LTC4354

8

4354f

APPLICATIO S I FOR ATIO

Figure 1. –36V to –72V/5A Design Example

LTC4354

DB

GA

DA

GB

–48V_RTN

FAULT

M2

IRF3710S

M1

IRF3710S

4354 F01

R3

33k

TO

MODULE

INPUT

12k

0.5W

R2

2k

R1

2k

D1

LED

1µF

2, 5

6

14

8

3

7

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.

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.

R

VV

mA

k

−

=

(.

)

.

36

11 5

2

12 25

The nearest lower 5% value is 12k.