LTC4211CS8 datasheet(20/36 Pages) LINER

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LTC4211

4211f

GATE

TIMER

VCC

3 4 5

GATE

VFB < VREF

>50mV

VREF

µA

4211 F10

tRESET

tFAULTSC

µA

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

start-up sequence where the LTC4211 is powered up into

a load overcurrent condition. Note that the circuit breaker

trips at Time Point B and is reset at Time Point 9A.

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 20

µs. During normal operation, the FILTER

output pin is held low as an internal 10

µA 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 2

µA pull-up

current source is connected to the FILTER pin by transis-

tor 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 10

µA 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:

SLOWCOMP

FILTER

+µ

1 236

.•

(7)

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

time = 638

µs. 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.

OPERATIO

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