SMH4811A(2000) 데이터 시트보기 (PDF) - Summit Microelectronics
부품명
상세내역
일치하는 목록
SMH4811A Datasheet PDF : 16 Pages
| |||
SMH4811A
DEVICE OPERATION
Power-Up Sequence
The SMH4811A 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 convert-
ers.
The SMH4811A hot-swap controller provides under-volt-
age and over-voltage monitoring of the host power supply,
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 SMH4811A
controller are within specification, the SMH4811A pro-
vides a “Power Good” logic output that may be used to turn
ON a load or drive an LED status light. There is a master
enable/temperature sense input and 2.5V and 5V refer-
ence outputs for expanding monitor functions.
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
SMH4811A starts up but does not immediately apply
power to the output load. Under-voltage and over-voltage
circuits 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 SMH4811A 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 SMH4811A 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/CB-
Sense 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 instan-
taneously if the voltage drop exceeds VQCB.
When the breaker trips, the VGATE output is turned off and
CBFault# will be driven LO. In 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. The MOSFET can be switched off by holding the
CBReset input LO.
The value of the over-current sense resistor is determined
by the following formula: RS = 50mV/IOC where Rs is the
value of the sense 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 through-
hole and surface mount. The resistor chips are surface
mount and offer excellent thermal characteristics. Both styles
are available in resistance ranges from 3mΩ to 1Ω.
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 SMH4811A is designed to control a single DC/DC
converter, or other loads, which incorporate on/off control.
The Power Good output activates the load when the
following conditions have been met: the input voltage to
the SMH4811A monitored by UV and OV is within user-
defined limits and the external MOSFET is switched ON.
The delays built into the SMH4811A allow correct se-
quencing of power to the loads. The delay time is factory
programmed.
The PG# output has a 12V withstand capability so high
voltages must not be connected to this pin. Inexpensive
bipolar transistors will boost the withstand voltage to that
of the host supply, see figure 5 for connections.
SUMMIT MICROELECTRONICS, Inc.
2044 4.1 8/1/00
7
Share Link: 