FA551 Datasheet, PDF(9/22 Page) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

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FA551 Datasheet, PDF (9/22 Pages) List of Unclassifed Manufacturers – CMOS IC For Switching Power Supply Control

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â Description of each circuit

1. Oscillator

The oscillator generates a triangular waveform by charging and

discharging the built-in capacitor. A desired oscillation

frequency can be set by the value of the resistor connected to

the RT pin (See Figure 1).

The built-in capacitor voltage oscillates between about 3V and

1V, with almost the same charging and discharging gradients

(Figure 2). You can set the desired oscillation frequency by

changing the gradients using the resistor connected to the RT

pin. (Large RT=Low frequency, small RT=High frequency)

The relationship between RT and the oscillation frequency is

approximately given by:

fOSC Ô¼

4880

RT + 1.4

[kHz]

........................................................

(1)

RT Ô¼

4880

fOSC

â 1.4 [kâ¦] ........................................................ (2)

fOSC: Oscillation frequency [kHz]

RT: Timing resistance [kâ¦]

The oscillator waveform cannot be observed from the outside

because the oscillator output is not pinned out. The oscillator

output is connected to a PWM comparator.

2. PWM comparator

The PWM comparator has four inputs as shown in Figure 3.

Oscillator output á¢ is compared with CS pin voltage á£, FB pin

voltage á¤, and DT voltage á¥. The lowest of three inputs á£,

á¤, and á¥ has priority and is compared with oscillator output á¢.

While the voltage is lower than the oscillator output, the

comparator output is high. While the voltage is higher than the

oscillator output, the PWM comparator output is low (see Figure

4). The IC OUT pin voltage is high while the PWM comparator

output is low. When the IC is powered up, CS pin voltage á£

controls soft start operation. The output pulse then begins to

widen gradually. During normal operation, the output pulse

width is determined within the maximum duty cycle (FA5510/14:

46%, FA5511/15: 70%) set by DT voltage á¥ under the

condition set by FB pin voltage á£, to stabilize the output

voltage.

FA551X

OSC

Fig. 1 Oscillator

RT=Small RT=Large

Fig. 2 Oscillator output

á¥ DT voltage

á¤ FB pin voltage

á£ CS pin voltage

á¢ Oscillator

output

PWM comparator

output

Fig. 3 PWM comparator

á¤FB pin voltage

á¥DT voltage

á¢Oscillator output

á£CS pin voltage

PWM comparator

output

OUT pin voltage

Fig. 4 PWM comparator timing chart

(4.0V)

▷