UCSR3654S

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

7 of 9

www.unisonic.com.tw

QW-R129-009.a



OPERATION DESCRIPTION (Cont.)

Constant Current Operation

When the sampled voltage is below 2.0V reference voltage and the error amplifier output reaches its maximum,

thus UTC UCSR3654S operates in constant-current (CC) mode. The CC point and maximum output power can be

externally adjusted by external current sense resistor Rcs. The larger Rcs, the smaller CC point is, and the smaller

output power becomes.

Large

Rcs

Small

Rcs

Figure 4. Secondary current waveform

In CC operation, the CC loop control function of UTC UCSR3654S 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 UCSR3654S 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.