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:

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 cur-

rent-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

fluctuates 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 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 CSSN to the voltage at

REF and GND. Connect CLS to REF for the full-scale

input current limit.

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

threshold, the MAX1908 resumes full-charge mode,

charging to the programmed voltage and current limits.

The MAX8724 does 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

can drive a P-channel MOSFET directly at the charger

input, providing a lower dropout voltage than a

Schottky diode (Figure 2).

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

.

Low-Cost Multichemistry Battery Chargers

16

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