© 1997 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

BATTERY

CHARGE CONTROLLER

SC1766

Janaury 30, 1998

Battery Voltage Divider

To ensure proper operation of the SC1766, selection of

resistor values for the battery voltage divider must

meet the following two crucial requirements:

1. When the battery pack is disconnected from the

charge circuitry, the voltage of the VBT pin must be

higher than 4.0V or lower than 0.5V to put the SC1766

in reset status, where the VOUT and the LED pins be-

come high impedance and the FLASH pin (only for

14-pin version) goes to high level.

2. When the battery pack is connected in normal oper-

ation, the VBT pin voltage must remain in the range of

0.8V to 3.5V even when the battery pack voltage

reaches its peak when near full charge. Improper set-

ting of the VBT pin voltage may cause the VOUT pin

voltage to fluctuate due to SC1766 internal protection

scheme.

Take charging an 8-cell battery pack as an example.

The highest pack voltage would be 16V when fully

charged if the highest voltage of a fully charged battery

cell is assumed to be 2V. Since the VBT pin voltage is

restricted to no higher than 3.5V, the battery voltage

divider ratio must be higher than 3.57 (16V/3.5V -1). In

other words, resistor R14 must be greater than 535k

Ω.

When the battery pack is disconnected from the charg-

ing circuitry the VBT pin voltage must be higher than

4.0V, dictating VBAT node voltage of the charging cir-

cuitry to be higher than 17.8V (3.9V x 4.57). Practically,

however, the charging circuit supply voltage +VIN

should be higher than 18.5 in order to ensure proper

operation. In case the supply voltage +VIN can not

meet this requirement, the application circuit of the fol-

lowing figure can be adopted to get around this prob-

lem.

PRINCIPLES OF OPERATION (cont.)

Experimental Results

Experiments have been conducted to verify the

SC1766 operation with NiMH and NiCd battery cells of

various brands. Actual results shown in the figures be-

low clearly indicate that the negative delta voltage de-

tector and the peak voltage timer of the SC1766 have

precisely detected the tiny cell voltage drops or the cell

voltage peaks and consequently terminated the fast

charge process after batteries are fully charged. The

battery cell temperatures were all under safety levels.

Note that the fast charge for the NiMH battery in the

figure “Charge Characteristics of NiMH Battery” is ter-

minated by the peak voltage timer (0

∆V) while the fast

charge for the NiCd battery in the figure “Charge Char-

acteristics of NiCd Battery” is terminated by the -

∆V

detector.

Battery Voltage can be raised to

VIN = 0.9V when being charged.