker will re-trip. |
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