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driven lower than 1V. In this area, decreasing the voltage by 1V will cause a 2.5% decrease of the
vertical sawtooth amplitude, which normally should be more than enough for compensation.
A resistive network should be introduced between ABL point and pin 18, with a twofold purpose:
●
set the bias voltage in the active area between 1 and 8V,
●
apply to pin 18 the ABL voltage variations with convenient attenuation.
The simplest schematic diagram (but sufficient in most cases) is represented in
Since Pin 18 has a very high input impedance, the attenuation of ABL voltage variations is set by
the resistive divider (R2 with the parallel combination of R3 and R4). In this case, gain is equal to:
Since the voltage variation on pin 18 is in the range of 1V, the bias of pin 18 mainly depends on the
R3/R4 ratio and on the voltage reference (you can use the one on pin 13, or any other stable and
well filtered reference). Choose R3 and R4 to bias pin 18 in the active range, then adjust R2 for
optimal breathing compensation. Manage to have R2 much higher than R1, in order not to disturb
ABL operation: for that purpose you can multiply R2, R3 and R4 by a same factor without affecting
compensation. Finally, verify that pin 18 will always remain in the active range between 1 and 8V.
Here the ABL voltage appears at the common point of R1 and C1, the time constant for ABL is
R1 x C1. Other circuitry, like C2, R6, may be added to improve transient operation.
Usually, the optimal compensation amount is not constant in relation to the H frequency. For
instance, if less compensation is needed above a certain frequency, switch S may connect an extra
resistor R5 in parallel to R4. As a result, the gain and bias voltage will be reduced in the same ratio.
After R2, R3 and R4 have been tuned for low-frequency compensation, set the value of R5 for
proper compensation in the high frequency range. Because R5 decreases not only the gain, but
also the amplitude, you will have to compensate by increasing the Vertical Amplitude setting. Check
that pin 18 always remains higher than 1V; for that purpose, it is advisable to set the initial bias
(R3/R4)xVref to approximately 8V. Switch S may be a transistor or a diode, controlled by the MCU.
The control of pin 17 for H breathing is similar, but may require a different value of gain and time
constants. Voltage variations on pin 17 may be as high as 2V, check that they remain in the range
between 1 and 8V!
Figure 12: Principle of Breathing Compensation
R3 R4
||
R2
R3 R4
||
(
)
+
----------------------------------------
EHT Transformer
EHT
DC 5...12V
To ABL
R1
R2
TDA9112
Vref
Breathing
C1
R3
R4
R5
C2
R6
