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LTC4216

4216f

Power-Up into an Output-Short

Figure 10 shows the timing diagram in the case when the

output is a dead short during power-up. As GATE ramps

up at time point 6, the MOSFET current increases due to

the output short causing the voltage drop across the sense

resistor to rise above 25mV. FILTER sources 60µA, charg-

ing the external capacitor. At time point 7, GATE regulates

to limit the output current to 40mV/RSENSE. If the output

continues to be in analog current limit when the FILTER

pin voltage reaches its threshold (1.253V) at time point

8, the circuit breaker trips and GATE is pulled low. The

device latches-off and ⎯F⎯A⎯U⎯L⎯T is pulled low, indicating a

fault condition. The FILTER capacitor discharges through

a 2.4µA pull-down until the device resets.

Resetting the Electronic Circuit Breaker

When the LTC4216’s electronic circuit breaker is tripped

during a fault condition, ⎯F⎯A⎯U⎯L⎯T is asserted low and the

⎯R⎯E⎯S⎯E⎯T, SS and GATE pins are all pulled to ground. This is

shown in the timing diagram of Figure 11. The LTC4216

remains latched-off until the external fault is cleared. To

clear the internal fault latch and restart the device, pull

the ON pin low (< 0.4V) at time point 4 for at least 100µs,

after which the ⎯F⎯A⎯U⎯L⎯T will go high at time point 5. Tog-

gling the ON pin from low to high (> 0.8V) initiates a new

start-up cycle.

Sense Resistor Considerations

The circuit breaker trip threshold of 25mV and the value of

the sense resistor, RSENSE,connectedbetweentheSENSEP

and SENSEN pins, determine the trip current level as given

by Equation (10). If the fault current level exceeds the

analog current limit, the current is limited to a value given

by Equation (11). Should the overload condition exist for

more than one fault ﬁlter delay as given by Equation (2),

the circuit breaker trips and the device is latched-off.

TRIP CB

CB TH

SENSE

()

∆

(10)

ACL

ACL TH

SENSE

∆

()

(11)

For a new circuit design, the sense resistor value is ﬁrst

calculated from the maximum operating load current under

normal conditions and the minimum circuit breaker trip

threshold. This is given by:

SENSE

CB TH MIN

LOAD MAX

∆

)

()

21 5

(12)

Figure 10. Power-Up into an Output-Short and

Circuit Breaker Trips

Figure 11. Mild Overcurrent Circuit Breaker Trips Followed by

Device Reset

TIMER

2.4

µA

40mV

25mV

µA

12 3

4 5 6 7

4216 F10

0.8V

TRACKS SS RAMP

FPD

µA

SENSEP-SENSEN

FILTER

GATE

VOUT

FAULT

RESET

VFILT(TH)

VTMR(TH)

VGATE – VOUT < VGS(TH)

FAULT

FILTER

DELAY

GATE

REGULATING

2.4

µA

<40mV

25mV

4216 F11

FPD

µA

tRST(ONLO)

VFILT(TH)

SENSEP-SENSEN

FILTER

FAULT

RESET

GATE

VOUT

0.4V

POWER BAD

VFB < 0.6V

MILD

OVERCURRENT

FAULT LATCH

RESET

CIRCUIT BREAKER TRIPS

AND LATCHED-OFF

RESET PULLED LOW

DUE TO POWER BAD

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