Datasheet

13
LTC4210-1/LTC4210-2
421012f
Circuit Breaker Timer Operation
When a current limit fault is encountered at time point A in
Figure 5, the circuit breaker timing is activated with a 60µA
pull-up. The circuit breaker trips at time point B if the fault
is still present and the TIMER pin voltage reaches the
COMP2 threshold and the LTC4210 shuts down. For a
continuous fault, the circuit breaker delay is:
tV
C
A
CBDELAY
TIMER
=
µ
13
60
.•
(13)
Intermittent overloads may exceed the current limit as in
Figure 6, but if the duration is sufficiently short, the TIMER
pin may not reach the COMP2 threshold and the LTC4210
will not shut down. To handle this situation, the TIMER
discharges with 2µA whenever (V
CC
– SENSE) voltage is
below the 50mV limit and the TIMER voltage is between
the COMP1 and COMP2 thresholds. When the TIMER
voltage falls below the COMP1 threshold, the TIMER pin is
discharged with an equivalent 7k resistor (normal mode,
100µA source) when (V
CC
– SENSE) voltage is below the
50mV limit. If the TIMER pin does not drop below the
COMP1 threshold, any intermittent overload with an ag-
gregate duty cycle of more than 3.8% will eventually trip
the circuit breaker. Figure 7 shows the circuit breaker
response time in seconds normalized to 1µF. The asym-
metric charging and discharging of TIMER is a fair gauge
of MOSFET heating.
t
C
sF
VF
AD A
TIMER
(/ )
.•
•–
µ=
µ
µ
()
µ
13 1
60 2
(14)
When the circuit breaker trips, the GATE pin is pulled low.
The TIMER enters latchoff mode with a 5µA pull-up for the
LTC4210-2 (latched-off version), while an autoretry “cool-
off” cycle begins with a 2µA pull-down for the LTC4210-1
(autoretry version). An autoretry cool-off delay of the
LTC4210-1 between COMP2 and COMP1 thresholds takes:
tV
C
A
COOLOFF
TIMER
=
µ
11
2
.•
(15)
APPLICATIO S I FOR ATIO
WUUU
V
TIMER
NORMAL
MODE
FAULT
MODE
100µA
COMP1
CIRCUIT BREAKER
TRIPS
LATCHED OFF (5µA PULL-UP)
OR RETRY (2µA PULL-DOWN)
COMP2
AB
60µA
4210 F05
Figure 5. A Continuous Fault Timing
OVERLOAD DUTY CYCLE, D (%)
10
NORMALIZED RESPONSE TIME (s/µF)
30
1
4210 F07
0.01
0.1
20 100
90
807060
50
40
0
(s/µF) =
t
C
TIMER
1.3V • 1µF
60µA • D – 2µA
Figure 7. Circuit Breaker Timer Response
for Intermittent Overload
A1 B1 A2 B2 A3 B3
~50mV/R
SENSE
60µA
CIRCUIT BREAKER
TRIPS
COMP1
4210 F06
COMP2
I
LOAD
V
TIMER
V
GATE
2µA
CB
FAULT
CB
FAULT
CB
FAULT
2µA
LATCHED OFF (5µA PULL-UP)
OR RETRY (2µA PULL-DOWN)
60µA
60µA
10µA 10µA
Figure 6. Mulitple Intermittent Overcurrent Conditon