LTC3455EUF datasheet(13/24 Pages) LINER

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LTC3455

3455f

APPLICATIO S I FOR ATIO

ceramic capacitance may be connected to the VMAX pin

without difficulty. More than 50

µF 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 VMAX Pin to Power Other Devices

The VMAX 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 VMAX will always draw power from the best available

input power source.

The internal PMOS connecting VMAX to the battery is

current limited to 900mA at startup (to minimize in-rush

current) and to 4A once VMAX has risen close to the battery

voltage. Because of the reduced startup current limit, the

turn-on of other devices powered from VMAX should

always be delayed to minimize the currrent initially needed

from the VMAX 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 VMAX pin has

reached the VBAT pin voltage. The VMAX pin is discharged

to ground when the LTC3455 is shut down, so that any

device supplied by VMAX 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 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 VMAX 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 VMAX pin has reached the VBAT

pin voltage.

Figure 2. Momentary Push-Button Operation

ON2

PWRON

PBSTAT

SWITCHER 2

ENABLED

SWITCHER 1

ENABLED

LTC3455

3455 F02

PUSH

BUTTON

µC

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