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LTC3455

3455f

Increasing 3.3V Output Current to 1.2A

With an internal current limit of 900mA, Switcher 2 typi-

cally provides a 3.3V, 600mA output. While this output

current is sufficient for many portable devices, some

applications need a 3.3V supply capable of providing more

than 1A. Figure 15 shows how to implement a higher

current 3.3V output using the LTC3455. By adding one tiny

SOT23 PMOS and using the AI/AO amplifier as an LDO, the

3.3V output now provides 1.2A of output current. Switcher

2 is programmed for an output voltage of 3.3V, and the

LDO is programmed for an output voltage of 3.2V (3%

lower). As long as the load current is low enough for

Switcher 2 to provide, the LDO is turned off completely.

This circuit is ideal for applications that need the higher

3.3V output current for only a brief time. Switcher 2 will

normally provide all of the output current, and the LDO will

turn on briefly to provide the higher load currents.

Figure 14. Standalone USB Power Supply with

Temporary Backup Power

µF

4700

µF

WALLFB

HSI

HSO

C1, C2, C3, C5, C6: X5R OR X7R CERAMIC

L1, L2: TOKO DB318C

ALL RESISTORS 1%

3.3V, HS

VMAX

Ω

4.7

µF

USB 5V

USB

SUSPEND

USBHP

SW2

FB2

249k

L2, 4.7

µH

80.6k

10pF

µF

3.3V

0.4A

SW1

FB1

100k

L1, 4.7

µH

80.6k

10pF

µF

1.8V

0.2A

TIMER

PROG

DROPOUT

VBAT

20k

10k

1.8V

VMAX

82.5k

µF

2.49k

1.8V

GND

CHRG

PBSTAT

RST

MODE

LTC3455

ON2

PWRON

HSON

3455 TA02

µC

USB

CONTROLLER

ON/OFF

TYPICAL APPLICATIO S

and AO pins provides 150mV of hysteresis (the dropout

indicator stays low until the VMAX pin rises back above

4.15V). A 4700

µF backup capacitor connected to the VBAT

pin briefly provides power to the system after the USB

supply has been removed, and also helps support tran-

sient loads that slightly exceed the USB current limit.

Connecting this large capacitance to the VBAT pin has

several advantages. It provides a large energy reservoir

that is isolated from both the USB pin (the USB specifica-

tion limits capacitance on the USB supply pin to 10

µF or

less) and the VMAX pin (using a very large capacitance on

this pin will delay the system turn-on), and it prevents

large inrush currents by using the battery charger to

slowly charge this capacitor (normally using such a large

capacitor would result in very large inrush currents). With

the TIMER pin tied to VMAX, the battery charger operates

in constant-current mode (the voltage-loop and timer

function are disabled), so the 4700

µF capacitor is always

fully charged to the available USB voltage.

Ω

µF

4.7

µF

WALL 5V

C1 TO C8: X5R OR X7R CERAMIC

L1, L2: TOKO DB318C

D1: ON SEMI MBRM110E

ALL RESISTORS 1%

3.3V, HS

Ω

4.7

µF

USB 5V

249k

L2, 4.7

µH

2.49k

80.6k

100pF

2x10

µF

3.3V

1.2A

FDN304P

Si2305DS

100k

L1, 4.7

µH

80.6k

10pF

µF

1.8V

0.4A

C8, 0.1

µF

VMAX

2.49k

1.8V

3455 F15

µC

USB

CONTROLLER

1.24k

3.32k

ON/OFF

WALLFB

HSI

HSO

VMAX

USB

SUSPEND

USBHP

SW2

FB2

SW1

FB1

TIMER

PROG

VBAT

GND

CHRG

PBSTAT

RST

MODE

LTC3455

ON2

PWRON

HSON

SINGLE

CELL Li-ION

3.3V TO 4.2V

Figure 15. LTC3455 Application with 3.3V Output Current

Increased to 1.2A

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