AN993 Datasheet, PDF (24/27 Pages) STMicroelectronics – Electronic ballast with PFC using L6574 and L6561

Dimming the lamp
6
Dimming the lamp
AN993
The lamp is dimmed by changing the working frequency. The minimum working frequency
is:
Equation 18
To change the working frequency, one has to change the current that flows from pin 4. Using
the circuitry shown in Figure 15, the current that flows from pin 4 also depends on the
amplifier’s out pin (pin 5) voltage: if V5 > V4, there will be no current in R₁₆ and the
frequency will be equal to f_min. On the other hand, when the amplifier’s out voltage goes
lower than V4, the current that flows from pin 4 will go into R₁₆ and R₁₈. Pin 4 sees the
parallel between R₁₈ and an equivalent resistor that depends on V₅ and on the voltage of
D₄.
Equation 19
4
4
So the working frequency will be:
Equation 20
and the max f_working is obtained when V5 = 0 V.
For example, we can calculate a maximum frequency of ~53 kHz (V4 = 2 V and assuming
Vdiode3 = 0.5 V).
The dimming level is set by changing the value of the P1 potentiometer, which causes the
amplifier’s positive reference to change from 20-30 mV to 110-120 mV. If R16 is lowered, the
maximum working frequency is increased and the dimming level lowered as well (the higher
the frequency, the lower the current in the lamp’s arc).
If one tries to dim the lamp towards a low power range (<20% of the electrical arc power) a
common effect is the presence of stationary waves along the lamp tube and/or some
flickering effects.
A common trick to get rid of these disturbances is to add a small continuous current flow
inside the lamp (a few mA). The easiest way to do this is to add a resistor in parallel to the
half battery capacitor (C17 in Figure 13 and Figure 18). For instance, one can add 50 kΩ
(I = 200 V/50 kΩ = 4 mA), which is already very effective. Of course, the resistor has to be
able to sustain the power dissipation caused by the current flow, so it is common to use
many resistors connected in parallel.
Remember that this is only a tip. It helps but is not enough, and should be used together
with the right frequency settings that must be chosen according to the type of lamp.
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Doc ID 5656 Rev 10