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43631

High Voltage Surge Stopper with Current Limit

Linear Technology 

LT4363

Applications Information

VIN

12V

D1*

SMAJ58CA

Q2

SI7461DP

R7

10k

5 VCC

Q1

IRLR2908

RSNS

10mΩ

D2

1N5245

R3

15V

10Ω

C1

47nF

4

3

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 F07

Figure 7. Overvoltage Regulator with P-channel MOSFET

Reverse Input Protection

Shutdown

The LT4363 can be shut down to a low current mode when

the voltage at the SHDN pin is pulled below the shutdown

threshold of 0.4V. The quiescent current drops down to

7µA with internal circuitry turned off.

The SHDN pin can be pulled up to 100V or below GND by

up to 60V without damage. Leaving the pin open allows

an internal current source to pull it up and turn on the part

while clamping the pin to 2.2V. The leakage current at the

pin should be limited to no more than 1µA if no pull up

device is used to help turn it on.

Supply Transient Protection

The LT4363 is tested to operate to 80V and guaranteed to

be safe from damage up to 100V. Nevertheless, voltage

transients above 100V may cause permanent damage.

During a short-circuit condition, the large change in cur-

rent flowing through power supply traces and associated

wiring can cause inductive voltage transients which could

exceed 100V. To minimize the voltage transients, the power

trace parasitic inductance should be minimized by using

wide traces. A small RC filter, in Figure 8, at the VCC pin

will clamp the voltage spikes.

VIN

R7

1k

Q1

RSNS

FDB33N25 10mΩ

C1

47nF

R3

10Ω

CL**

22µF

VOUT

C2

0.1µF

D1*

SMAJ58A

R4

374k

R5

90.9k

R6

10k

54 3

VCC GATE SNS

2

R1

100k

OUT

FB 1

6 SHDN

LT4363DE-2

R2

4.99k

8 UV

ENOUT 11

VCC

DC/DC

CONVERTER

SHDN

7 OV

GND TMR

FLT 10

FAULT

GND

9

12

4363 F08

CTMR

47nF *DIODES INC.

**SANYO 25CE22GA

Figure 8. Overvoltage Regulator with Input Voltage Detection

Another way to limit transients above 100V at the VCC

pin is to use a Zener diode and a resistor, D1 and R7 in

Figure  8. The Zener diode limits the voltage at the pin while

the resistor limits the current through the diode to a safe

level during the surge. However, D1 can be omitted if the

filtered voltage, due to R7 and C1, at the VCC pin is below

100V. The inclusion of R7 in series with the VCC pin will

increase the minimum required voltage at VIN due to the

extra voltage drop across it. This voltage drop is due to the

supply current of the LT4363 and the leakage current of D1.

A total bulk capacitance of at least 22µF low ESR electro-

lytic is required close to the source pin of MOSFET Q1. In

addition, the bulk capacitance should be at least 10 times

larger than the total ceramic bypassing capacitor on the

input of the DC/DC converter.

Layout Considerations

To achieve accurate current sensing, Kelvin connection

to the current sense resistor (RSNS in Figure 8) is recom-

mended. The minimum trace width for 1 oz copper foil is

0.02" per amp to ensure the trace stays at a reasonable

temperature. 0.03" per amp or wider is recommended.

Note that 1oz copper exhibits a sheet resistance of about

530µΩ/square. Small resistances can cause large errors in

high current applications. Noise immunity will be improved

16

For more information www.linear.com/LT4363

4363fb

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