LOW COST POWER-SAVING MODE PWM CONTROLLER FOR FLYBACK CONVERTERS

DESCRIPTION

The UTC UC3838 provides a CCM switching mixed mode

operation for better efficiency performance. The operation mode

stays at CCM at heavy load, and switch to power-saving mode at

light load.

The UTC UC3838 is a high performance current mode PWM

controller ideally suited for low standby power. Low VCC startup

current make the power reliable on startup design and a large

value resistor could be used in the startup circuit to minimize the

standby power. At no load condition, the IC operates in

power-saving mode for lower standby power, decreasing frequency

for Higher conversion efficiency at light load condition.

The UTC UC3838 contains protection with automatic recovery

including OLP (over load protection), OCP (cycle-by-cycle current

limiting), and UVLO (VCC over voltage clamp and under voltage

lockout). It also provides the protections including OTP (over

temperature protection), OVP (VCC or DC output over voltage

protection) with automatic recovery. To protect the power

MOSFET, Gate-drive output is fixed up to 17V max.

The internal slope compensation improves system stability at

high PWM duty cycle output. Leading-edge blanking on current

sense input removes the signal glitch, which offering minima

external component count in the design. Excellent EMI

performance is achieved with UTC proprietary frequency hopping

technique together with soft driver control. Audio noise is

eliminated due to switch frequency more than 20kHz during

operation.

UTC UC3838 is packaged by using tiny SOT-26 package. It has

such applications as: battery charger, power adaptor, set-top box

power supplies, ink jet printers, open-frame SMPS.

FEATURES

* Proprietary frequency hopping for Improved EMI performance

* Cycle-by-cycle current limiting

* CCM Power-saving mode Switching Operation

* Fixed switch frequency 65kHz TYP.

* Dynamic peak current limiting for constant output power

* Built-in synchronized slope compensation

* Gate output voltage clamped at 17V

* Adjustable DC output OVP/OTP

* OLP/VCC OVP/Internal OTP (automatic recovery)

* Internal Soft Start

* OTP through a NTC (latch mode)