LTC4210 -1/LTC4210 -2
14
421012fa
For more information www.linear.com/LTC4210-1
APPLICATIONS INFORMATION
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:
t
CBDELAY
=1.3V •
TIMER
60µA
(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
aggregate 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.
C
TIMER
(s / µF)=
60µA •D
( )
– 2µA
(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 auto-retry “cool-
off” cycle begins with a 2µA pull-down for the LTC4210-1
(auto-retry version). An auto-retry cool-off delay of the
LTC4210-1 between COMP2 and COMP1 thresholds takes:
t
COOLOFF
=1.1V •
TIMER
2µA
(15)
Figure 5. A Continuous Fault Timing
Figure 7. Circuit Breaker Timer Response
for Intermittent Overload
Figure 6. Multiple Intermittent Overcurrent Condition
TIMER
NORMAL
FAULT
100µA
COMP1
CIRCUIT BREAKER
TRIPS
LATCHED OFF (5µA PULL-UP)
COMP2
60µA
4210 F05
~50mV/R
SENSE
60µA
CIRCUIT BREAKER
TRIPS
COMP1
4210 F06
I
LOAD
V
TIMER
V
GATE
2µA
CB
CB
CB
2µA
LATCHED OFF (5µA PULL-UP)
OR RETRY (2µA PULL-DOWN)
60µA
60µA
10µA 10µA
OVERLOAD DUTY CYCLE, D (%)
10
NORMALIZED RESPONSE TIME (s/µF)
30
0.01
0.1
20 100
90
807060
50
40
0
(s/µF) =
t
C
TIMER
1.3V • 1µF
60µA • D – 2µA