USR3651S

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 9

www.unisonic.com.tw

QW-R103-085.E



OPERATION DESCRIPTION

The UTC USR3651S is a primary control switch mode charger and adapter applications. It operates in

primary-side sensing and regulation. Opto-coupler and TL431 could be eliminated. Proprietary built-in CV and CC

control can achieve high precision CV/CC performance. The device integrates PWM controller to enhance the

performance of discontinuous conduction mode (DCM) flyback converters.

Startup control

be used to minimize the power loss in application because the start current of UTC USR3651S is very low. When the

Operating Current

The Operating current of UTC USR3651S is as low as 1.6mA. Good efficiency and very low standby power can be

achieved.

Constant Voltage Operation

The output voltage is defined by the transmission ratio between the secondary and auxiliary winding. The UTC

USR3651S

captures the auxiliary winding feedback voltage at INV pin and operates in constant-voltage (CV) mode

to regulate the output voltage. The auxiliary voltage reflects the output voltage is given by:

）

（

V

∆

+

V

×

N

N

=

V

O

S

A

AUX

(1)

△

Where

V indicates the drop voltage of the output diode.

Figure 2. Auxiliary voltage waveform

and it is hold until the next sampling. The sampled voltage is compared with 2.0V reference voltage and the error is

output voltage, thus constant output voltage can be achieved.

Constant Current Operation

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

thus UTC USR3651S 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.