L6566BH View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566BH' PDF : 51 Pages View PDF
Application information
Figure 19. Frequency modulation circuit
L6566BH
L6566BH
13
6
1V
1.5 V
OSC
FMOD
0V
RMOD
0.5 V
RT
CMOD
AM11495v1
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
β = ∆fsw
fm
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
fsw _ min
=
fsw
−
1
2
∆fsw
;
fsw _ max
= fsw
+
1
2
∆fsw
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
RMOD
=
2 ⋅ 103
∆fsw
C MOD
=
75
fm
30/51
Doc ID 16610 Rev 2
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