5

LNK520

E

2/05

Output Voltage

Tolerance envelope

without optocoupler

Inherent

CC to CV

transition

point

Load variation

during battery

charging

Voltage

feedback

threshold

Characteristic with

optocoupler

Typical inherent

characteristic without

optocoupler

PI-2788-092101

Output Current

Output Voltage

Output Current

Tolerance envelope

without optocoupler

Characteristic with

optocoupler

Power supply peak

output power curve

Typical inherent

characteristic without

optocoupler

PI-2790-112102

Inherent

CC to CV

transition

point

Load variation

during battery

charging

Characteristic observed with

load variation often applied during

laboratory bench testing

Voltage

feedback

threshold

Figure 7. Influence of the Optocoupler on the Power Supply Output Characteristic.

Figure 8. Output Characteristic with Optocoupler Regulation (Reduced Voltage Feedback Threshold).

is the characteristic seen if the voltage feedback threshold is

above the output voltage at the inherent CC to CV transition

point also indicated in Figure 7.

Figure 8 shows a case where the voltage feedback threshold

is set below the voltage at the inherent CC to CV transition

point. In this case, as the output voltage rises, the secondary

feedback circuit takes control before the inherent CC to CV

transition occurs. In an actual battery charging application, this

simply limits the output voltage to a lower value. However, in

laboratory bench tests, it is often more convenient to test the

power supply output characteristic starting from a low output

current and gradually increasing the load. In this case, the

optocoupler feedback regulates the output voltage until the

peak output power curve is reached as shown in Figure 8. Under

theseconditions,theoutputcurrentwillcontinuetoriseuntilthe

peak power point is reached and the optocoupler turns off. Once

the optocoupler is off, the CONTROL pin feedback current is

determined only by R1 and the output current therefore folds

back to the inherent CC characteristic as shown. Since this type

of load transition does not normally occur in a battery charger,

theoutputcurrentneverovershootstheinherentconstantcurrent

value in the actual application.

In some applications it may be necessary to avoid any output

currentovershoot,independentofthedirectionofloadvariation.