Datasheet

LTC3553
24
3553fb
OPERATION
regulator, the LDO’s load capability remains unchanged.
However, the LDO’s transient response is slowed, as il-
lustrated in Figure 5 and Figure 6.
measures should be taken to ensure that the buck is not
operated outside the specifi ed BVIN input supply range,
as operation beyond this range is not guaranteed.
LDO Regulator UVLO Considerations
The LDO regulator’s bias current is supplied via an internal
connection to the USB PowerPath V
OUT
pin. The V
OUT
UVLO
shuts down the LDO when V
OUT
drops below about 2.6V
in order to prevent the LDO from operating incorrectly
due to too low a bias supply voltage.
The LDO power input pin, V
INLDO
, can be driven with as
little as 1.65V. There is, however, no UVLO to enforce
this requirement. It is thus recommended that V
INLDO
be
tied to either the buck regulator output (programmed to
regulate at least 1.65V), or to the USB PowerPath V
OUT
pin, to ensure proper operation.
PUSHBUTTON INTERFACE
State Diagram/Operation
Figure 7 shows the LTC3553 pushbutton state diagram.
The pushbutton state machine has a clock with a 1.82ms
period.
Upon fi rst application of power, V
BUS
or BAT, an inter-
nal power on reset (POR) signal places the pushbutton
circuitry into the power-down (PDN1) state. One second
Figure 7. Pushbutton State Diagram
3553 F07
PUP2
PDN1
PDN2
HRST
HRST
HRST
POR
UVLO AND
EITHER BUCK_ON
OR LDO_ON
BUCK_ON
OR LDO_ON
EXTPWR OR
PB400MS
1SEC
5SEC
5SEC
1SEC
PON
PUP1
HR
UVLO OR
BOTH BUCK_ON
AND LDO_ON
EXTPWR OR
PB400MS
POFF
Figure 5. LDO Load Step Response in Normal Operation
LDO REGULATING 3.3V
4.7µF OUTPUT CAPACITOR
STBY LOW
LDO OUTPUT
VOLTAGE
AC-COUPLED
0.1V/DIV
150mA
5mA
I
LDO
3553 F05
50µs/DIV
LDO REGULATING 3.3V
4.7µF OUTPUT CAPACITOR
STBY HIGH
LDO OUTPUT
VOLTAGE
AC-COUPLED
0.1V/DIV
150mA
5mA
3553 F06
50µs/DIV
I
LDO
V
OUT
UNDERVOLTAGE LOCKOUT (V
OUT
UVLO)
An undervoltage lockout circuit on the USB PowerPath
V
OUT
pin shuts down and prevents both the buck and
the LDO from enabling when the V
OUT
pin voltage drops
below about 2.6V.
Buck Regulator UVLO Considerations
It is recommended that the buck regulator input supply
(BVIN pin) be connected directly to the USB PowerPath
output (V
OUT
pin). With this connection, the V
OUT
UVLO
prevents the buck regulator from operating at low input
supply voltages where loss of regulation or other unde-
sirable operation may occur. In applications where the
buck input is supplied from other than the V
OUT
pin, other
Figure 6. LDO Load Step Response in Standby Mode