L6566B datasheet(31/51 Pages) STMICROELECTRONICS

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L6566B

Application information

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between the detected event and the oscillator, occasionally the device could stop after the

third detection.

This condition is latched as long as the device is supplied. While it is disabled, however, no

energy is coming from the self-supply circuit; hence the voltage on the Vcc capacitor will

decay and cross the UVLO threshold after some time, which clears the latch. The internal

start-up generator is still off, then the Vcc voltage still needs to go below its restart voltage

before the Vcc capacitor is charged again and the device restarted. Ultimately, this will result

in a low-frequency intermittent operation (Hiccup-mode operation), with very low stress on

the power circuit. This special condition is illustrated in the timing diagram of Figure 18 on

page 30

5.8

Frequency modulation

To alleviate converter’s EMI emissions and reduce cost and size of the line filter, it is

advantageous to modulate its switching frequency, so that the resulting spread-spectrum

action distributes the energy of each harmonic of the switching frequency over a number of

side-band harmonics. Their overall energy will be unchanged but the individual amplitudes

will be smaller. This is what naturally occurs with QR operation, due to the twice-mains-

frequency ripple appearing on the input bulk capacitor, which translates into different DCM-

CCM boundary frequencies.

The L6566B is provided with a dedicated pin, FMOD (6), to perform this function if FF mode

is selected.

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). Hence, the voltage across CMOD will be a symmetric triangle, whose frequency fm

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

sourced by pin OSC will be modulated according a triangular profile at a frequency fm. If

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

triangle will be 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:

L6566B

OSC

FMOD

RMOD

CMOD

1.5 V

0.5 V

0 V

1 V

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