Switch-Mode Lithium-Ion

Battery Charger

_______________________________________________________________________________________

7

BATT

1/4

IBAT

DCIN

CURRENT

SENSE

ON

CS

SETI

CCI

VADJ

CCV

CELL0

VL

BST

VL

STATUS

REF

DHI

LX

DLO

PGND

GND

GMV

GMI

CELL1

PWM

LOGIC

5.4V

REG

4.2

REF

CELL

LOGIC

CLAMP

REF

2

THM/SHDN

Figure 2. Functional Diagram

To set currents below full scale without changing

R1, adjust the voltage at SETI according to the follow-

ing formula:

A capacitor at CCI sets the current-feedback loop’s

dominant pole. While the current is in regulation, CCV

voltage is clamped to within 80mV of the CCI voltage.

This prevents the battery voltage from overshooting

when the voltage setting is changed. The converse is

true when the voltage is in regulation and the current

setting is changed. Since the linear range of CCI or

CCV is about 2V (1.5V to 3.5V), the 80mV clamp results

in negligible overshoot when the loop switches from

voltage regulation to current regulation, or vice versa.

Monitoring Charge Current

The battery-charging current can be externally moni-

IBAT and GND. IBAT is the output of a voltage-con-

trolled current source, with output current given by:

resistor (in millivolts) given by:

2V for the maximum charging current. Connect IBAT to

GND if unused.

PWM Controller

The battery voltage or current is controlled by a

current-mode, PWM DC/DC converter controller. This

controller drives two external N-channel MOSFETs,

which control power from the input source. The con-

troller sets the switched voltage’s pulse width so that it

supplies the desired voltage or current to the battery.

Total component cost is reduced by using a dual,

N-channel MOSFET.

The heart of the PWM controller is a multi-input com-

parator. This comparator sums three input signals to

determine the switched signal’s pulse width, setting the

battery voltage or current. The three signals are the

current-sense amplifier’s output, the GMV or GMI error

amplifier’s output, and a slope-compensation signal

that ensures that the current-control loop is stable.

The PWM comparator compares the current-sense

amplifier’s output to the lower output voltage of either

the GMV or GMI amplifiers (the error voltage). This cur-

rent-mode feedback reduces the effect of the inductor

on the output filter LC formed by the output inductor

(L1) and C1 (Figure 1). This makes stabilizing the cir-

cuit much easier, since the output filter changes to a

first-order RC from a complex, second-order RLC.

V

=

0.9 A

I

x

R

R

IBAT

CHG

IBAT

1

µ