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LTC4216I

Ultralow Voltage Hot Swap Controller

Linear Technology 

LTC4216

Applications Information

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 FAULT 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:

tCB(TRIP)

=

1.253V • C3

60µA

+

20µs

(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.

t

C3

(s

/ µF)

=

1.253

(60•D)– 2.4

(3)

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

and FAULT 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.

VFILTER

A

B

1.253V

CIRCUIT BREAKER TRIPS

60µA 2.4µA

NORMAL FAULT

MODE MODE

4216 F04

Figure 4. A Continuous Fault Timing

ILOAD

VFILTER

VGATE

A1

B1

A2

B2

A3

B3

25mV/RSENSE

60µA

60µA

2.4µA

1.253V

2.4µA

60µA

2.4µA

CIRCUIT

BREAKER

TRIPS

CB

FAULT

CB

FAULT

CB

FAULT

Figure 5. Multiple Intermittent Overcurrent Condition

For more information www.linear.com/LTC4216

4216fa

11

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