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LTC4216

4216f

Electronic Circuit Breaker

The LTC4216 features an electronic circuit breaker function

that protects the external MOSFET against short-circuits or

excessive load current conditions on the supply. An external

sense resistor connected between SENSEP and SENSEN

pins is used to measure the load current. If the voltage

across the sense resistor exceeds the circuit breaker trip

threshold of 25mV for more than a fault ﬁlter delay, the

gate of the MOSFET is pulled low, turning it off.

The fault ﬁlter delay is determined by a capacitor, C3, con-

nected between the FILTER pin and ground as in Equation

(2). The FILTER pin sources 60µA pull-up current when

the sense voltage across the sense resistor exceeds 25mV.

Otherwise, it pulls down with 2.4µA. When the FILTER

pin voltage exceeds VFILT(TH) threshold (1.253V), there

is an internal 20µs delay before the GATE pulls low and

the ⎯F⎯A⎯U⎯L⎯T pin will be pulled low. If no FILTER capacitor

is used, the ﬁlter fault delay defaults to 20µs. The circuit

breaker response time or fault ﬁlter delay with the FILTER

capacitor, C3, is given by:

CB TRIP

()

.•

+µ

1 253

(2)

The FILTER capacitor, C3, should be chosen so that the

fault ﬁlter delay is not too short to trip the circuit breaker

as the MOSFET current charges up a large output load

capacitance in analog current limit during power-up. It

also should not be too long to exceed the safe operating

area (SOA) of the external MOSFET.

Intermittent overloads may exceed the current limit as in

Figure 5, but if the duration is sufﬁciently short, the FILTER

pin voltage may not reach the VFILT(TH) threshold and the

device will not shut off. To handle this situation, the FILTER

discharges with 2.4µA whenever voltage across the sense

resistor is below 25mV. Any intermittent overload with

an aggregate duty cycle of more than 4% will eventually

trip the circuit breaker. Figure 6 shows the circuit breaker

response time in seconds normalized to 1µF as given by

Equation (3). The asymmetric charging and discharging

of FILTER is a fair gauge of MOSFET heating.

1 253

2 4

)

(• )– .

µ=

(3)

Following a circuit breaker trip, the device is latched-off

and ⎯F⎯A⎯U⎯L⎯T is pulled low until the fault latch is cleared by

pulling the ON pin low (< 0.4V) for at least 100µs. The

FILTER pin is pulled low by an internal N-channel switch

to discharge the capacitor quickly when the ON pin volt-

age falls below 0.4V and pulls down with 2.4µA when the

ON pin voltage rises above 0.8V to initiate a new start-up

cycle. The new timing cycle will not start until the FILTER

pin voltage is below 0.2V. The electronic circuit breaker

is disabled during the ﬁrst timing cycle upon start-up and

any short-circuit faults will be ignored.

Figure 4. A Continuous Fault Timing

Figure 5. Multiple Intermittent Overcurrent Condition

VFILTER

CIRCUIT BREAKER TRIPS

1.253V

NORMAL

MODE

FAULT

MODE

2.4µA

60µA

4216 F04

ILOAD

VFILTER

VGATE

CIRCUIT

BREAKER

TRIPS

1.253V

FAULT

25mV/RSENSE

60µA

2.4µA

APPLICATIO S I FOR ATIO

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