Low-Cost Multichemistry Battery Chargers

18

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Setting the Charging-Current Limit

The ICTL input sets the maximum charging current. The

current is set by current-sense resistor RS2, connected

between CSIP and CSIN. The full-scale differential

voltage between CSIP and CSIN is 75mV; thus, for a

0.015

Ω sense resistor, the maximum charging current

is 5A. Battery-charging current is programmed with

ICTL using the equation:

The MAX1908/MAX8724 shut down if ICTL is forced

does not.

Connect ICTL to LDO to select the internal default full-

scale, charge-current sense voltage of 45mV. The

charge current when ICTL = LDO is:

where RS2 is 0.015

Ω, providing a charge-current set

point of 3A.

The current at the ICHG output is a scaled-down replica

of the battery output current being sensed across CSIP

and CSIN (see the

Current Measurement section).

When choosing the current-sense resistor, note that the

voltage drop across this resistor causes further power

loss, reducing efficiency. However, adjusting ICTL to

reduce the voltage across the current-sense resistor

can degrade accuracy due to the smaller signal to the

input of the current-sense amplifier. The charging-

current-error amplifier (GMI) is compensated at CCI

(see the

Compensation section).

Setting the Input Current Limit

The total input current (from an AC adapter or other DC

source) is a function of the system supply current and

the battery-charging current. The input current regulator

limits the input current by reducing the charging

current when the input current exceeds the input

current-limit set point. System current normally fluc-

tuates as portions of the system are powered up or

down. Without input current regulation, the source must

be able to supply the maximum system current and the

maximum charger input current simultaneously. By using

the input current limiter, the current capability of the AC

adapter can be lowered, reducing system cost.

The MAX1908/MAX8724/MAX8765/MAX8765A limit the

battery charge current when the input current-limit

threshold is exceeded, ensuring the battery charger

does not load down the AC adapter voltage. An internal

amplifier compares the voltage between CSSP and

resistive divider between REF and GND. Connect CLS

to REF for the full-scale input current limit. The CLS volt-

age range for the MAX1908/MAX8724 is from 1.6V to

REF, while the MAX8765/MAX8765A CLS voltage is

from 1.1V to REF.

The input current is the sum of the device current, the

charger input current, and the load current. The device

current is minimal (3.8mA) in comparison to the charge

and load currents. Determine the actual input current

required as follows:

where

η is the efficiency of the DC-DC converter.

the maximum allowable input current. Calculate the

input current limit as follows:

Once the input current limit is reached, the charging

current is reduced until the input current is at the

desired threshold.

When choosing the current-sense resistor, note that the

voltage drop across this resistor causes further power

loss, reducing efficiency. Choose the smallest value for

RS1 that achieves the accuracy requirement for the

input current-limit set point.

Conditioning Charge

The MAX1908 includes a battery-voltage comparator

that allows a conditioning charge of overdischarged Li+

battery packs. If the battery-pack voltage is less than

3.1V

× number of cells programmed by CELLS, the

MAX1908 charges the battery with 300mA current when

using sense resistor RS2 = 0.015

Ω. After the

battery voltage exceeds the conditioning charge thresh-

old, the MAX1908 resumes full-charge mode, charging

to the programmed voltage and current limits. The

MAX8724/MAX8765/MAX8765A do not offer this feature.

AC Adapter Detection

Connect the AC adapter voltage through a resistive

divider to ACIN to detect when AC power is available,

as shown in Figure 1. ACIN voltage rising trip point is

output and is high impedance when ACIN is less than

I

V

VRS

INPUT

CLS

REF

=×

0 075

1

.

II

IV

V

INPUT

LOAD

CHG

BATT

IN

=+

×

×

⎛

⎝⎜

⎞

⎠⎟

η

I

V

RS

0 045

2

.

I

V

VRS

CHG

ICTL

REFIN

=×

0 075

2

.