DER-33

3.9W CC/CV TNY266P Charger

April 1, 2004

Page 7 of 24

Power Integrations

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5.5

Output Voltage Sensing and Feedback

Transistor Q1, resistors R4, R5, R6, R7, R8, R9, diode D8, Zener diode VR2 and opto-

isolator U2 form the CV, CC, and cable drop compensation circuit. Q1, R6, R7, R8, R9,

VR2, D8 and U2 comprise the Constant Voltage (CV) mode control loop and cable

compensation control loop while R4, R5 and U2 make up the Constant Current (CC)

mode control loop.

CC Mode Operation

The CC mode set-point is determined by the voltage drop on the optocoupler LED and

the voltage drop on R5. The voltage drop on R4 is quite small and can be ignored. The

TinySwitch-II has an EN pin current that is very constant with power delivery, so therefore

the current in the optocoupler LED is very constant. For this reason the CC set-point

does not change with load voltage.

CV Mode and Cable Drop Compensation Operation

The CV mode set-point is set by the voltage drops on VR1, R7, and the Vbe of Q1. The

voltage on R7 depends on the operation of the cable drop compensation circuit. In order

to have a regulated voltage at the end of the cable, the load current produces a voltage

drop on R9 which feeds to the Base of Q1, through R8. The net effect is that the voltage

set-point increases as the load increases, canceling the voltage drop in the output cable.

D6 provides temperature compensation for the temperature coefficient of Q1.