LTC3455EUF datasheet(21/24 Pages) LINER

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LTC3455

3455f

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 is 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

is shut down.

Gain Block

The LTC3455 contains 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 13. 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.

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 is turned on. This

APPLICATIO S I FOR ATIO

is not a problem for most applications since the LTC3455

usually powers the microcontroller and all other intelli-

gence 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 compo-

nent placement. The VBAT capacitor, VMAX capacitor, and

both inductors must all be placed as close as possible to

the LTC3455. These components, along with both DC/DC

converter output capacitors, should be placed on the same

side of the circuit board as the LTC3455, with their

connections made on that top layer. Place a local, unbro-

ken ground plane below these components that is tied to

the exposed pad of the LTC3455. The exposed pad (pin 25)

must be soldered to the PCB (to system ground) for proper

operation.

Figure 13. Low-Battery Detector and LDO Using the Gain Block

1.8V

LBO

LOW-BATTERY

DECTECTOR

VBAT

2.49M

806k

3.3V

100k

3455 F14

LDO

2.5V

169k

100pF

80.6k

LTC3455

µF

TYPICAL APPLICATIO S

Standalone USB Power Supply

with Temporary Backup Power

Although designed primarily for Li-Ion powered portable

applications, the LTC3455 is also a good choice for

systems that are always powered by a USB supply or wall

adapter. The battery charger can then be used to charge up

a large capacitor or backup battery, which briefly provides

power to the system after the external power has been

removed. This gives the microcontroller enough time to

follow proper shutdown procedures when the main power

source is abruptly removed. Figure 14 shows a standalone

power supply for USB high power applications (500mA

maximum USB current) using the LTC3455. The total

system power should be kept below 1.8W to ensure clean

operation even under worst-case USB conditions. With

the resistor values shown, the low-battery indicator (AI

and AO pins) triggers when the VMAX pin voltage drops to

4V, notifying the microcontroller of an impending dropout

condition. The 1M

Ω resistor connected between the AI

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