SO16N datasheet(30/51 Pages) STMICROELECTRONICS

30 / 51 page

Application information

L6566BH

30/51

Doc ID 16610 Rev 2

Figure 19.

Frequency modulation circuit

With reference to

Figure 19, the capacitor CMOD is connected from FMOD to ground and is

alternately charged and discharged between 0.5 and 1.5 V by internal current generators

sourcing and sinking the same current (three times the current defined by the resistor RT on

pin OSC). Therefore, the voltage across CMOD is a symmetric triangle, whose frequency fm

is determined by CMOD. By connecting a resistor RMOD from RMOD to OSC, the current

sourced by the OSC pin is modulated according to a triangular profile at a frequency fm. If

RMOD is considerably higher than RT, as is normal, both fm and the symmetry of the triangle

is little affected.

With this arrangement it is possible to set, nearly independently, the frequency deviation

∆fsw and the modulating frequency fm, which define the modulation index:

Equation 9

which is the parameter that the amplitude of the generated side-band harmonics depends

on.

The minimum frequency fsw_min (occurring on the peak of the triangle) and the maximum

frequency fsw_max (occurring on the valley of the triangle) is symmetrically placed around the

centre value fsw, so that:

Equation 10

Then, RT is found from (5) (see Section 5.2: Zero-current detection and triggering block;

oscillator block), while RMOD and CMOD can be calculated as follows:

Equation 11

AM11495v1

L6566BH

OSC

FMOD

RMOD

CMOD

1.5 V

0.5 V

0 V

1 V

∆

max

min

;

∆

−

MOD

∆

⋅

MOD

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