LTC4355



4355fa

applicaTions inForMaTion

System Power Supply Failure

The LTC4355 automatically supplies load current from

the system input supply with the higher voltage. If this

supply shorts to ground, reverse current begins to flow

through the pass transistor temporarily and the transis-

tor begins to turn off. When this reverse current creates

–25mV of voltage drop across the drain and source pins

of the pass transistor, a fast pulldown circuit engages to

drive the gate low faster.

The remaining system power supply delivers the load cur-

rent through the body diode of its pass transistor until the

channel turns on. The LTC4355 ramps the gate up with

20µA, turning on the N-channel MOSFET to reduce the

voltage drop across it.

When the capacitances at the inputs and output are very

small, large changes in current can cause inductive tran-

sients that exceed the 100V Absolute Maximum Ratings

of the pins. A surge suppressor (TransZorb) at the output

will minimize this ringing.

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 less than 1000pF gate capacitance

is chosen, a 1000pF compensation capacitor connected

across the gate and source pins might be required.

Design Example

The following design example demonstrates the calcula-

tions involved for selecting components in a 36V to 72V

system with 5A maximum load current (see Figure 1).

First, choose the N-channel MOSFET. The 100V, FDS3672

good solution. The maximum voltage drop across it is:

DV = 5A • 22mW = 110mV

The maximum power dissipation in the MOSFET is a mere:

P = 5A • 110mV = 0.55W

Next, select the resistive dividers that guarantee the

PWRFLT pins willl not assert when the input supplies are

R1 = 12.7k

W. Then,

I

V

R MIN

I

V

R

MONx TH

MONx TH MAX

2

1

1 245

1

=

+

=

( )

( )(

)

(

)

.

22 7

1

1

100

.

( %)

k

A

A

W −

+

=

µ

µ

R

V

V

A

k

2

36

1 245

100

348

=

−

=

.

µ

W

4355 F01

LTC4355

GND

GATE1

GATE2

IN1

OUT

IN2

MON1

MON2

SET

R1

12.7k

R2

340k

R3

12.7k

R4

340k

R5

33k

R7

33k

R9

33k

R6

33k

R8

33k

D1

D3

D2

D4

D5

GND

M1

FDS3672

M2

FDS3672

TO

LOAD

VDSFLT

FUSEFLT1

FUSEFLT2

PWRFLT1

PWRFLT2

F1

7A

F2

7A

GREEN LEDs

PANASONIC LN1351C

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