FSL136HR

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8

FUNCTIONAL DESCRIPTION

Startup

At startup, an internal high−voltage current source

supplies the internal bias and charges the external capacitor

switch begins switching and the internal high−voltage

current source is disabled. The power switch continues

normal switching operation and the power is provided from

the stop voltage of 8 V.

Figure 14. Startup Circuit

Oscillator Block

The oscillator frequency is set internally and the power

switch has a random frequency fluctuation function.

Fluctuation of the switching frequency of a switched power

supply can reduce EMI by spreading the energy over a wider

frequency range than the bandwidth measured by the EMI

test equipment. The amount of EMI reduction is directly

related to the range of the frequency variation. The range of

frequency variation is fixed internally; however, its

selection is randomly chosen by the combination of external

feedback voltage and internal free−running oscillator. This

randomly chosen switching frequency effectively spreads

the EMI noise nearby switching frequency and allows the

use of a cost− effective inductor instead of an AC input line

filter to satisfy the world−wide EMI requirements.

Figure 15. Frequency Fluctuation Waveform

Feedback Control

FSL136HR employs current−mode control, as shown in

Figure 16. An opto−coupler (such as the FOD817A) and

shunt regulator (such as the KA431) are typically used to

implement the feedback network. Comparing the feedback

possible to control the switching duty cycle. When the shunt

regulator reference pin voltage exceeds the internal

reference voltage of 2.5 V, the optocoupler LED current

duty cycle is reduced. This typically occurs when the input

voltage is increased or the output load is decreased.

Figure 16. Pulse−Width−Modulation Circuit

Leading−Edge Blanking (LEB)

At the instant the internal SENSEFET is turned on, the

primary−side capacitance and secondary−side rectifier

diode reverse recovery typically cause a high−current spike

through the SENSEFET. Excessive voltage across the

current−mode PWM control. To counter this effect, the

power switch employs a leading−edge blanking (LEB)

circuit (see the Figure 16). This circuit inhibits the PWM

turned on.

Protection Circuit

The power switch has several protective functions, such

as overload protection (OLP), over−voltage protection

(OVP), output−short protection (OSP), under−voltage

lockout (UVLO), abnormal over−current protection

(AOCP), and thermal shutdown (TSD). Because these

various protection circuits are fully integrated in the IC

without external components, the reliability is improved

without increasing cost. Once a fault condition occurs,

switching is terminated and the SENSEFET remains off.

In this manner, the auto−restart can alternately enable and

disable the switching of the power SENSEFET until the

fault condition is eliminated.