EVAL6574B STMicroelectronics, EVAL6574B Datasheet - Page 24

• Manufacturer Part Number: EVAL6574B
• Description: EVAL BOARD FOR HF TL BALLAST
• Manufacturer: STMicroelectronics

Specifications of EVAL6574B

• Mfg Application Notes: EVAL6574B AppNote
• Design Resources: EVAL6574B Gerber Files
• Main Purpose: Lighting, Ballast Control
• Embedded: No
• Utilized Ic / Part: L6574
• Primary Attributes: 58W, 85 ~ 265 VAC, Dimmable, PFC
• Secondary Attributes: Preheating Ignition
• Processor To Be Evaluated: L6574
• Operating Supply Voltage: 600 V
• Core Chip: L6574
• Kit Contents: Board Docs
• Development Tool Type: Hardware - Eval/Demo Board
• Supported Devices: L6574 And L6562
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: 497-3823

Dimming the lamp
6
24/27
Dimming the lamp
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
amplifier’s out pin (pin 5) voltage: if V5 > V4, there will be no current in R
frequency will be equal to f
lower than V4, the current that flows from pin 4 will go into R
parallel between R
D
Equation 19
So the working frequency will be:
Equation 20
and the max f
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
(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.
4
.
working
18
is obtained when V5 = 0 V.
and an equivalent resistor that depends on V
Figure
min
15, the current that flows from pin 4 also depends on the
Figure 13
. On the other hand, when the amplifier’s out voltage goes
Doc ID 5656 Rev 10
and
4
Figure
4
18). For instance, one can add 50 kΩ
16
and R
5
and on the voltage of
18
. Pin 4 sees the
16
and the
AN993