LTC3455_15 Datasheet, PDF (15/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

APPLICATIONS INFORMATION
ON2 19
PBSTAT 23
ON 24
PWRON 22
V_BAT 9
+
3V –
LTC3455/LTC3455-1
LTC3455/LTC3455-1
SWITCHER 2
ENABLED
SWITCHER 1
ENABLED
WALLFB 11
+
1.23V –
CHARGER
ENABLED
USB 8
+
3.9V –
SUSPEND 6
USB POWER
CONTROLLER
ENABLED
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Figure 3. Turn-On Logic Diagram
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/LTC3455-1
are 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/LTC3455-1.
the 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 whenever the
LTC3455/LTC3455-1 are first turned on, either by the ON
or PWR pins, or by the application of external power. The
RST signal is also pulled low whenever the entire LTC3455/
LTC3455-1 are in shutdown, ensuring no false starts for
the microcontroller as the output voltages are rising or
collapsing.
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
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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