NiCd/NiMH Battery

Fast-Charge Controllers

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13

terminal. In this case, use the configuration shown in

Figure 9b. Thermistors T2 and T3 can be replaced by

standard resistors if absolute temperature charge cut-

off is acceptable. All resistance values in Figures 9a

and 9b should be chosen in the 10k

Ωto 500kΩrange.

__________Applications Information

Switch-Mode Operation

For applications where the power dissipation in the

pass transistor cannot be tolerated (ie., where heat

sinking is not feasible or is too costly), a switch-mode

charger is recommended.

Switch-mode operation can be implemented simply by

using the circuit of Figure 19. The circuit of Figure 19

uses the error amplifier at the CC pin as a comparator

with the 33pF capacitor adding hysteresis. Figure 19 is

shown configured to charge two cells at 1A. Lower

charge currents and a different number of cells can be

PGM0–PGM3 connections (Tables 2 and 3).

The input power-supply voltage range is 8V to 15V and

must be at least 2V greater than the peak battery

voltage, under fast charge. As shown in Figure 19, the

source should be capable of greater than 1.3A of

output current. The source requirements are critical

because if violated, proper termination of the fast-

charge cycle might not occur. For input voltages

greater than 15V, see the MAX713SWEVKIT data sheet.

MAX712

MAX713

0.022µF

0.022µF

NOTE: FOR ABSOLUTE TEMPERATURE CHARGE CUTOFF, T2 AND T3 CAN BE

REPLACED BY STANDARD RESISTORS.

1µF

TEMP

TLO

IN THERMAL

CONTACT WITH

BATTERY

AMBIENT

TEMPERATURE

AMBIENT

TEMPERATURE

T3

+2.0V

T2

T1

R3

R4

R5

HOT

REF

THI

BATT-

COLD

Figure 9a. Temperature Comparators

MAX712

MAX713

0.022µF

0.022µF

NOTE: FOR ABSOLUTE TEMPERATURE CHARGE CUTOFF, T2 AND T3 CAN BE

REPLACED BY STANDARD RESISTORS.

1µF

TEMP

TLO

AMBIENT

TEMPERATURE

AMBIENT

TEMPERATURE

IN THERMAL

CONTACT WITH

BATTERY

T3

+2.0V

T1

T2

R4

R3

R5

HOT

REF

THI

BATT-

COLD

Figure 9b. Alternative Temperature Comparator Configuration

11

6

0

200

600

1000

7

10

400

LOAD CURRENT (mA)

800

9

8

120Hz RIPPLE

LOW PEAK

HIGH PEAK

Figure 10. Sony Radio AC Adapter AC-190 Load Characteristic,

9VDC 800mA