Datasheet

LTC4216
11
4216fa
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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 V
FILT(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:
t
VC
A
s
CB TRIP()
.•
=
µ
+µ
1 253 3
60
20
(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 V
FILT(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
C
sF
D3
1 253
60
24
(/ )
.
(•)– .
µ=
(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.
Figure 4. A Continuous Fault Timing
Figure 5. Multiple Intermittent Overcurrent Condition
V
FILTER
CIRCUIT BREAKER TRIPS
A B
1.253V
NORMAL
MODE
FAULT
MODE
2.4µA
60µA
4216 F04
A1
I
LOAD
V
FILTER
V
GATE
CIRCUIT
BREAKER
TRIPS
1.253V
B1
CB
FAULT
CB
FAULT
CB
FAULT
A2 B2 A3 B3
25mV/R
SENSE
60µA
60µA
60µA
2.4µA
2.4µA
2.4µA