Datasheet

LTC4227
10
422712fa
For more information www.linear.com/LTC4227
applicaTions inForMaTion
High availability systems often employ parallel-connected
power supplies or battery feeds to achieve redundancy
and enhance system reliability. Power ORing diodes are
commonly used to connect these supplies at the point of
load, but at the expense of power loss due to significant
diode forward voltage drop. The LTC4227 minimizes this
power loss by using external N-channel MOSFETs for the
pass elements, allowing for a low voltage drop from the
supply to the load when the MOSFETs are turned on (see
Figure 1). When the input source voltage drops below the
output common supply voltage, the appropriate MOSFET is
turned off, thereby matching the function and performance
of an ideal diode. By adding a current sense resistor and
a Hot Swap MOSFET after the parallel-connected ideal
diode MOSFETs, the LTC4227 enhances the ideal diode
performance with inrush current limiting and overcurrent
protection. This allows the boards to be safely inserted
and removed from a live backplane without damaging
the connector.
Internal V
CC
Supply
The LTC4227 can operate with input supplies from 2.9V
to 18V at the IN pins. The power supply to the device is
internally regulated at 5V by a low dropout regulator (
LDO)
with an output at the INTV
CC
pin. An internal diode-OR
circuit selects the highest of the supplies at the IN and OUT
pins to power the device through the LDO. The diode-OR
scheme permits the device’s power to be temporarily kept
alive by the OUT load capacitance when the IN supplies
have collapsed or shut off.
An undervoltage lockout circuit prevents all of the MOSFETs
from turning on until the INTV
CC
voltage exceeds 2.2V. A
0.1µF capacitor is recommended between the INTV
CC
and
GND pins, close to the device for bypassing. No external
supply should be connected at the INTV
CC
pin so as not
to affect the LDO’s operation. A small external load of less
than 500µA can be connected at the INTV
CC
pin.
Turn-On Sequence
The board power supply at the OUT pin is controlled with
external N-channel MOSFETs (M
D1
, M
D2
and M
H
). The
ideal diode MOSFETs connected in parallel on the supply
side function as a diode-OR, while M
H
on the load side
acts as a Hot Swap controlling the power supplied to the
output load. The sense resistor, R
S
, monitors the load
current
for overcurrent detection. The HGATE capacitor,
C
HG
, controls the gate slew rate to limit the inrush current.
Resistor R
HG
with C
HG
compensates the current control
loop, while R
H
prevents high frequency oscillations in the
Hot Swap MOSFET.
Figure 1. Card Resident Diode-OR with Hot Swap Application
BACKPLANE
CONNECTOR
V
IN1
12V
V
IN2
12V
CARD
CONNECTOR
CPO1
D2ON
ON
FAULT
PWRGD
C
L
680µF
12V
7.6A
R1
20k
INTV
CC
Z2
SMAJ13A
GND
IN1 DGATE1 DGATE2
LTC4227
4227 F01
M
D2
SiR462DP
M
H
Si7336ADP
R
S
0.006Ω
HGATE OUTSENSE
+
SENSE
–
CPO2
C
CP2
0.1µF
IN2
+
R2
137k
R3
100k
R4
100k
R
H
10Ω
R
HG
47Ω
C
HG
15nF
C
CP1
0.1µF
C1
0.1µF
M
D1
SiR462DP
EN
C
T
0.1µF
C
F
10nF
Z1
SMAJ13A
TMR