ACE4703

Fully 5A, 3cell Standalone Li-ion Battery Charger

VER 1.2

9

Gate Drive

The ACE4703

’s gate driver can provide high transient currents to drive the external pass transistor. The

rise and fall times are typically 40ns when driving a 2000pF load, which is typical for a P-channel

MOSFET with Rds(on) in the range of 50mΩ.

A voltage clamp is added to limit the gate drive to 8V max. below VCC. For example, if VCC is 20V, then

the DRV pin output will be pulled down to 12V min. This allows low voltage P-channel MOSFETs with

superior Rds(on) to be used as the pass transistor thus increasing efficiency.

Loop Compensation

In order to make sure that the current loop and the voltage loop are stable, the following compensation

components are necessary:

(1) A 470pF capacitor from the COM1 pin to GND

(2) A series 220nF ceramic capacitor and 120

Ω resistor from the COM2 pin to GND

(3) An 100nF ceramic capacitor from the COM3 pin to GND

Battery Detection

ACE4703 does not provide battery detection function, when the battery is not present, the charger

charges the output capacitor to the regulati

on voltage quickly, then the BAT pin’s voltage decays slowly to

recharge threshold because of low leakage current at BAT pin, which results in a ripple waveform at BAT

pin, in the meantime, CHRG

pin outputs a pulse to indicate that the battery’s absence. The pulse’s

frequency is around 10Hz when a 10uF output capacitor is used.

It is generally not a good practice to connect a battery while the charger is running. The charger may

provide a large surge current into the battery for a brief time.

Input and Output Capacitors

Since the input capacitor is assumed to absorb all input switching ripple current in the converter, it must

have an adequate ripple current rating. Worst-case RMS ripple current is approximately one-half of output

charge current.

The selection of output capacitor is primarily determined by the ESR required to minimize ripple voltage

and load step transients. Generally speaking, a 10uF ceramic capacitor can be used.

Inductor Selection

During P-

channel MOSFET’s on time, the inductor current increases, and decreases during P-channel

MOSFET’s off time, the inductor’s ripple current increases with lower inductance and higher input voltage.

Higher inductor ripple current results in higher charge current ripple and greater core losses. So the

inductor’s ripple current should be limited within a reasonable range.

The inductor’s ripple current is given by the following formula: