AN4599 datasheet(5/26 Pages) STMICROELECTRONICS

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DocID026993 Rev 1

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AN4599

Circuit description

Procedure to set oscillator components:

•

nominate the start frequency (do not exceed 300 kHz fstart)

•

find the corresponding value of CF in Table 1

•

with a chosen fstart of 156 kHz, the corresponding CF is 560 pF in Table 1

•

choose the minimum frequency and determine RFmin using the equation:

Equation 1

The chosen Fmin is 49.6 kHz, CF is 560 pF and thus RFmin = 12 kΩ

Determine RSS using equation:

Equation 2

For the chosen Fstart of 156 kHz, CF = 560 pF, RFmin = 12 kΩ and consequently

RSS = 5.6 kΩ

Verify the following relationships:

Equation 3

Choose Fmax and determine RFmax using the formula:

Equation 4

For a chosen Fmax of 150 kHz, RFmax = 2 kΩ

Calculate the CSS using the formula CSS = 3 * 0.001 / RSS

In the application, it may be necessary to increase this value to optimize start-up procedure

by minimizing the inrush current and charging current of the output capacitor.

Good performance is achieved with CSS = 4.7 µF.

AN4599 Datasheet(PDF) 5 Page - STMicroelectronics