LTC3455_15 Datasheet, PDF(23/28 Page) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

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LTC3455_15 Datasheet, PDF (23/28 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

pin to GND. Since the charge cycle is terminated only by

the charge timer, external charge termination is required

when using either of these methods. Use an external NMOS

to ï¬oat the PROG pin and disable charging.

Constant-current-only mode is a good choice for systems

that are always powered by a USB supply or wall adapter,

and the charger can be used to charge a super-cap or

backup battery. Disabling the voltage ampliï¬er allows the

super-cap/backup battery to charge up fully to the available

USB or wall adapter voltage.

Hot Swap Output

A current limited Hot Swap output is provided for power-

ing memory cards or other external devices that can be

hot-plugged into the system. Typically connected to the

3.3V supply, this output provides isolation to prevent the

external device from disturbing the 3.3V supply when

inserted. The Hot Swap output can only operate when

the LTC3455/LTC3455-1 are on, and is enabled using the

HSON pin. If this hot-plugging protection is not needed,

this output can be used as a load switch for other devices

within the system. The HSO pin is discharged to ground

when the LTC3455/LTC3455-1 are shut down.

Gain Block

The LTC3455/LTC3455-1 contain a gain block (pins AI

and AO) that can be used as either a low-battery indicator,

or as an LDO with the addition of an external PNP. Both

circuits are shown in Figure 14. The LDO is convenient for

applications needing a third output (possibly a low current

2.5V or a quiet 3V supply). The AO pin can sink around

1mA, which typically limits the LDO current to 100mA

or less (due to the current gain of the PNP). An external

PMOS can be used for the LDO, but a much larger output

capacitor is needed to ensure stability at light loads.

LOW-BATTERY

DECTECTOR

1.8V

3.3V

LBO

VBAT

LTC3455/

2.5V

LTC3455-1

2.49M

10Î¼F

806k

LDO

100k

169k

80.6k

100pF

LTC3455/

LTC3455-1

3455 F14

Figure 14. Low-Battery Detector and LDO Using the Gain Block

The gain block is alive whenever switcher 1 is enabled, and

is turned off during shutdown to minimize battery drain.

This means that the low-battery detector will not report

a low-battery condition until the LTC3455/LTC3455-1 are

turned on. This is not a problem for most applications since

the LTC3455/LTC3455-1 usually power the microcontroller

and all other intelligence in the system.

PCB Layout Considerations

As with all DC/DC converters, careful attention must be paid

to the printed circuit board (PCB) layout and component

placement. The VBAT capacitor, VMAX capacitor, and both

inductors must all be placed as close as possible to the

LTC3455/LTC3455-1. These components, along with both

DC/DC converter output capacitors, should be placed on the

same side of the circuit board as the LTC3455/LTC3455-1,

with their connections made on that top layer. Place a local,

unbroken ground plane below these components that is

tied to the Exposed Pad of the LTC3455/LTC3455-1. The

Exposed Pad (pin 25) must be soldered to the PCB (to

system ground) for proper operation. Figure 15 shows

the recommended placement for the power sections of

the LTC3455/LTC3455-1.

3455fc

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