FAN7554 Ver la hoja de datos (PDF) - Fairchild Semiconductor

Número de pieza

componentes Descripción

Fabricante

FAN7554

Versatile PWM Controller

Fairchild Semiconductor 

FAN7554

S/S

2

100uA

5V

Ds

2R

1mA

R

Cs

Cfb

Output drive

5uA 5V

Vcc

1

FB

FAN7554

Figure 13. Soft Start Circuit & Circuit Flow

Oscillator

As shown in figure14, the oscillator frequency is programmed by values selected for timing components Rt and Ct. Capacitor

Ct is charged to almost 2.8V through resistor Rt from the 5V reference and discharged to 1.2V by an internal current source.

The oscillator generates the clock signal while the timing capacitor Ct is discharged. The gate drive output becomes low during

the clock time. Rt and Ct selection determine the oscillator frequency and maximum duty cycle. Charge and discharge times

can be calculated through the equations below.

Charging time : tc = 0.55×Rt×Ct

Discharging time : td = Rt×Ct×ln[(0.0063×Rt - 2.8) / (0.0063×Rt - 3.8)]

where the oscillator frequency : fosc = (tc + td)-1 (±10%)

When Rt > 5kΩ, fosc = 1 / (0.55×Rt×Ct) = 1.8 / (Rt×Ct)

Vref

8

Rt

CT

Ct 4

Ct

+

Discharge

-

Discharge

2.8V

/1.2V

Clock

Gate Drive

FAN7554

Figure 14. Oscillator Circuit

Vhigh(2.8V)

Sawtooth waveform

Vlow(1.2V)

tc

td

[ Rt > 5kΩ]

Internal clock

Vhigh(2.8V)

Sawtooth waveform

Vlow(1.2V)

Internal clock

td

tc

[ Rt < 5kΩ]

Figure 15. Sawtooth & Clock Waveform

9

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