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LT4356IDE-3-TR

Surge Stopper with Fault Latchoff

Linear Technology 

LT4356-3

APPLICATIONS INFORMATION

Fault Timer Current

The timer current starts at around 2µA with 0.5V or less

of VDS, increasing linearly to 50µA with 75V of VDS dur-

ing an overvoltage fault (Figure 1). During an overcurrent

fault, it starts at 4µA with 0.5V or less of VDS but increases

to 260µA with 80V across the MOSFET (Figure 2). This

arrangement allows the pass transistor to turn off faster

during an overcurrent event, since more power is dissipated

during this condition. Refer to the Typical Performance

Characteristics section for the timer current at different

VDS in both overvoltage and overcurrent events.

VTMR(V)

1.35

1.25

ITMR = 5µA

VDS = 75V

(ITMR = 50µA)

When the voltage at the TMR pin, VTMR, reaches the 1.25V

threshold, the FLT pin pulls low to indicate the detection

of a fault condition and provide warning to the load of

the impending power loss. In the case of an overvoltage

fault, the timer current then switches to a fixed 5µA. The

interval between FLT asserting low and the MOSFET turn-

ing off is given by:

t WARNING

=

CTMR • 100mV

5µA

ITMR = 5µA

VDS = 10V

(ITMR = 8µA)

0.50

tFLT

= 15ms/µF

tWARNING

= 20ms/µF

tFLT = 93.75ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tWARNING

= 20ms/µF

Figure 1. Overvoltage Fault Timer Current

TIME

43563 F01

VTMR(V)

1.35

1.25

VDS = 80V

(ITMR = 260µA)

VDS = 10V

(ITMR = 35µA)

0.50

tFLT

= 2.88ms/µF

tWARNING

= 0.38ms/µF

tFLT = 21.43ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tWARNING

= 2.86ms/µF

Figure 2. Overcurrent Fault Timer Current

TIME

43563 F02

43563fb

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