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43631

High Voltage Surge Stopper with Current Limit

Linear Technology 

LT4363

Applications Information

Limiting Inrush Current and GATE Pin Compensation

The LT4363 limits the inrush current to any load capacitance

by controlling the GATE pin voltage slew rate. An external

capacitor can be connected from GATE to ground to reduce

the inrush current at the expense of slower turn-off time.

The gate capacitor is set at:

C1=

IGATE(UP)

IINRUSH

•

CL

The LT4363 does not need extra compensation compo-

nents at the GATE pin for stability during an overvoltage or

overcurrent event. With transient input voltage slew rates

faster than 5V/µs, a gate capacitor, C1, to ground is needed

to prevent self enhancement of the N-channel MOSFET.

The extra gate capacitance slows down the turn off time

during fault conditions and may allow excessive current

during an output short event. An extra resistor, R1, in series

with the gate capacitor can improve the turn off time. A

diode, D1, should be placed across R1 with the cathode

connected to C1 as shown in Figure 5.

Q1

D1

1N4148W

R3

R1

C1

GATE

LT4363

4363 F05

Figure 5. External GATE network

Undervoltage/Overvoltage Comparators

The LT4363 has both undervoltage and overvoltage com-

parators that can be used to sense the input supply volt-

age. When the voltage at the UV pin is below the 1.275V

threshold, the GATE pin is held low to keep the external

MOSFET off. The supply voltage at the VCC pin should be

at least 4V for the UV comparator to function.

The overvoltage comparator prevents the LT4363-2 from

restarting if the voltage at the OV pin is above the 1.275V

threshold during a fault. The pass transistor is not allowed

to turn back on even after the cool down period has finished.

This prevents the pass transistor from cycling between ON

and OFF states when the input voltage stays at an elevated

level for a long period of time, reducing the stress on the

N-channel MOSFET. For the latch-off version, LT4363-1,

the overvoltage comparator function is not available.

Reverse Input Protection

A blocking diode is commonly employed to protect the

load when reverse input is possible, such as in automo-

tive applications. This diode causes extra power loss,

generates heat, and reduces the available supply voltage

range. During cold crank, the extra voltage drop across

the diode is particularly undesirable.

The LT4363 is designed to withstand reverse voltage with-

out damage to itself. The VCC, SHDN, UV, and OV pins can

withstand up to 60V of DC voltage below the GND potential.

Back-to-back MOSFETs must be used to block the current

path through Q1’s body diode (Figure 6). Figure 7 shows

the approach with a P-channel MOSFET in place of Q2.

VIN

12V

D1*

SMAJ58CA

Q2

IRLR2908

Q1

IRLR2908

RSNS

10mΩ

Q3

R4 R5 R3

2N3904 10Ω 1M 10Ω

D2

1N4148

R7 C1

10k 47nF

5 VCC

43

GATE SNS

2

OUT

FB 1

VOUT

12V, 3A

CLAMPED

AT 16V

R1

57.6k

R2

4.99k

6 SHDN

8 UV

7 OV

*DIODES INC.

LT4363DE-2

GND

9

ENOUT 11

TMR

FLT 10

12

CTMR

0.1µF

4363 F06

Figure 6. Overvoltage Regulator with N-channel MOSFET

Reverse Input Protection

For more information www.linear.com/LT4363

4363fb

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