Detailed Description

The MAX1908/MAX8724 include all the functions neces-

sary to charge Li+ batteries. A high-efficiency synchro-

nous-rectified step-down DC-DC converter controls

charging voltage and current. The device also includes

input source current limiting and analog inputs for set-

ting 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 D/As

or microcontrollers (µCs). Ratiometric ICTL and VCTL

improve the accuracy of the charge current and voltage

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 feature a voltage-regulation

loop (CCV) and two current-regulation 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 current-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 use a high-accuracy voltage

regulator for charging voltage. The VCTL input adjusts

the charger output voltage. VCTL control voltage can

adjust range to 10% of the regulation voltage, the exter-

nal 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 1 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

.

Low-Cost Multichemistry Battery Chargers

12

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Table 1. Cell-Count Programming

CELLS

CELL COUNT

GND

2

Float

3

4