lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 22

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

LT1976/LT1976B
APPLICATIO S I FOR ATIO
(I
voltage on the external capacitor reaches an internal clamp
(V
22
CT
CT
100k TO V
500mV/DIV
500mV/DIV
) from the C
), the PG pin becomes a high impedance node. The
2V/DIV
V
V
SHDN
V
OUT
PG
CT
IN
PG at 80% V
T
V
pin into the external capacitor. When the
OUT
Figure 9. Power Good
LT1976
LT1976
U
PGFB
PGFB
Disconnect at 80% V
V
with 100ms Delay
V
TIME (10ms/DIV)
PG
PG
FB
C
FB
C
IN
IN
T
T
U
OUT
with 100ms Delay
0.27μF
0.27μF
200k
W
200k
153k
12k
100k
153k
12k
100k
OUT
1976 F09
C
C
Figure 10. Power Good Circuits
OUT
OUT
V
U
OUT
= 3.3V
V
OUT
= 3.3V
V
resultant PG delay time is given by t = C
the voltage on the PGFB pin drops below its V
be discharged rapidly and PG will be active low with a
200μA sink capability. If the SHDN pin is taken below its
threshold during normal operation, the C
discharged and PG inactive, resulting in a non Power Good
cycle when SHDN is taken above its threshold. Figure 9
shows the power good operation with PGFB connected to
FB and the capacitance on C
a limited amount of drive capability and is susceptible to
noise during start-up and Burst Mode operation. If erratic
operation occurs during these conditions a small filter
capacitor from the PGOOD pin to ground will ensure
proper operation. Figure 10 shows several different con-
figurations for the LT1976 Power Good circuitry.
OUT
PG at V
LT1976
LT1976
Disconnect 3.3V Logic Signal
PGFB
PGFB
IN
with 100μs Delay
V
V
PG
PG
FB
C
FB
C
IN
IN
> 4V with 100ms Delay
T
T
511k
200k
0.27μF
270pF
200k
200k
165k
100k
866k
100k
1976 F10
T
C
OUT
= 0.1μF. The PGOOD pin has
C
V
OUT
OUT
= 3.3V
V
OUT
= 12V
CT
• (V
T
PGFB
pin will be
CT
)/(I
, C
CT
CT
1976bfg
). If
will