LTC3553-2_15 Datasheet, PDF (27/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
Power-Up Via Asserting the BUCK_ON Pin
Figure 8 shows the LTC3553-2 powering up by driving
BUCK_ON high. For this example the pushbutton circuitry
starts in the HR state with a battery connected. Once
BUCK_ON goes high, the pushbutton circuitry enters the
PON state and the buck powers up. Also, as the part exits
the hard reset state the LDO will power up simultaneously.
The PGOOD is initially low and will go high once both regu-
lators are within 8% of their regulation voltage for 1.8ms.
Powering up via asserting the BUCK_ON pin is useful
for applications containing an always-on μC that’s not
powered by the LTC3553-2 regulators. That μC can power
the application up and down for housekeeping and other
activities not needing the user’s control.
Power-Down by De-Asserting BUCK_ON
Figure 9 shows the LTC3553-2 powering down by μC/
μP control. For this example the pushbutton circuitry
starts in the PON state with a battery connected and both
regulators enabled. The user presses the pushbutton (ON
low) for at least 50ms, which generates a debounced, low
impedance pulse on the PBSTAT output. After receiving
the PBSTAT signal, the μC/μP software decides to drive
the BUCK_ON input low in order to power down. After
the BUCK_ON input goes low, the pushbutton circuitry
will enter the PDN2 state. In the PDN2 state a one second
wait time is initiated after which the pushbutton circuitry
enters the POFF state. During this one second time, the
ON, and BUCK_ON inputs as well as external power ap-
plication are ignored. Though the above assumes a battery
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
BUCK_ON
0
1
BUCK
0
1
LDO
0
1
PGOOD
0
STATE
HR
1.8ms
PON
35532 F08
Figure 8. Power-Up via Asserting the BUCK_ON Pin
1
BAT
0
1
V_BUS
0
1
ON (PB)
0
1
PBSTAT
0
1
PGOOD
0
1
BUCK_ON
0
1
BUCK
0
1
LDO
0
STATE
PON
50ms
1s
μC/μP CONTROL
PDN2
POFF
35532 F09
Figure 9. Power-Down via De-Assertion of BUCK_ON
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