LTC1421 datasheet(21/40 Pages) LINER

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LTC4211

4211fb

OPERATION

GATE

TIMER

VCC

3 4 5

GATE

VFB < VREF

>50mV

VREF

10μA

4211 F10

tRESET

tFAULTSC

2μA

FPD

VOUT

FAST COMPARATOR ARMED

CIRCUIT BREAKER TRIPS

CIRCUIT BREAKER RESET

SLOW COMPARATOR ARMED

VOUT

VCC – VSENSE

FILTER

FAULT

RESET

VSENSE = 50mV

REGULATING

LOAD CURRENT

Figure 10. Power-Up in Overcurrent, Slow Comparator Trips the Circuit Breaker

ADJUSTING SLOW COMP’S RESPONSE TIME

The response time of SLOW COMP is adjusted using a

capacitor connected from the LTC4211’s FILTER pin to

ground. If this pin is left unused, SLOW COMP’s delay

defaults to 20μs. During normal operation, the FILTER

output pin is held low as an internal 10μA pull-down

current source is connected to this pin by transistor M4.

This pull-down current source is turned off when an

overcurrent load condition is detected by SLOW COMP.

During an overcurrent condition, the internal 2μA pull-

up current source is connected to the FILTER pin by

transistor M5, thereby charging CFILTER. As the charge

on the capacitor accumulates, the voltage across CFILTER

increases. Once the FILTER pin voltage increases to 1.236V,

the electronic circuit breaker trips and the LTC4211’s

GATE pin is switched quickly to ground by transistor M3.

After the circuit breaker is tripped, M5 is turned OFF, M4

is turned ON and the 10μA pull-down current then holds

the FILTER pin voltage low.

The SLOW COMP response time from an overcurrent fault

condition to when the circuit breaker trips (GATE OFF) is

given by Equation 7:

tSLOWCOMP = 1.236V •

CFILTER

2µA

+ 20µs

(7)

For example, if CFILTER = 1000pF, SLOW COMP’s response

time = 638μs. As a design aid, SLOW COMP’s delay time

(tSLOW COMP) versus CFILTER for standard values of CFILTER

from 100pF to 1000pF is illustrated in Table 2.

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