LTC3455(RevA) 查看數據表（PDF） - Linear Technology

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LTC3455
(Rev.:RevA)

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

Linear Technology 

LTC3455

APPLICATIO S I FOR ATIO

Soft-Start for each Switcher

Soft-start is accomplished by gradually increasing the

peak inductor current for each switcher. This allows each

output to rise slowly, helping minimize the battery in-rush

current. Figure 10 shows the battery current during startup.

A soft-start cycle occurs whenever each switcher first

turns on, or after a fault condition has occurred (thermal

shutdown or UVLO).

In-Rush Current Limiting

When the LTC3455 is battery-powered, an internal 0.15Ω

PMOS switch connects the battery (VBAT pin) to the VMAX

pin to provide power for both switchers and other internal

circuitry. This PMOS switch is turned off in shutdown, and

the VMAX pin discharges to ground, providing output

disconnect for all outputs. At startup, this PMOS must first

charge up any capacitance present on the VMAX pin to the

battery voltage. To minimize the in-rush current needed

from the battery, the PMOS switch is current-limited to

900mA and both switchers are disabled while the VMAX

voltage is ramping up. Once VMAX reaches the battery

voltage, the PMOS current-limit increases to 4A and both

switchers are allowed to turn on. Figure 10 shows the

startup battery current for the LTC3455, which stays well-

controlled while VMAX is ramping up and while both

switchers outputs are rising.

Battery Charger General Information

The battery charger and Switcher 1 will always be enabled

whenever USB or wall power is present (as sensed by the

VMAX

2V/DIV

VOUT1 (1.8V)

2V/DIV

VOUT2 (3.3V)

2V/DIV

IBAT

500mA/DIV

100µs/DIV

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Figure 10. In-Rush Current at Startup

USB and WALLFB pins). This ensures that the battery can

be charged and that the microcontroller is alive whenever

external power is available. For some applications, it may

be undesirable for the charger to become active immedi-

ately when external power is applied. For such applica-

tions, an NMOS switch can be used to disconnect the

RPROG resistor and allow the PROG pin to float high,

turning off the charger. In this manner, charging occurs

only when allowed by the microcontroller.

The LTC3455 battery charger is a constant-current, con-

stant-voltage charger. In constant-current mode, the maxi-

mum charge current is set by a single external resistor.

When the battery approaches the final float voltage, the

charge current begins to decrease as the charger switches

to constant-voltage mode. The charge cycle is terminated

only by the charge timer.

Charge and Recharge Cycles

When external power is first applied, a new charge cycle is

always initiated. The battery will continue charging until

the programmed charge time is reached. If the battery

voltage is below 4.05V at the end of this cycle, the

LTC3455 will start a new charge cycle. This action will

continue until the battery voltage exceeds the 4.05V thresh-

old. This operation is typically seen only when charging

from USB power. Because the charge current can vary

dramatically when the LTC3455 is USB powered, it takes

considerably longer to charge a battery using the USB

supply (as compared to a wall adapter). If the timer

capacitor is chosen correctly, the battery should be fully

charged on one cycle when wall power is available.

If the battery is above the 4.05V threshold when a charge

cycle has expired, charging will stop. At this point, a

recharge cycle is initiated if any of the following occurs:

The battery voltage drops below 4.05V, external power is

removed and reapplied, the PROG pin is floated tempo-

rarily, or the SUSPEND pin is temporarily pulled high (if the

LTC3455 is under USB power).

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