Low-Cost Multichemistry Battery Chargers

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Detailed Description

The MAX1908/MAX8724/MAX8765 include all the func-

tions necessary to charge Li+ batteries. A high-efficien-

cy 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 charge current

ence of the host. For standard applications, 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

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 feature a voltage

regulation loop (CCV) and two current regulation loops

(CCI and CCS). The CCV voltage regulation loop moni-

tors BATT to ensure that its voltage does not exceed

the voltage set by VCTL. The CCI battery current regu-

lation 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 use a high-accuracy

voltage regulator for charging voltage. The VCTL input

adjusts the charger output voltage. VCTL control volt-

the adjust range to 10% of the regulation 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 termina-

tion voltage is a function of the battery chemistry.

Consult the battery manufacturer to determine this volt-

age. Connect VCTL to LDO to select the internal default

× 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

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

Float

3

4