LTC3455(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3455
(Rev.:RevA)

Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

Linear Technology 

LTC3455

APPLICATIO S I FOR ATIO

Handling Large Transient Loads when USB Powered

Many portable devices have nominal loads that can easily

be supported by the USB supply, but they have brief

transient loads that can exceed the maximum available

USB power. The LTC3455 is designed to handle these

overloads while drawing as much power as possible from

the USB port. If the USB bus is providing power but the

LTC3455 (or any other devices connected to the VMAX pin)

needs more total power than the USB bus can supply, the

battery charger turns off completely and the USB power

controller becomes a 500mA (or 100mA) current source

and the VMAX voltage begins to decrease. At this point, the

capacitance connected to the VMAX pin provides the addi-

tional current needed by the system. As long as the USB

pin stays above 3.9V, the USB bus will continue to provide

as much current as possible. Once the VMAX pin drops just

below the VBAT voltage, the battery will provide the addi-

tional current needed. This operation allows the LTC3455

to withstand load transients that briefly demand more

power than can be provided by the USB bus.

The oscilloscope photographs in Figure 1 show how the

LTC3455 handles load transients when USB powered. The

top photo shows a brief transient load that turns off the

charger but does not dip the VMAX voltage. The bottom

photo shows a prolonged transient condition that turns off

the charger and completely dips the VMAX voltage to the

point where the battery must provide current. For both

cases, normal operation resumes as soon as the transient

passes.

Extra capacitance can be connected to the VMAX pin to act

as a reservoir to help support large transient currents. For

most systems this is not necessary, as the LTC3455

cleanly handles heavy transients. For some designs, how-

ever, it may be desirable to use a larger capacitor con-

nected to VMAX to act as a larger reservoir. Up to 50µF of

ceramic capacitance may be connected to the VMAX pin

without difficulty. More than 50µF 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.

VMAX

2V/DIV

IMAX

500mA/DIV

IUSB

500mA/DIV

IBAT

500mA/DIV

100µs/DIV

3455 F01a

USB Maximum Current Condition

VMAX

2V/DIV

IMAX

500mA/DIV

IUSB

500mA/DIV

IBAT

500mA/DIV

100µs/DIV

3455 F01b

USB Heavy Over-Current Condition

Figure 1. Handling Load Transients when USB Powered

3455fa

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