Datasheet

LTC3455/LTC3455-1
14
3455fc
Figure 2. Momentary Push-Button Operation
Using the V
MAX
Pin to Power Other Devices
The V
MAX
pin can be used to provide power for other
devices within the system. This pin is connected to the
battery when no external power is available, and it is
connected to either the USB bus or the wall adapter when
either are available. This ensures that all devices powered
from V
MAX
will always draw power from the best available
input power source.
The internal PMOS connecting V
MAX
to the battery is current
limited to 900mA at startup (to minimize in-rush current)
and to 4A once V
MAX
has risen close to the battery voltage.
Because of the reduced startup current limit, the turn-on
of other devices powered from V
MAX
should always be
delayed to minimize the currrent initially needed from the
V
MAX
pin. The best choice is to enable these devices from
either switcher output, since the turn-on of both switch-
ers is always delayed until the V
MAX
pin has reached the
V
BAT
pin voltage. The V
MAX
pin is discharged to ground
when the LTC3455/LTC3455-1 are shut down, so that
any device supplied by V
MAX
will have its input grounded
during shutdown. This ensures output disconnect for all
supply voltages within the system.
Startup and Shutdown when Battery-Powered
When only battery power is available, the LTC3455/
LTC3455-1 turn on when either the ON pin is pulled low
or the PWRON pin is pulled high. Either of these pins will
keep the device running, but typically the ON and PWRON
pins are used together to provide turn-on and turn-off us-
ing a single momentary-on push-button switch. Figure 2
shows the method for using a momentary-on pushbutton
to turn the LTC3455/LTC3455-1 off and on.
When the momentary-on switch is fi rst pressed, short-
ing the ON pin to ground, PBSTAT goes low and the
LTC3455/LTC3455-1 fi rst bring up the V
MAX
pin, then
enables Switcher 1 to power the microcontroller. Once
up and running, the microcontroller provides the PWRON
signal to keep the LTC3455/LTC3455-1 turned on after the
push-button is released. When the push-button is pressed
again to turn off the device, the PBSTAT pin is pulled low
to notify the microcontroller that the push-button has been
pressed. The microcontroller prepares for shutdown then
pulls the PWRON signal low. When the push-button is re-
leased, the ON pin goes high and the LTC3455/LTC3455-1
turn off. The ON and PWRON pins enable Switcher 1 (along
with all the internal circuits needed for normal operation),
and the ON2 pin enables Switcher 2. Switcher 2 can only
operate when Switcher 1 is also enabled. The turn-on of
both switchers is always delayed until the V
MAX
pin has
reached the V
BAT
pin voltage.
APPLICATIONS INFORMATION
24
ON
23
19
ON2
22
PWRON
PBSTAT
SWITCHER 2
ENABLED
SWITCHER 1
ENABLED
LTC3455/LTC3455-1
3455 F02
PUSH
BUTTON
μC