US3651A

Preliminary

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 9

www.unisonic.com.tw

QW-R129-014.a



OPERATION DESCRIPTION

The UTC US3651A 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 US3651A is very low. When the

Operating Current

The Operating current of UTC US3651A 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

US3651A

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 US3651A 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.