USRB02

Preliminary

`

LINEAR INTEGRATED CIRCUIT

UNISONICTECHNOLOGIESCO.,LTD

4 of 7

www.unisonic.com.tw

QW-R103-087.a



OPERATION DESCRIPTION

The UTC USRB02 is a primary control unit for 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. It uses Pulse Frequency Modulation (PFM) and Pulse Width

Modulation (PWM) method to build discontinuous conduction mode (DCM) flyback power supplies.

Startup control

Operating Current

The Operating current of UTC USRB02 is as low as 720μA. 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

USRB02

captures the auxiliary winding feedback voltage at FB 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 2.5V reference voltage and the error is

amplified. The error amplifier output reflects the load condition and controls the Toff time to regulate the output

voltage, thus constant output voltage can be achieved.

Constant Current Operation

When the sampled voltage is below 2.5V reference voltage and the error amplifier output reaches its maximum,

thus UTC USRB02 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.