US3651A

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R129-014.a



OPERATION DESCRIPTION(Cont.)

Large

Rcs

Small

Rcs

Figure 4. Secondary current waveform

In CC operation, the CC loop control function of UTC US3651A will keep a fixed proportion between secondary

inductance de-magnetization time (Tdemag) and switching cycle time (Tsw). The fixed proportion is

2

1

=

Tsw

Tdemag

(2)

Thus the output current is given by:

PK

PK

OUT

I

S

N

P

N

4

1

Tsw

Tdemag

I

S

N

P

N

2

1

I















(3)

Programmable Cable Drop Compensation

UTC US3651A has a built-in cable voltage drop compensation to achieve good load regulation. An offset voltage

is generated at INV pin by an internal current flowing into the resistor divider. The current is inversely proportional to

the voltage of COMP pin. As a result, it is inversely proportional to the output load current. The voltage drop across

the cable is compensated by this offset voltage at INV pin. It can also be programmed by adjusting the resistance of

the divider to compensate the drop for various cable lines used.

Current Sensing and Leading Edge Blanking

Cycle-by-cycle current limiting is offered in UTC US3651A. The switch current is detected by a sense resistor into

the CS pin. When the power switch is turned on, a turn-on spike will occur on this resistor. A 900ns leading-edge

blanking is built in to avoid false-termination of the switching pulse so that the external RC filtering is no longer

needed.