FSL518APG datasheet(16/27 Pages) ONSEMI

16 / 27 page

FSL518H, FSL538H, FSL518A, FSL538A

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Figure 37. Soft-start Operation

1.25 ms

8 Steps

ILIM

Drain Current

IDRAIN

VCOMP

Pull high (5 V)

VSS− LIMIT

Main Control Frequency Reduction

Operating frequency of switching operation is

synchronized with COMP-pin voltage, VCOMP. When

VCOMP drops, operating frequency will also decrease. This

helps reducing switching losses and thus improve light-load

efficiency operation. The operating frequency will not be

decreased below 22-kHz so acoustic noise can be avoided.

PWM Control

The FSL5x8 operates with peak−current mode to regulate

output voltage. The duty cycle of PWM is determined by

comparing drain peak current and VCOMP. The VCOMP can

be controlled by either the input signal of error amplifier or

the signal delivered via opto−coupler and feedback loop for

output regulation.

Slope Compensation

Built-in slope compensation is added into the PWM

procedure when duty cycle is higher than 45%. It helps to

avoid sub-harmonic oscillation of peak-current control.

Figure 38. Slope Compensation

VSENSE

VSENSE+COMP

VSLOPE

45 Duty

Burst−mode Operation

As loading of the power converter decreases, VCOMP

decreases, thus reducing switching frequency of the

oscillator. When minimum operating frequency is reached,

to further reduce delivered output power, the device goes

into burst−mode. In burst−mode, switching operation is

halted when VCOMP is lower than VBURL and resumed when

VCOMP is higher than VBURH. By skipping un-needed

switching cycles, the FSL5x8 drastically reduced the power

wasted during light load conditions.

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