6

SMH4811A

2044 6.

6 03/27/09

SUMMIT MICROELECTRONICS, Inc.

Preliminary

FUNCTIONAL DESCRIPTION

GENERAL OPERATION

The SMH4811A is an integrated power controller for hot

swappable add-in cards. The device operates from a wide

supply range and generates the signals necessary to drive

an isolated output DC/DC converter. As a typical add-in

board is inserted into the powered backplane physical

connections must first 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 device

starts up, but does not immediately apply power to the

output load. Under-voltage and over-voltage circuits

inside the controller check to see that the input voltage is

within a user-specified range, and pin detection signals

determine whether the card is seated properly.

These requirements must be met for a Pin Detect Delay

enables VGATE to turn on the external power MOSFET

allowing the slew rate to be easily modified using external

passive components. During the controlled turn-on period

SENSE input. When the drain sense drops below 2.5V,

can turn on the DC/DC controller. A Power Good Enable

input may be used to activate or deactivate an output load.

Steady state operation is maintained as long as all condi-

tions are normal. Any of the following events may cause

the device to disable the DC/DC controller by shutting

down the power MOSFET: an under-voltage or over-

voltage condition on the host power supply; an over-

current event detected on the CBSENSE input; a failure of

the power MOSFET sensed via the DRAIN SENSE pin;

the pin detect signals becoming invalid; or the master

enable (EN/TS) falling below 2.5V. The SMH4811A may

be configured so that after any of these events occur the

VGATE output shuts off and either latches into an off state

that prevents the voltage from exceeding 12V. It is

the device and prevent possible damage. The dropper

resistor allows the device to operate across a wide range

of system supply voltages, and also helps protect the

device against common-mode power surges. Refer to the

tance value.

System Enables

There are several enabling inputs, which allow a host

system to control the SMH4811A. The Pin Detect pins

(PD1# & PD2#) are two active low enables that are

generally used to indicate that the add-in circuit card is

properly seated. This is typically done by clamping the

switch, or alternatively through the use of a staggered pins

at the card-cage interface. Two shorter pins arrayed at

opposite ends of the connector force the card to be fully

seated (not canted) before both pin detects are enabled.

Care must be taken not to exceed the maximum voltage

rating of these pins during the insertion process. Refer to

details in the Applications Section for proper circuit imple-

mentation.

The EN/TS input provides an active high comparator input

that may be used as a master enable or temperature

sense input. These inputs must be held low for a period of

Under-/Over-Voltage Sensing

The Under-Voltage (UV) and Over-Voltage (OV) inputs

provide a set of comparators that act in conjunction with an

external resistive divider ladder to sense when the host

supply voltage exceeds the user defined limits. If the input

to the UV pin rises above 2.5V, or the input to the OV pin

spurious start-up sequences while the card is being in-

serted. If UV falls below 2.5V or OV rises above 2.5V, the

PG and VGATE outputs will be shut down immediately.

Figure 1. Under-/Over-Voltage Filter Timing

2044 Fig01 5.0

OV / UV

FAULT#

2.5V