Single Stage PFC and Flyback LED Controller

ICL8002G

Functional Description

Data Sheet

13

Rev1.0, 2012-02-28

4.2

Soft-start

At the time ton, the IC begins to operate with a soft-start. By this soft-start the switching stresses for the switch,

diode and transformer are minimised. The soft-start implemented in ICL8002G is a digital time-based function. The

preset soft-start time is 12ms with 4 steps. If not limited by other functions, the peak voltage on CS pin will increase

step by step from 0.32 V to 1 V finally.

Figure 5

Maximum current sense voltage during softstart

4.3

Normal Operation

The PWM controller during normal operation consists of a digital signal processing circuit including a comparator,

and an analog circuit including a current measurement unit and a comparator. The switch-on and -off time points

are each determined by the digital circuit and the analog circuit, respectively. As input information for the switch-

on determination, the zero-crossing input signal is needed, while the voltages sense signal at pin VR and the

PWM controller in normal operation are illustrated in the following paragraphs.

4.3.1

Zero Crossing

In the system, the voltage from the auxiliary winding is applied to the zero-crossing pin through a RC network,

which provides a time delay to the voltage from the auxiliary winding. Internally, this pin is connected to a clamping

network, a zero-crossing detector, an output overvoltage detector and a ringing suppression time controller.

During on-state of the power switch a negative voltage applies to the ZCV pin. Through the internal clamping

network, the voltage at the pin is clamped to -0.3V.

The voltage

To achieve the switch-on at voltage valley, the voltage from the auxiliary winding is fed to a time delay network

(the RC network consists of Dzc, Rzc1, Rzc2 and Czc as shown in typical application circuit) before it is applied

to the zero-crossing detector through the ZC pin. The needed time delay to the main oscillation signal Dt should

be approximately one fourth of the oscillation period (by transformer primary inductor and drain-source capacitor)

369

12

0.32

0.49

0.66

0.83

1.00

Vcs_sst

(V)

Time(ms)