LTC4224-1/LTC4224-2

6

422412fa

OPERATION

The LTC4224 is designed to control power on a live back-

plane, allowing boards to be safely inserted and removed. It

between 1V and 6V via two external N-channel MOSFETs.

For applications where the total load current is 5A or less,

dual MOSFETs such as the FDS6911 can be used to save

board area. The two supplies can be turned on and off

independently using the active low ON1 and ON2 pins.

Pulling the ON pin low turns on a charge pump which

sources 10μA at the GATE pin thereby ramping up the gate

of the external MOSFET. When the MOSFET turns on, the

inrush current is limited at a level set by an external sense

resistor. Inrush current can be further reduced, if desired,

by adding a capacitor from GATE to GND. To protect the

external MOSFET, the GATE pins are clamped to about

5.5V above the higher of the two supplies.

Each supply is continuously monitored for undervoltage

and overcurrent conditions. The undervoltage monitor

shuts off the external MOSFET when the corresponding

supply is too low.

Current is monitored by an active current limit amplifier

(ACL) and a timed electronic circuit breaker (ECB). Like

all timing delays in the LTC4224, the ECB delay of 5ms is

generated internally without requiring any external timing

capacitors. The ECB threshold is slightly below the ACL

threshold and shuts off the external MOSFET after 5ms.

FAULT is latched low to indicate an overcurrent fault.

The LTC4224-1 remains latched off until reset by either

turning off and then on the affected supply or its ON pin.

The LTC4224-2 automatically restarts after four seconds

to allow time for the MOSFET to cool down.

The typical LTC4224 application is in a high availability

system where two positive supply voltages are distributed

to power individual cards. The LTC4224 detects board

presence during insertion and removal, allowing power

to be delivered in a controlled manner without damaging

the connector. It reports overcurrent faults to the system

controller through its FAULT pin, which can light an LED

or can be monitored by a system controller.

Figure 1. Typical Application

LTC4224

SENSE1

SENSE2

GATE1 GATE2

GND

ON1

ON2

FAULT

Q1

Q2

FDS6911

R2

0.010Ω

R1

0.015Ω

R3

390

D1

5V

BULK SUPPLY

BYPASS CAPACITOR

BULK SUPPLY

BYPASS CAPACITOR

3.3V

1k

GND

5V

1A

3.3V

2A

CONNECTOR

PLUG-IN CARD

MOD DETECT

APPLICATIONS INFORMATION

The basic LTC4224 application circuit is shown in Figure 1.

turn-on and turn-off sequence, various fault conditions

and recovery from fault situations. External component

selection is discussed in detail in the Design Example

section.