LT1977 Linear Technology, LT1977 Datasheet - Page 19

• Manufacturer Part Number: LT1977
• Description: Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
the PGFB pin drops below V
discharged resetting the delay period. The PGFB voltage is
typically generated by a resistive divider from the regu-
lated output or input supply.
The capacitor on the C
delay time between the PGFB pin exceeding its threshold
(V
PGFB
100k TO V
500mV/DIV
500mV/DIV
2V/DIV
V
) and the PG pin set to a high impedance state.
V
SHDN
OUT
V
PG
CT
IN
PG at 80% V
V
OUT
Figure 9. Power Good
LT1977
LT1977
U
PGFB
PGFB
Disconnect at 80% V
TIME (10ms/DIV)
with 100ms Delay
V
V
PG
PG
T
FB
C
FB
C
IN
IN
T
T
U
pin determines the amount of
OUT
PGFB
with 100ms Delay
0.27µF
0.27µF
200k
W
the C
200k
153k
12k
100k
153k
12k
100k
T
OUT
1977 F09
pin is rapidly
C
C
Figure 10. Power Good Circuits
OUT
OUT
U
V
OUT
= 3.3V
V
OUT
= 3.3V
When the PGFB pin rises above V
(I
voltage on the external capacitor reaches an internal clamp
(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
several different configurations for the LT1977 Power
Good circuitry.
LAYOUT CONSIDERATIONS
As with all high frequency switchers, when considering
layout, care must be taken in order to achieve optimal
V
CT
PG at V
OUT
CT
) from the C
), the PG pin becomes a high impedance node. The
LT1977
LT1977
Disconnect 3.3V Logic Signal
PGFB
PGFB
IN
with 100µs Delay
V
V
PG
PG
FB
C
FB
C
IN
IN
T
T
> 4V with 100ms Delay
T
511k
200k
pin into the external capacitor. When the
0.27µF
270pF
200k
200k
165k
100k
866k
100k
1977 F10
C
T
OUT
= 0.1µF. Figure 10 shows
C
V
OUT
OUT
PGFB
V
= 3.3V
OUT
current is sourced
= 12V
CT
• (V
LT1977
T
PGFB
pin will be
CT
)/(I
, C
19
CT
CT
). If
will
1977f