Datasheet

LTC4354
8
4354fc
LTC4354
TO
MODULE
INPUT
DB GADA GB
V
SS
V
B
V
A
–48V_RTN
FAULT
M2
IRF3710S
M1
IRF3710S
4354 F02
V
CC
R3
33k
R
IN
12k
0.5W
D1
LED
1 8 4
3
6 2, 5
7
R1
2k
R2
2k
C
IN
1µF
applicaTions inForMaTion
System Power Supply Failure
LTC4354 automatically supplies load current from the
system supply with the more negative input potential. If
this supply is shorted to the return side, a large reverse
current flows from its pass transistor. When this reverse
current creates –140mV of voltage drop across the drain
and source pins of the pass transistor, the LTC4354 drives
the gate low fast and turns it off.
The remaining system power supply will deliver the load
current through the body diode of its pass transistor until
the channel turns on. The LTC4354 ramps the gate up and
turns on the N-channel MOSFET to reduce the voltage drop
across it, a process that takes less than 1ms depending
on the gate charge of the MOSFET.
Drain Resistor
Tw o resistors are required to protect the DA and DB pins
from transient voltages higher than 80V. In the case
when the supply with the lower potential is shorted to the
return side due to supply failure, a reverse current flows
briefly through the pass transistor to the other supply to
discharge the output capacitor. This current stores energy
in the stray inductance along the current path. Once the
pass transistor
is turned off, this energy forces the drain
terminal
of the FET high until it reaches the breakdown
voltage. If this voltage is higher than 80V, the internal
ESD devices at the DA and DB pins might break down
and become damaged. The external drain resistors limit
the current into the pins and protect the ESD devices. A
2k resistor is recommended for 48V applications. Larger
resistor values increase the source drain sense threshold
voltage due to the input current at the drain pins.
Loop Stability
The servo loop is compensated by the parasitic capacitance
of the power N-channel MOSFET. No further compensation
components are normally required. In the case when a
MOSFET with very small parasitic capacitance is chosen,
a 1000pF compensation capacitor connected across the
gate and source pins might be required.
Design Example
The following demonstrates the calculations involved for
selecting components in a –36V to –72V system with 5A
maximum load current, see Figure 2.
First, select the input dropping resistor. The resistor should
allow 2mA of current with the supply at –36V.
R
IN
≤
(36V − 11.5V)
2mA
= 12.25k
The nearest lower 5% value is 12k.
Figure 2. –36V to –72V/5A Design Example