DETAILED DESCRIPTION

BATTERY VOLTAGE REGULATION

x

x

VADJ

BAT

REF

V

V

= cell count

4V + 0.512

V

é

ù

æ

ö÷

ç

ê

ú

÷

ç

÷

ê

ú

ç

÷

çè

ø

ê

ú

ë

û

(1)

BATTERY CURRENT REGULATION

x

SRSET

CHARGE

REF

SR

V

0.10

I

=

V

R

(2)

INPUT ADAPTER CURRENT REGULATION

bq24705

The bq24705 uses a high-accuracy voltage regulator for charging voltage. The internal default battery voltage

of the regulation voltage, the external resistor mismatch error is reduced from ±1% to ±0.1%. Therefore, an

overall voltage accuracy as good as 0.5% is maintained, while using 1% mis-match resistors. Ratio-metric

conversion also allows compatibility with D/As or microcontrollers (

µC). The battery voltage is programmed

through VADJ and VREF using Equation 1.

CELLS pin is the logic input for selecting cell count. Connect CELLS to charge 2,3, or 4 Li+ cells. When charging

other cell chemistries, use CELLS to select an output voltage range for the charger.

CELLS

CELL COUNT

Float

2

AGND

3

VREF

4

The per-cell battery termination voltage is function of the battery chemistry. Consult the battery manufacturer to

determine this voltage.

The BAT pin is used to sense the battery voltage for voltage regulation and should be connected as close to the

battery as possible, or directly on the output capacitor. A 0.1-

µF ceramic capacitor from BAT to AGND is

recommended to be as close to the BAT pin as possible to decouple high frequency noise.

between SRP and SRN. The full-scale differential voltage between SRP and SRN is 100 mV. Thus, for a 0.010

Ω

sense resistor, the maximum charging current is 10 A. SRSET is ratio-metric with respect to VREF using

Equation 2:

The input voltage range of SRSET is between 0 and VREF, up to 3.3 V.

values can also be used. A larger the sense resistor, gives a larger sense voltage, and a higher regulation

accuracy, but at the expense of higher conduction loss.

The total input from an AC adapter or other DC sources is a function of the system supply current and the battery

charging current. System current normally fluctuates as portions of the systems are powered up or down. Without

Dynamic Power Management (DPM), the source must be able to supply the maximum system current and the

maximum charger input current simultaneously. By using DPM, the input current regulator reduces the charging

current when the input current exceeds the input current limit set by ACSET. The current capability of the AC

adapter can be lowered, which may reduce the system cost.

and ACN. The maximum value is set by ACSET, which is a ratio-metric with respect to VREF, using Equation 3.

Copyright © 2007–2009, Texas Instruments Incorporated

Submit Documentation Feedback

15

Product Folder Link(s) :bq24705

Not Recommended for New Designs