Datasheet

LTC3677-3
37
36773f
operaTion
Power-Up via Pushbutton Timing
The timing diagram, Figure 13, shows the LTC3677-3
powering up through application of the external pushbut-
ton. For this example the pushbutton circuitry starts in
the POFF state with V
OUT
not in UVLO and Buck1, Buck2
and LDO2 disabled. Pushbutton application (ON LOW) for
50ms transitions the pushbutton circuitry into the PUP
state which sequences up LDO2, Buck1 and Buck2 in that
order. PWR_ON must be driven HIGH before the 1 second
PUP period is over to keep supplies up. If PWR_ON is
LOW or goes LOW after the 1 second PUP period Buck1,
Buck2, and LDO2 will be shut down together. PGOOD is
asserted once Buck1, Buck2 and LDO1 are within 8% of
their regulation voltage for 230ms.
Buck3 can be enabled and disabled at any time via EN3
once in the PUP or PON states. The PWR_ON input can
be driven via a μP/μC or by one of the sequenced outputs
through a high impedance (100kΩ typ). PBSTAT goes
LOW 50ms after the initial pushbutton application and
will stay LOW for 50ms minimum. PBSTAT will go HIGH
coincident with ON going HIGH unless ON goes HIGH
before the 50ms minimum LOW time.
Power-Up via PWR_ON Timing
The timing diagram, Figure 14, shows the LTC3677-3
powering up by driving PWR_ON HIGH. For this example
the pushbutton circuitry starts in the POFF state with V
OUT
not in UVLO and Buck1, Buck2 and LDO2 disabled. 50ms
after PWR_ON goes HIGH the pushbutton circuitry transi-
tions into the PUP state which sequences up LDO2, Buck1
and Buck2 in that order. PWR_ON must be driven high
before the 1 second PUP period is over to keep supplies
up. If PWR_ON is LOW or goes LOW after the 1 second
PUP period Buck1, Buck2 and LDO2 will be shut down
together. PGOOD is asserted once Buck1, Buck2 and LD01
are within 8% of their regulation voltage for 230ms.
Buck3 can be enabled and disabled at any time via EN3
once in the PUP or PON states.
Powering up via PWR_ON is useful for applications
containing an always on microcontroller. This allows the
microcontroller to power the application up and down
for house keeping and other activities outside the user’s
control.
V
OUT
UVLO
PBSTAT
LDO2
BUCK1
BUCK2
PGOOD
PWR_ON
STATE POFF PUP PON
36773 F13
ON (PB)
50ms
230ms
14ms
1 SEC
Figure 13. Power-Up via Pushbutton
V
OUT
UVLO
PBSTAT
LDO2
BUCK1
BUCK2
PGOOD
PWR_ON
STATE POFF PUP PON
36773 F14
ON (PB)
1 SEC
50ms
14ms
230ms
Figure 14. Power-Up via PWR_ON