AN1593

4

MOTOROLA ANALOG IC APPLICATIONS INFORMATION

Figure 3. Low Cost Switching Regulator Performs Constant–Current/Constant–Voltage 3 Cell Charging Function.

Version with Full Short–Circuit Regulation Control Capability.

R4

1.0 k

W

L1

400

mH

D1

1N5819

C2

330

mF/16 V

ON/OFF

Control

(from Controller)

+

+

+Vin

ON/OFF

R3

27

W

R2

39

W

R1

10 k

W

C3

33 nF

RS

0.22

W

3 Battery Cells

Under Charge

Charge Current

Control

(from Controller)

–VO

+VO

+

–

U1

LM2575–ADJ

U2

MC33341

Gnd

C1

100

mF/50 V

Feedback

Output

1

2

4

5

3

D2

1

24

5

3

6

7

8

S1

1

2

A

R5

560

W

R6

100

W

D3

1N4001

1N4745A

Threshold Voltage

Control

(from Controller)

S2

1

2

C4

680 nF

Unregulated

DC – Input

Vin = 10 to 40 V

+

Gnd

Figure 4. Constant–Voltage/Constant–Current Charging Characteristic

of the Improved Current Source for Battery Chargers

IO, OUTPUT CURRENT (A)

0.4

0

0.2

0.6

0.8

1.0

0

1

2

3

4

5

6

Figure 4 shows the V/I charging characteristic of the

improved circuit (see schematic in Figure 3). The small

voltage drop in the beginning of the V/I characteristic is

caused by dynamic resistance of the diode D2. This

phenomenon can be eliminated either by reconnection of the

resistor R2 from the point “A” to the cathode of the diode D2,

or this diode might be replaced by shorting wire. Both actions

would cause also another effect on the V/I characteristic of

this circuit. When the output voltage drops below

approximately 1.2 V, the output current will fall down

accordingly, thus creating a typical “foldback” V/I

characteristic.