Datasheet

LTC2953
17
2953f
TYPICAL APPLICATIONS
Push Button Controlled μP Reset
The circuit of Figure 19 can be used to keep a μP in reset
for 200ms after the push button has enabled system power.
After system power has stabilized, the voltage monitor
input continues to monitor the supply at the load end.
ON/OFF
ENV
IN
VM
RST
LTC2953-2
R3
499k
R2
100k
2953 F19
PB
VM
RST
PB
R5
100k
R9
100k
R11
510k
3.3V
TO LOAD
200ms
0.5V
ON/OFF
ENV
IN
VM
RST
LTC2953-2
DC/DC
#1
R3
866k
R2
200k
2953 F20
PFI
PFO
PB
R5
100k
R9
100k
DC/DC
#2
R15
604k
R16
200k
SHDN
SHDN
V
OUT
V
OUT
V
IN
V
IN
V3
1.8V
V2
2.5V
V1
3.3V
V
TH
= 2.66V
V
TH
= 2.01V
3.3V
Push Button Controlled Supply Sequencing
The circuit in Figure 20 uses the LTC2953-2 to sequence
3 supply rails. Power on sequencing begins by pressing
the push button for 32ms. This asserts the
⎯
E
⎯
N output low,
which turns on the V1 supply. 200ms after V1 reaches 80%
of its fi nal value (2.66V), the V2 supply is enabled. When
the V2 DC voltage reaches 80% of its fi nal value (2V), the
V3 supply is enabled. Note that there is no internal delay
from the
⎯
P
⎯
F
⎯
I input to the
⎯
P
⎯
F
⎯
O output and so V3 is enabled
at the same time V2 rises above 2V.
A power down supply sequence begins when any of these
inputs is asserted:
⎯
P
⎯
B , UVLO or
⎯
K
⎯
I
⎯
L
⎯
L . When
⎯
E
⎯
N pulls up
to V
IN
, V1 disconnects fi rst. When V1 decays to 2.66V, V2
is immediately disabled (there is no 200ms delay from VM
to
⎯
R
⎯
S
⎯
T during power down). When V2 decays to 2V, V3 is
immediately disabled. See Figure 21 timing diagram.
32ms
200ms
PB
V1
EN
2953 F21
V2
V3
POWER ON POWER OFF
80%
80%
80%
80%
Figure 19. Push Button Controlled μP Reset
Figure 21. Push Button Controlled Supply Sequence Timing
Figure 20. Push Button Controlled Supply Sequencing