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PL3325BD_ASEMI电源芯片2020版资料(英)

作者:深圳市强元芯电子有限公司 来源:asemi888 发布时间:2020-01-04 浏览:53

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Application Notes PL3325B provides a cost effective solution for low power adaptor/charger applications. The innovative CV and CC control can remove the need for secondary feedback circuitry while achieving excellent CV/CC for much more strict requirements.

9.1Startup & Operating Current PL3325B has very slight startup current , thus the larger value startup resistor and smaller

VDD capacitor can be used to minimize the power loss in application.

The operating current of PL3325B is as low as 1mA. Together with ‘Muti-mode’control features we could get high efficiency especially in light load.

9.2 UnderVoltageLockout(UVLO) An UVLO detector is implemented in it to detect the voltage on the VDD pin. It would assure the

Datasheet PL3325B

V1.1 © 2019 www.pmicro.com.cn 7

supply voltage enough to turn on the PWM controller and further to drive the power mos. a hysteresis is built in to prevent the shutdown from the voltage dip during startup. The turn-on and turn-off threshold level are set at 16 V and 8 V,typically.

9.3Realization ofCV/CC The CV/CC control is based on the system working in DCM. In the DCM fly-back converter, the output voltage can be sensed via the auxiliary winding. During MOSFET turn-on time, the current in the primary winding ramps up. When MOSFET turns off, the primary current transfers to the secondary at the amplitude of

(1) PpkI is the current in the primary winding at the point MOSFET turns off.

Through coupling between the secondary and auxiliary winding the output voltage is given by V N VNsV aux aux o (2) Where aux V is the voltage of the auxiliary

winding and V indicates the drop voltage of the output diode.

Fig.2.VoltageofAuxiliary-windingWaveform

Via a resistor divider connected between the auxiliary winding and FB, the auxiliary voltage is sampled based on the internal timing control. Through the built-in error amplifier the sampled voltage could be regulated at a preset level, thus constant output voltage can be achieved.

During the CC mode of operation PL3325B will regulate the output current at a constant level regardless of the output voltage, while avoiding continuous conduction mode.

9.4 Adjustable CC Point and Output Power In application of less equal than 12W, we could change the Rs to get different CC points and the maximum output power only using one model .The larger Rs, the smaller CC point is, and the smaller output power becomes.

9.5 Switching Frequency and Lm Tolerance Compensation The system load condition and the operation mode decide the switching frequency of PL3325B.The maximum switching frequency is set by system design. In DCM assuming the efficiency to 100%, the output power is given by

OPpkswm IVoIfLPo

(3) Where Lm indicates the inductance of the primary winding and Ppk I is the peak current of the primary winding. Refer to the equation 3, the change of Lm results in the change of Po and the constant output current in the CC mode. The tolerance of Lm will make CC worse in mass production. To smooth it away, we use an internal Lm compensation circuit to correct the offset of inductance. Whatever Lm is changed to, the CC point is constant.

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