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SMH4803CHMP Datasheet(PDF) 10 Page - Summit Microelectronics, Inc. |
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SMH4803CHMP Datasheet(HTML) 10 Page - Summit Microelectronics, Inc. |
10 / 22 page 10 SMH4803 2041 8.4 6/15/00 SUMMIT MICROELECTRONICS DEVICE OPERATION Power-Up Sequence The SMH4803 is an integrated power controller for hot swappable add-in cards. The device operates from a single supply ranging from 20V to 500V and generates the signals necessary to drive isolated output DC/DC con- verters. The SMH4803 hot-swap controller provides under-volt- age and over-voltage monitoring of the host power sup- ply, it drives an external power MOSFET switch that connects the supply to the load. It also protects against over-current conditions that might disrupt the host supply. When the input and output voltages to the SMH4803 controller are within specification, the SMH4803 provides three “Power Good” logic outputs that may be used to turn ON loads or drive an LED status light. The SMH4803 provides three separate “Power Good” logic outputs that activate loads in a programmable timed sequence. There is a master enable/temperature sense input and 2.5V and 5V reference outputs for expanding monitor functions. There are two “Power Good” enable inputs that may be used to activate or deactivate output loads, and duty- cycle or reset over-current protection modes to provide automatic or manual restart of the controller after over- current load conditions. Insertion Process As the add-in board is inserted into the backplane, physi- cal connections must be made with the chassis to dis- charge any electrostatic voltage potentials. The board then contacts the long pins on the backplane that provide power and ground. As soon as power is applied the SMH4803 starts up but does not immediately apply power to the output load. Under-voltage and over-voltage cir- cuits inside the controller check to see if the input voltage is within a user-specified range, and pin detection signals determine whether the card is seated properly. tPDD after these requirements are met, the hot-swap controller enables Vgate to turn on the power MOSFET switch. The Vgate output is current limited to IVGATE, allowing the slew rate to be easily modified using external passive components. During the controlled turn-on pe- riod, the Vds of the MOSFET is monitored by the drain sense input. When Vds drops below a user-specified voltage the power output is considered to be ON. The resistor and diode in series with the drain sense input determine Vds(ON). Provided there is no sustained over-current condition during start-up, the SMH4803 turns on the loads with the Power Good logic outputs. Three DC/DC converters can be connected to the outputs and their turn-on is sequenced by pre-programmed delays. If a sustained over-current condition occurs during or after the insertion process, then Vgate is shorted to Vss and the MOSFET switch is turned off to protect the host supply. Circuit Breaker Operation The SMH4803 provides a circuit breaker function to protect against over current conditions. A sustained over- current event could damage the host supply and/or the load circuitry. The board’s load current passes through a series resistor connected between MOSFET source/ CBSense and Vss on the controller. The breaker will trip whenever the voltage drop across the series resistor is greater than 50mV for more than tCBD, and will trip instantaneously if the voltage drop exceeds VQCB. When the breaker trips, the Vgate output is turned off and CBFault# will be driven LO. The circuit breaker can be reset by taking CBReset# LO and then back HI when the circuit breaker is in the reset mode. In the duty-cycle mode, the circuit breaker resets automatically after a fixed time period. If the over current condition still exists after reset, the circuit will re-trip. In both operating modes of the circuit breaker, the MOSFET can be switched off by holding the CBReset input LO. The value of the over-current shunt resistor is determined by the following formula: Rs = 50mV/Ioc where Rs is the value of the shunt resistor and Ioc is the over current limit determined by the board’s power requirement or the limit of the host supply. Current Sense Resistors Current sense resistors are available from a number of sources and come in two basic formats: open air sense resistors and current sense resistor chips. The open air resistors are metal strips that are available as both thru-hole and surface mount. The resistor chips are surface mount and offer excellent thermal characteristics. Both styles are available in resistance ranges from 3 milliohm to 1 ohm. IRC (www.irctt.com) is one source for these resistors. The open air sense resistors can be found in their OARS series, and the chip resistors are found in their LRC series. Load Control The SMH4803 is designed to control three or more DC/ DC converters, or other loads, which incorporate ON/OFF control. The Power Good outputs activate the loads when the following conditions have been met: the input voltage to the SMH4803 monitored by UV and OV is within user- defined limits and the external MOSFET is switched ON. The SMH4803 has three Power Good enable outputs, PG1#, PG2#, and PG3#, that turn on the DC/DC con- verter loads in sequence. Output PG1# is activated first, followed by PG2# after a delay of PGD, and finally PG3# |
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