UPSRB02 Datasheet, PDF (4/7 Pages) Unisonic Technologies – HIGH PRECISION CC/CV PRIMARY SIDE SWITCHING REGULATOR

UPSRB02
Preliminary
LINEAR INTEGRATED CIRCUIT
 OPERATION DESCRIPTION
The UTC UPSRB02 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
The V_DD pin of UTC UPSRB02 is connected to the line input through a resistor. A large value startup resistor can
be used to minimize the power loss in application because the start current of UTC UPSRB02 is very low. When the
V_DD voltage reaches V_{TH (ON)}, the internal startup circuit is disabled and the IC turns on.
Operating Current
The Operating current of UTC UPSRB02 is as low as 400μA. 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
UPSRB02 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_AUX
=
N_A
N_S
×（V_O
+
∆V）
(1)
Where △V indicates the drop voltage of the output diode.
Figure2. Auxiliary voltage waveform
Via a resistor divider connected between the auxiliary winding and FB, the V_AUX is sampled at the Tsampling end
and it is hold until the next sampling. The sampled voltage is compared with 2V 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 2V reference voltage and the error amplifier output reaches its maximum, thus
UTC UPSRB02 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.
UNISONIC TECHNOLOGIES CO., LTD
www.unisonic.com.tw
4 of 7
QW-R103-079.a