UPSRB03

Preliminary

LINEAR INTEGRATED CIRCUIT

UNISONIC TECHNOLOGIES CO., LTD

5 of 7

www.unisonic.com.tw

QW-R129-020.a



OPERAT I ON DESCRI PT I ON (Cont .)

Figure4. Secondary current waveform

In CC operation, the CC loop control function of UTC UPSRB03 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)

Current Sensing and Leading Edge Blanking

Cycle-by-cycle current limiting is offered in UTC UPSRB03. 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.

Programmable Cable Drop Compensation

UTC UPSRB03 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.

Built-in Load and Line CC Compensation

I

n conventional PSR system, the output CC point can vary with output and line voltage. UTC UPSRB03 has

built-in blocks to compensate the variation. The device can adjust CC point based on sensed output voltage and

PFM duty. In this way, CC accuracy can be improved.