LTC2913 datasheet(11/24 Pages) AD

11 / 24 page

LT4356-3

Rev D

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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

duringanovercurrentevent,sincemorepowerisdissipated

during this condition. Refer to the Typical Performance

Characteristics section for the timer current at different

VDS in both overvoltage and overcurrent events.

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:

tWARNING =

CTMR • 100mV

5µA

APPLICATIONS INFORMATION

Figure 1. Overvoltage Fault Timer Current

Figure 2. Overcurrent Fault Timer Current

tFLT

= 15ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tWARNING

= 20ms/µF

tFLT = 93.75ms/µF

tWARNING

= 20ms/µF

VTMR(V)

ITMR = 5µA

VDS = 75V

(ITMR = 50µA)

VDS = 10V

(ITMR = 8µA)

1.35

1.25

TIME

43563 F01

0.50

tFLT

= 2.88ms/µF

TOTAL FAULT TIMER = tFLT + tWARNING

tFLT = 21.43ms/µF

tWARNING

= 2.86ms/µF

tWARNING

= 0.38ms/µF

VTMR(V)

VDS = 10V

(ITMR = 35µA)

1.35

1.25

0.50

TIME

43563 F02

VDS = 80V

(ITMR = 260µA)

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