UPSRB04

Preliminary

CMOS IC

UNISONIC TECHNOLOGIES CO., LTD

5 of 7

www.unisonic.com.tw



OPERAT I ON DESCRI PT I ON (Cont .)

Figure4. Secondary current waveform

In CC operation, the CC loop control function of UTC UPSRB04 will keep a fixed proportion between secondary

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

2

1

=

T

T

SW

demag

(2)

Thus the output current is given by:

PK

S

P

SW

demag

PK

S

P

out

I

×

N

N

×

4

1

=

T

T

×

I

×

N

N

×

2

1

=

I

(3)

Programmable Cable drop Compensation

UTC UPSRB04 has a built-in cable voltage drop compensation to achieve good load regulation. An offset voltage

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

the output load current. The voltage drop across the cable is compensated by this offset voltage at FB 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 UPSRB04. 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 500ns leading-edge

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

needed.