© 1997 SEMTECH CORP.

652 MITCHELL ROAD NEWBURY PARK CA 91320

BATTERY

CHARGE CONTROLLER

SC1766

Janaury 30, 1998

PRINCIPLES OF OPERATION

Battery Properties

The basic principle of rechargeable battery systems,

including NiMH and NiCd cells, is that the processes of

charge and discharge are reversible. The charge char-

acteristics of NiMH and NiCd cells look similar in that

1) the cell voltage at the end of charge drops and 2)

the cell temperature increases rapidly near the end of

charge. The figure below shows the charge voltage

and charge temperature characteristics of NiMH and

NiCd cells. Notice that the cell voltage decline of NiMH

cells at the end of charge is less pronounced than for

NiCd cells.

Fast battery chargers are available where recharging

takes only 1 hour or less with a simple control circuit.

One main purpose of the control circuit is to terminate

the fast charge process to prevent the temperature and

internal pressure of the battery cell from building to a

damaging level which degrades or even destroys the

battery cell.

The SC1766 is a battery fast charge controller IC that

utilizes the following methods to terminate the fast

charge process for NiMH or NiCd battery cells:

1

Negative delta voltage cut-off (-

∆V ),

2

Peak voltage timer cut-off (0

∆V ),

3

Maximum temperature cut-off (TCO),

4

Maximum voltage cut-off (VCO),

5

Safety timer cut-off.

The principle of operation of the SC1766 is described

in the following section.

SC1766 Operation

When power is first applied to the charge system, con-

sisting of rechargeable battery cells, charge current

source, the SC1766 and its associated external circuit,

all internal digital circuit blocks of the SC1766 are reset

by internal power-on-reset circuitry. The internal control

unit then checks the battery condition to prevent fast

charge from taking place under battery fault conditions,

i.e. cell voltage fault (VBT< 0.65V or VBT> 3.7V) or cell

temperature fault (

αNTC > αNTCL or αNTC < αNTCH

for the 14-pin version). Temperature fault limits corre-

sponding to

αNTCL and αNTCH are determined by an

external thermistor divider circuit as included in the typ-

ical application circuit. After the battery passes condi-

tion fault checks, the V

charge while the initial timer and safety timer of the

SC1766 start counting. Both the negative delta voltage

detector and the peak voltage timer, however, are dis-

abled until the initial stage of a charge cycle elapses.

The SC1766 constantly monitors the voltage at the

VBT pin, which rises as battery cells are being fast-

charged until the battery full condition is nearly ap-

proached. The battery temperature is also constantly

sensed to guard against abnormal temperature situa-

tions. The V

pull-up device and the fast charge process will be

switched to trickle charge when one of the following

situations is encountered:

detected compared to its peak value (

−∆V),

duration determined by the peak voltage timer set-

ting (0

∆V),

voltage protection limit (VCO),

divider, exceeds the fault temperature range

(TCO),

TIMER PIN

The timer pin can be used as follows to select one of

the preset safety timer periods and its corresponding

periods of initial timer and peak voltage timer:

TIMER pin Safety timer

Peak V timer

Initial timer

VDD

40 min.

2 min.

1.5 min.

GND

80 min.

4 min.

3 min.

Floating

160 min.

8 min.

3 min.