Datasheet

14
LTC1642
1642fb
APPLICATIO S I FOR ATIO
WUU
U
The undervoltage monitor behaves differently if FB is
above its threshold when the GATE begins ramping:
RESET goes high as soon as the GATE ramp begins.
RESET goes low immediately if V
CC
falls below the chip’s
2.5V internal undervoltage lockout threshold.
To disable the undervoltage monitor, tie FB to REF and
ground RESET.
D1
1N4705
18V
GATEON
14
SENSE
15
FB
7
RESET
5
4
V
CC
16
R2
0.015Ω
Q1
FDS6630A
GND
LTC1642
8
RST TMR
3
1642 F12
R3
100Ω
R4
330Ω
C2
0.047µF
R9
95.3k
1%
R8
12.4k
1%
+
C1
0.33µF
ALL RESISTORS ±5% UNLESS NOTED.
FB COMPARATOR TRIPS AT V
OUT
= 10.7V
V
IN
12V
2.5A
V
OUT
C
LOAD
Figure 12. Undervoltage Monitoring Circuitry Figure 13. Supply Monitor Waveforms
REFERENCE CURRENT
100µA 1mA 10mA
MINIMUM REF COMPENSATION (µF)
1642 F14
0.1
0.2
0.4
1.0
2.0
4.0
10.0
Figure 14. Minimum REF Compensation vs REF Current
Reference
The LTC1642’s internal voltage reference is buffered and
brought out to the REF pin. The buffer amplifier should be
compensated with a capacitor connected between REF
and ground. If no DC current is drawn from REF, 0.1µF
ensures an adequate phase margin, but the minimum
compensation increases if REF sources a substantial DC
current, as shown in Figure 14.
t
1
t
2
t
3
t
4
t
5
250ms/DIV 1642 F09
V
IN
20V/DIV
V
RSTTMR
1V/DIV
V
RESET
10V/DIV