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
controls two supplies (V
CC1
, V
CC2
) with operating voltages
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.
Internal 10µA current sources pull these pins to V
CC
.
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 amplifi er
(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
V
CC1
SENSE1 SENSE2 GATE1
GATE2
GND
ON1
ON2FAULT
V
CC2
Q1
Q2
FDS6911
R2
0.010Ω
R1
0.015Ω
R3
390
D1
5V
BULK SUPPLY
BYPASS CAPACITOR
BULK SUPPLY
BYPASS CAPACITOR
3.3V
R
MOD_DET
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.
The following sections cover V
CC
selection, the normal
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.
