UC3846
LINEAR INTEGRATED CIRCUIT
OPERATION DESCRIPTION
The UTC UC3846 devices integrate many useful designs into one controller for low-power switch-mode power
supplies. The following descriptions highlight some of the features of the UTC UC3846 series.
Start-up Current
The start-up current is only 22μA. Low start-up current allows a start-up resistor with a high resistance and a
low-wattage to supply the start-up power for the controller. For AC/DC adaptor with universal input range design, a 1
MΩ, 1/8 W startup resistor could be used together with a VDD capacitor to provide a fast startup and low power
dissipation solution.
Power-Saving Mode Operation
The proprietary Power-Saving Mode function provides linearly decrease the switching frequency under light-load
conditions for higher efficiency. The feedback voltage, which is sampled from the voltage feedback loop, is taken as
the reference. Once the feedback voltage exceeds the threshold voltage, the switching frequency starts to decrease.
This Power-Saving Mode function dramatically reduces power consumption under light-load conditions. The
frequency control also eliminates the audio noise at any loading conditions.
Burst Mode Operation
At zero load condition, majority of the power dissipation in a switching mode power supply is from switching loss
on the MOSFET transistor, the core loss of the transformer and the loss on the snubber circuit. The magnitude of
power loss is in proportion to the number of switching events within a fixed period of time. Reducing switching events
leads to the reduction on the power loss and thus conserves the energy. The UTC UC3846 enter burst mode at
standby condition to minimize the switching loss and reduces the standby power consumption. The frequency control
also eliminates the audio noise at any loading conditions. Power supplies using the UTC UC3846 can easily meet
even the strictest regulations regarding standby power consumption.
Switch Frequency Set
The maximum switch frequency is set to 68kHz. Switch frequency is modulated by output power POUT during IC
operating. At no load or light load condition, most of the power dissipation in a switching mode power supply is from
switching loss on the MOSFET transistor, the core loss of the transformer and the loss on the subber circuit. The
magnitude of power loss is in proportion to the number of switching events within a fixed period of time. So lower
switch frequency at lower load, which more and more improve IC’s efficiency at light load. At from no load to light
load condition, The IC will operate at from Burst mode to Reducing Frequency Mode. The relation curve between fSW
and POUT/POUT (MAX) as followed FIG.1.
FIG.1 The relation curve between fSW and relative output power POUT/ POUT (MAX)
UNISONIC TECHNOLOGIES CO., LTD
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QW-R103-053.B
www.unisonic.com.tw
UTC [ Unisonic Technologies ]
