LTC3455
APPLICATIO S I FOR ATIO
ceramic capacitance may be connected to the V
MAX
pin
without difficulty. More than 50µF requires using a capaci-
tor with some ESR or adding some resistance in series
with some of the ceramic capacitance. This is necessary to
ensure loop stability in the battery charger loop when
under USB power.
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 switchers 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 is 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.
PBSTAT
ON
µC
PUSH
BUTTON
PWRON
ON2
Figure 2. Momentary Push-Button Operation
3455f
U
Startup and Shutdown when Battery-Powered
When only battery power is available, the LTC3455 turns
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 using a single
momentary-on push-button switch. Figure 2 shows the
method for using a momentary-on pushbutton to turn the
LTC3455 off and on.
When the momentary-on switch is first pressed, shorting
the ON pin to ground, PBSTAT goes low and the LTC3455
first brings 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 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 micro-
controller that the push-button has been pressed. The
microcontroller prepares for shutdown then pulls the
PWRON signal low. When the push-button is released, the
ON pin goes high and the LTC3455 turns 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.
LTC3455
23
24
22
19
SWITCHER 1
ENABLED
SWITCHER 2
ENABLED
3455 F02
W
U
U
13
LINER [ LINEAR TECHNOLOGY ]
