Power supply, UVLO and voltage reference

AN937

4/29

Doc ID 5655 Rev 15

2

Power supply, UVLO and voltage reference

The device is provided with an internal stabilized power supply (of about 12 V typ.) that

powers the analog and digital control blocks and the bootstrap section.

From this pre regulator, a 3.3V reference voltage ±2%, is internally available.

2.1

Oscillator and voltage feed forward

Just one pin is necessary to implement the oscillator function, with inherent voltage

feedforward.

Figure 4.

Oscillator internal circuit

A resistor Rosc and a capacitor Cosc connected as shown in Figure 4, allow the setting of

the desired switching frequency in agreement with the below formula:

(Q1) and during this period the internal threshold is set at 1 V by a second MOS, Q2. When

the oscillator voltage capacitor reaches the 1V threshold the output comparator turn-off the

The oscillator block, shown in Figure 4, generates a sawtooth wave signal that sets the

switching frequency of the system.

This signal, compared with the output of the error amplifier, generates the PWM signal that

will modulate the conduction time of the power output stage.

The way the oscillator has been integrated, does not require additional external components

to benefit of the voltage feed forward function.

Osc

TO PWM

COMPARATOR

1V

R

5R

CLOCK

D97IN655A

-

+

F

SW

1

R

oasc

C

osc

⋅

()In 6

5

---

⎝⎠

⎛⎞

100 C

osc

⋅

+

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=