LTC4354 Linear Technology, LTC4354 Datasheet - Page 8

• Manufacturer Part Number: LTC4354
• Description: Negative Voltage Diode-OR Controller and Monitor
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC4354
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 –60mV 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 depend-
ing on the gate charge of the MOSFET.
Drain Resistor
Two 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 dis-
charge 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 termi-
nal of the FET high until it reaches the breakdown voltage.
8
U
U
–48V_RTN
V
V
A
B
W
Figure 1. –36V to –72V/5A Design Example
DA
1
R1
2k
U
DB
8
R2
2k
IRF3710S
0.5W
R
12k
M1
GA
IN
LTC4354
V
4
CC
3
IRF3710S
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 capaci-
tance of the power N-channel MOSFET. No further com-
pensation 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 1.
First, select the input dropping resistor. The resistor
should allow 2mA of current with the supply at –36V.
The nearest lower 5% value is 12k.
GB
M2
R
6
IN
FAULT
≤
V
SS
(
2, 5
1µF
C
36
IN
7
V
2
−
mA
11 5
D1
LED
R3
33k
4354 F01
.
V
TO
MODULE
INPUT
)
=
12 25
.
k
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