AN1593

3

MOTOROLA ANALOG IC APPLICATIONS INFORMATION

non–inverting input of the transconductance amplifier inside

the MC33341. This voltage is divided by resistor divider R1,

R2 to the 1.2 V internally fixed level Vth. By this arrangement

the battery charger output voltage threshold can be set.

Moreover, in the low–side current sensing configuration

(refer to Figure 3) this threshold level can be externally

adjusted over a range of 0 to 1.2 V with respect to the U2

ground at Pin 4. The maximum battery charger output voltage

Vreg (the voltage at the point A with respect to Pin 4 of U2)

can be calculated as follows:

Vreg + Vth

R2

R1

) 1 + 1.2 R2

R1

) 1

The current control loop is closed by connecting Pin 8 of

U2 directly to the feedback input of the LM2575 (Pin 4 of U1).

Under normal working conditions this pin is held at 1.23 V,

resistor R4 is added to convert the MC33341 output current

to this voltage. The diode D2 protects the batteries against

discharge through U2 when the power source U1 is switched

off. For the 1N4001 diode, used in this design, the typical

forward voltage drop is 0.9 V. This value must be added to the

voltage of three fully charged battery cells in series when the

output voltage threshold level is chosen. Capacitor C3 is

used for frequency compensation of an internal

transconductance amplifier.

The circuit shown in Figure 2 provides high efficiency

battery charging with protection against short circuit

accomplished by the LM2575 internal current limiting.

Because it is possible to set the output voltage threshold by a

simple resistor divider, various types of battery cells, as well

as various number of cells, can be charged. Nonetheless,

this circuit has some limitations. In the high–side current

sensing configuration, shown in Figure 2

, the VCC Pin 7 of the

MC33341 is connected to the output of the power supply

circuit. Such a configuration offers the advantage of a

common return path for both ICs, the LM2575–ADJ and the

MC33341, but it has also a drawback. The low limit of the

MC33341 supply voltage is 1.9 V. That implies that charging

of a single 1.2 V cell may not be possible (depending on the

voltage drop on D2, if used). Also the current limit in the case

of fully discharged cells or shorted output is given only by the

internal current limiting of the LM2575, as mentioned above.

This drawback can be solved by using the circuit shown in

Figure 3.

Improved Battery Charger

This circuit is very similar to the previous one. It uses the

same source IC, the LM2575 and also the same charger

control IC, the MC33341 but now in the low–side current

sensing configuration.

The second difference is a different connection of the VCC

pin of U2 which is the supply voltage pin of the MC33341.

Now this VCC pin is connected directly to the unregulated dc

input voltage through the supply current limiting resistor R5

and resistor R6. The use of the coupling capacitor C4 is

essential to assure a stable operation of the whole system.

C4 transfers the ac part of the LM2575 output voltage (or the

LM2575 output ripple voltage) through the MC33341 VCC

Pin 7 and output Pin 8 into the feedback Pin 4 of the LM2575.

The way the LM2575 operates makes this connection

necessary. Since the maximum allowable supply voltage of

the MC33341 is 18 V, the Zener diode D3 has to be used to

clamp the supply voltage of the MC33341to its operating limit

when the input voltage exceeds that value. Use of such an

arrangement assures that the charging control circuit U2 will

always have a supply voltage high enough, even under short

circuit conditions at the output of the battery charger. Switch

S1 can be used the same way as in the previous design.

Switch S2 is used to alter the output voltage threshold. When

S2 is in position “1”, the voltage threshold on Pin 5 is set

internally to 1.2 V and consequently the output voltage

threshold can be set only by the resistor divider R1, R2.

Switching S2 to position “2” allows an external control of the

feature contributes to the universality of this battery charger.