LTC4221IGN datasheet(17/28 Pages) LINER

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LTC4221

4221f

duty cycle of more than 1.8% will eventually trip the circuit

breaker. Figure 8 shows the circuit breaker response time

in seconds normalized to 1µF.Theasymmetricchargingand

discharging of FILTER is a fair gauge of MOSFET heating.

FILTER µ

()

124

105

1 8

•– .

(2)

The fast comparators trip the circuit breaker to protect

against fast load overcurrents if VRSENSE is greater than

VSENSE(FC) (100mV) for 1µs. The response time of each fast

comparator is fixed at 1µs nominal. The timing diagram in

Figure 9 illustrates the operation of the LTC4221 when the

load current conditions cause VRSENSE of channel 1 to ex-

ceed 100mV for more than 1µs between time points 7 and

8. Figure 9 also illustrates when the LTC4221’s electronic

circuit breaker is armed. After the initial timing cycle, it is

armed at time point 3. Arming the circuit breaker at time

point 3 ensures that the system is protected against an over-

current condition during the channel start-up cycle. At time

point 4, the slow comparators are armed when the internal

control loop is disengaged.

Autoretry After a Fault

Once the LTC4221 circuit breaker is tripped, FAULT is

latched low and both GATE pins are pulled to ground. To

clear the internal FAULT latch and to restart the LTC4221,

its ON1 pin must be pulsed below its reset threshold

(VON(RESET) = 0.4V) for at least 15µs.

APPLICATIO S I FOR ATIO

TIMER

FAULT

FILTER

0.851V

GATE

SENSE

VOUTn

RESET

INITIAL TIMING

1.9µA

VTMR(L)

VTMR(H)

ELECTRONIC CIRCUIT BREAKER ARMED

SLOW COMPARATORS ARMED

20µA

1.8µA

0.4V

105µA

9.5µA

VFILTER(TH)

VSENSE(FC)

VSENSE(SC)

6 7

RESET

4221 F09

CHANNEL

START-UP

NORMAL

FAULT

Figure 9. Fast Comparator Trip Timing Waveforms

Figure 8. Circuit Breaker Filter Response

for Intermittent Overload

OVERLOAD DUTY CYCLE, D (%)

0.01

0.1

102030405060708090 100

4221 F08

CFILTER (µF)

1.24V

105 • D – 1.8

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