Datasheet

LTC4211
21
4211fb
OPERATION
GATE
TIMER
V
CC
12 3 45 6
GATE
V
FB
< V
REF
>50mV
V
REF
10μA
4211 F10
t
RESET
t
FAULTSC
2μA
FPD
V
OUT
78 B 9 9A 1
FAST COMPARATOR ARMED
CIRCUIT BREAKER TRIPS
CIRCUIT BREAKER RESET
SLOW COMPARATOR ARMED
V
OUT
V
CC
– V
SENSE
FILTER
FAULT
RESET
ON
ONON
V
SENSE
= 50mV
REGULATING
LOAD CURRENT
Figure 10. Power-Up in Overcurrent, Slow Comparator Trips the Circuit Breaker
ADJUSTING SLOW COMP’S RESPONSE TIME
The response time of SLOW COMP is adjusted using a
capacitor connected from the LTC4211’s FILTER pin to
ground. If this pin is left unused, SLOW COMP’s delay
defaults to 20μs. During normal operation, the FILTER
output pin is held low as an internal 10μA pull-down
current source is connected to this pin by transistor M4.
This pull-down current source is turned off when an
overcurrent load condition is detected by SLOW COMP.
During an overcurrent condition, the internal 2μA pull-
up current source is connected to the FILTER pin by
transistor M5, thereby charging C
FILTER
. As the charge
on the capacitor accumulates, the voltage across C
FILTER
increases. Once the FILTER pin voltage increases to 1.236V,
the electronic circuit breaker trips and the LTC4211’s
GATE pin is switched quickly to ground by transistor M3.
After the circuit breaker is tripped, M5 is turned OFF, M4
is turned ON and the 10μA pull-down current then holds
the FILTER pin voltage low.
The SLOW COMP response time from an overcurrent fault
condition to when the circuit breaker trips (GATE OFF) is
given by Equation 7:
t
SLOWCOMP
= 1.236V •
C
FILTER
2µA
+ 20µs
(7)
For example, if C
FILTER
= 1000pF, SLOW COMP’s response
time = 638μs. As a design aid, SLOW COMP’s delay time
(t
SLOW COMP
) versus C
FILTER
for standard values of C
FILTER
from 100pF to 1000pF is illustrated in Table 2.