Low-Cost Multichemistry Battery Chargers

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Detailed Description

The MAX1908/MAX8724/MAX8765/MAX8765A include

all the functions necessary to charge Li+ batteries. A

high-efficiency synchronous-rectified step-down DC-DC

converter controls charging voltage and current. The

device also includes input-source current limiting and

analog inputs for setting the charge current and charge

voltage. Control charge current and voltage using the

ICTL and VCTL inputs, respectively. Both ICTL and

VCTL are ratiometric with respect to REFIN, allowing

compatibility with DACs or microcontrollers (µCs).

Ratiometric ICTL and VCTL improve the accuracy of the

tions, internal set points for ICTL and VCTL provide 3A

charge current (with 0.015

Ω sense resistor), and 4.2V

(per cell) charge voltage. Connect ICTL and VCTL to

LDO to select the internal set points. The MAX1908

safely conditions overdischarged cells with 300mA (with

0.015

Ω sense resistor) until the battery-pack voltage

exceeds 3.1V

× number of series-connected cells. The

SHDN input allows shutdown from a microcontroller or

thermistor.

The DC-DC converter uses external n-channel

MOSFETs as the buck switch and synchronous rectifier

to convert the input voltage to the required charging

current and voltage. The

Typical Application Circuit

shown in Figure 1 uses a µC to control charging cur-

rent, while Figure 2 shows a typical application with

charging voltage and current fixed to specific values

for the application. The voltage at ICTL and the value of

RS2 set the charging current. The DC-DC converter

generates the control signals for the external MOSFETs

to regulate the voltage and the current set by the VCTL,

ICTL, and CELLS inputs.

The MAX1908/MAX8724/MAX8765/MAX8765A feature

a voltage regulation loop (CCV) and two current regula-

tion loops (CCI and CCS). The CCV voltage regulation

loop monitors BATT to ensure that its voltage does not

exceed the voltage set by VCTL. The CCI battery cur-

rent regulation loop monitors current delivered to BATT

to ensure that it does not exceed the current limit set by

ICTL. A third loop (CCS) takes control and reduces the

battery-charging current when the sum of the system

load and the battery-charging input current exceeds

the input current limit set by CLS.

Setting the Battery-Regulation Voltage

The MAX1908/MAX8724/MAX8765/MAX8765A use a

high-accuracy voltage regulator for charging voltage.

The VCTL input adjusts the charger output voltage.

voltage. By limiting the adjust range to 10% of the regu-

lation voltage, the external resistor mismatch error is

reduced from 1% to 0.05% of the regulation voltage.

Therefore, an overall voltage accuracy of better than

0.7% is maintained while using 1% resistors. The per-

cell battery termination voltage is a function of the bat-

tery chemistry. Consult the battery manufacturer to

determine this voltage. Connect VCTL to LDO to select

× number of

cells, or program the battery voltage with the following

equation:

CELLS is the programming input for selecting cell count.

Connect CELLS as shown in Table 2 to charge 2, 3, or 4

Li+ cells. When charging other cell chemistries, use

CELLS to select an output voltage range for the charger.

The internal error amplifier (GMV) maintains voltage

regulation (Figure 3). The voltage error amplifier is

compensated at CCV. The component values shown in

Figures 1 and 2 provide suitable performance for most

applications. Individual compensation of the voltage reg-

ulation and current regulation loops allows for optimal

compensation (see the

Compensation section).

V

CELLS

V

V

V

BATT

VCTL

REFIN

=×

+

×

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

40 4

.

Table 2. Cell-Count Programming

DESCRIPTION

MAX1908

MAX8724

MAX8765/

MAX8765A

Conditioning

Charge Feature

Yes

No

No

ICTL Shutdown

Mode

Yes

Yes

No

ACOK Enable

Condition

REFIN must

be ready

REFIN must

be ready

Independent

of REFIN

Table 1. Versions Comparison

CELLS

CELL COUNT

GND

2

Open

3

4