LTC3455 Datasheet, PDF (14/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
APPLICATIO S I FOR ATIO
ON2 19
PBSTAT 23
ON 24
PWRON 22
V_BAT 9
+
3V –
LTC3455
SWITCHER 2
ENABLED
SWITCHER 1
ENABLED
WALLFB 11
+
1.23V –
CHARGER
ENABLED
USB 8
+
3.9V –
SUSPEND 6
Figure 3. Turn-On Logic Diagram for LTC3455
USB POWER
CONTROLLER
ENABLED
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Startup and Shutdown When USB or Wall Powered
Whenever USB or wall power is present (as sensed by the
USB and WALLFB pins), Switcher 1 and the battery
charger will always be enabled. If the LTC3455 is off and
external power is applied, both the charger and Switcher
1 will start independent of the state of the ON and PWRON
pins. This provides maximum battery run-time by always
allowing the battery to charge whenever external power is
available, and ensures that the microcontroller is always
alive when external power is available (this is important for
designs that utilize coulomb-counting or other battery
monitoring techniques). Switcher 2 starts only if ON2 is
also pulled high. Figure 3 shows the turn-on logic diagram
for the LTC3455.
output sequencing when both switchers are enabled at
startup with the ON2 pin tied to V_MAX. The turn-on of both
switchers is always delayed until the V_MAX pin has reached
the V_BAT pin voltage.
Reset Signal (RST)
A 200ms reset signal (the RST pin is pulled low) is
provided for proper initialization of a microcontroller when-
ever the LTC3455 is first turned on, either by the ON or
PWRON pins, or by the application of external power. The
RST signal is also pulled low whenever the LTC3455 is in
shutdown, ensuring no false starts for the microcontroller
as the output voltages are rising or collapsing. In the event
of a fault condition the RST pin will be pulled low.
Starting Switcher 2/Power Supply Sequencing
Switcher 2 can operate only when Switcher 1 is also
enabled and in regulation. The ON2 pin can be driven by a
logic signal for independent control of Switcher 2. If both
outputs always operate together, tie the ON2 pin to the
V_MAX pin. This will enable Switcher 2 after the output of
Switcher 1 has reached 90% of its final value. This power-
up delay ensures proper supply sequencing and reduces
the peak battery current at startup. Figure 4 shows the
PWRON/ON2
2V/DIV
V_MAX
2V/DIV
V_OUT1 (1.8V)
2V/DIV
V_OUT2 (3.3V)
2V/DIV
100µs/DIV
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Figure 4. Sequencing for Switcher 1 and 2 Outputs
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