ltc3455euf-trpbf Linear Technology Corporation, ltc3455euf-trpbf Datasheet - Page 15

• Manufacturer Part Number: ltc3455euf-trpbf
• Description: Dual Dc/dc Converter With Usb Power Manager And Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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.
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
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
MAX
pin. This will enable Switcher 2 after the output of
U
U
SUSPEND
WALLFB
PBSTAT
PWRON
V
ON2
USB
BAT
ON
19
23
24
22
11
9
8
6
1.23V
3.9V
W
3V
Figure 3. Turn-On Logic Diagram for LTC3455
+
–
+
–
+
–
U
output sequencing when both switchers are enabled at
startup with the ON2 pin tied to V
switchers is always delayed until the V
the V
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 entire LTC3455
is in shutdown, ensuring no false starts for the
microcontroller as the output voltages are rising or
collapsing.
PWRON/ON2
V
V
OUT1
OUT2
BAT
Figure 4. Sequencing for Switcher 1 and 2 Outputs
2V/DIV
2V/DIV
2V/DIV
2V/DIV
(1.8V)
(3.3V)
V
pin voltage.
MAX
SWITCHER 2
ENABLED
SWITCHER 1
ENABLED
CHARGER
ENABLED
USB POWER
CONTROLLER
ENABLED
LTC3455
3455 F03
100µs/DIV
MAX
. The turn-on of both
MAX
LTC3455
pin has reached
3455 F04
15
3455fb