Datasheet

17
LTC4221
4221fa
duty cycle of more than 1.8% will eventually trip the circuit
breaker. Figure 8 shows the circuit breaker response time
in seconds normalized to 1μF. The asymmetric charging and
discharging of FILTER is a fair gauge of MOSFET heating.
t
CF
V
AD A
FILTER
μ
()
=
μ
()
μ
124
105 1 8
.
•–.
(2)
The fast comparators trip the circuit breaker to protect
against fast load overcurrents if V
RSENSE
is greater than
V
SENSE(FC)
(100mV) for 1μs. The response time of each fast
comparator is fixed at 1μs nominal. The timing diagram in
Figure 9 illustrates the operation of the LTC4221 when the
load current conditions cause V
RSENSE
of channel 1 to ex-
ceed 100mV for more than 1μs between time points 7 and
8. Figure 9 also illustrates when the LTC4221’s electronic
circuit breaker is armed. After the initial timing cycle, it is
armed at time point 3. Arming the circuit breaker at time
point 3 ensures that the system is protected against an over-
current condition during the channel start-up cycle. At time
point 4, the slow comparators are armed when the internal
control loop is disengaged.
Autoretry After a Fault
Once the LTC4221 circuit breaker is tripped, FAULT is
latched low and both GATE pins are pulled to ground. To
clear the internal FAULT latch and to restart the LTC4221,
its ON1 pin must be pulsed below its reset threshold
(V
ON(RESET)
= 0.4V) for at least 15μs.
APPLICATIO S I FOR ATIO
WUUU
TIMER
FAULT
FILTER
ON
n
0.851V
GATE
n
SENSE n
V
OUT n
RESET INITIAL TIMING
21
1.9μA
V
TMR(L)
V
TMR(H)
ELECTRONIC CIRCUIT BREAKER ARMED
SLOW COMPARATORS
ARMED
3
20μA
20μA
1.8μA
0.4V
105μA
9.5μA
V
FILTER(TH)
V
SENSE(FC)
V
SENSE(SC)
45 6 7 8 9
RESET
4221 F09
CHANNEL
START-UP
NORMAL FAULT
Figure 9. Fast Comparator Trip Timing Waveforms
Figure 8. Circuit Breaker Filter Response
for Intermittent Overload
OVERLOAD DUTY CYCLE, D (%)
NORMALIZED RESPONSE TIME (s/μF)
1
0.01
0.1
0 102030405060708090100
4221 F08
t
C
FILTER
(μF)
1.24V
105 • D – 1.8
=