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FAN6861 • Rev. 1.0.1

10

Operation Description

Startup Operation

Figure 17 shows the typical startup circuit and

transformer auxiliary winding for FAN6861 application.

Before

FAN6861

begins

switching

operation,

it

consumes only startup current (typically 8μA) and the

current supplied through the startup resistor charges

and the current consumed increases to 3mA. Then, the

power required is supplied from the transformer

provides more holdup time, which allows using small

connected to AC line for a fast reset of latch protection.

Figure 17. Startup Circuit

Green-Mode Operation

indicator of the output load and modulates the PWM

frequency, as shown in Figure 18, such that the

switching frequency decreases as load decreases. In

heavy load conditions, the switching frequency is

PWM frequency starts to linearly decrease from 65KHz

is fixed at 22.5kHz and FAN6861 enters into deep

green mode, where the operating current reduces to

2.2mA (maximum), further reducing the standby power

output voltage starts to drop, which causes the

switching resumes. Burst mode alternately enables and

disables switching, thereby reducing switching loss in

standby mode, as shown in Figure 19.

Figure 18. PWM Frequency

Figure 19. Burst Mode Operation

Frequency Hopping

EMI reduction is accomplished by frequency hopping,

which spreads the energy over a wider frequency range

than the bandwidth measured by the EMI test

equipment. An internal frequency hopping circuit

changes the switching frequency between 60.8kHz and

69.2kHz with a period of 4.4ms, as shown in Figure 20.

Figure 20. Frequency Hopping