AN-9719 データシートの表示（PDF） - Fairchild Semiconductor

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AN-9719

Applying Fairchild Power Switch (FPS™) FSL1x7 to Low- Power Supplies

Fairchild Semiconductor 

AN-9719

2. Device Block Description

2.1 Startup Circuit

For startup, the HV pin is connected to the line input or bulk

capacitor through external resistor RHV, as shown in Figure

2. Typical startup current is 3.5mA and it charges the VDD

capacitor (CDD) through resistor RHV. The startup current

turns off when the VDD capacitor voltage reaches VDD-ON.

The VDD capacitor maintains VDD until the auxiliary

winding of the transformer provides the operating current.

APPLICATION NOTE

2.3 Green Mode Operation

The FSL-series uses feedback voltage (VFB) as an indicator

of the output load and modulates the PWM frequency, as

shown in Figure 4, such that the switching frequency

decreases as load decreases. In heavy-load conditions, the

switching frequency is 100kHz. Once VFB decreases below

VFB-N (2.5V), the PWM frequency starts to linearly decrease

from 100kHz to 18kHz to reduce the switching losses. As

VFB decreases below VFB-G (2.4V), the switching frequency

is fixed at 18kHz and FSL-series enters “deep” green mode

to reduce the standby power consumption. As VFB decreases

below VFB-ZDC (2.1V), FSL-series enters into burst-mode

operation. When VFB drops below VFB-ZDC, FSL-series stops

switching and the output voltage starts to drop, which

causes the feedback voltage to rise. Once VFB rises above

VFB-ZDC, switching resumes. Burst mode alternately enables

and disables switching, thereby reducing switching loss to

improve power saving, as shown in Figure 5.

Figure 2. Startup Circuit

2.2 Soft-Start

The FSL-series has internal soft-start circuit that slowly

increases the SenseFET current during startup. The typical

soft-start time is 5ms, during which the VLimit level is

increased in six steps to smoothly establish the required

output voltage, as shown in Figure 3. It also helps prevent

transformer saturation and reduce stress on the secondary

diode during startup.

Figure 4. PWM Frequency

Figure 3. Soft-Start Function

Figure 5. Bust-Mode Operation

© 2010 Fairchild Semiconductor Corporation

Rev. 1.0.0 • 11/2/10

2

www.fairchildsemi.com

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