USRB01

Preliminary

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LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

5 of 7

www.unisonic.com.tw

QW-R103-080.b



OPERATION DESCRIPTION (Cont.)

Iout

Vout

Large

Rcs

Small

Rcs

Figure3. Adjustable output power by changing Rcs

Figure4. Secondary current waveform

In CC operation, the CC loop control function of UTC USRB01 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 USRB01 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

voltage of VC 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 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 USRB01. 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.