LTC3455_15 Datasheet, PDF(14/28 Page) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

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LTC3455_15 Datasheet, PDF (14/28 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

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LTC3455/LTC3455-1

APPLICATIONS INFORMATION

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 switch-

ers is always delayed until the VMAX pin has reached the

VBAT pin voltage. The VMAX pin is discharged to ground

when the LTC3455/LTC3455-1 are 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/

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 ï¬rst pressed, short-

ing the ON pin to ground, PBSTAT goes low and the

LTC3455/LTC3455-1 ï¬rst bring 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/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 VMAX pin has

reached the VBAT pin voltage.

LTC3455/LTC3455-1

PBSTAT

Î¼C

PUSH

BUTTON

PWRON

ON2

SWITCHER 1

ENABLED

SWITCHER 2

ENABLED

3455 F02

Figure 2. Momentary Push-Button Operation

3455fc

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