Obsolete Product(s) - Obsolete Product(s)
DC/DC Converter Section
AN1290
48/62
STMicroelectronics Confidential
The capacitor value should be low enough not to switch the MOS transistor ON during a power
switch-ON. A Zener diode protects the gate at this time. On the other side, the surge current will be
sunk by one of the buffer transistors.
To obtain E/W modulation, a current carrying the E/W data should be injected into pin 15. Choose
the resistor value to obtain the correct amplitude. A DC current may have to be injected or sunk in
order to obtain the proper B+ value.
7.1.8
Structure of the Regulation Loop (Step-down, Voltage mode)
We will describe an implementation with an N-MOS transistor controlled through a transformer. Of
course, it is also possible to control a P-MOS transistor just as in the previous paragraph.
In the TDA9112 only, connecting pin 16 to a voltage higher than 6V (for instance pin 13)
automatically modifies the internal structure, allowing an application such as the one shown in
The DC voltage source is now in the 200V range (higher than the highest needed B+ value).
A filtering cell [L- Cb] is not really necessary. Nevertheless, because of the poor frequency response
of the voltage-mode loop, speed cannot be high. Consequently, this structure is not well suited to
modulating the B+ converter using the E/W signal. It can be used together with a diode modulator.
Choose the correct output polarity, taking into account the phase of the transformer, to apply
convenient pulse polarity to the gate.
The triggering point is automatically the starting point of the HFocus ramp, inside the flyback pulse.
Figure 19: Regulation Loop in Step-down (Current mode, MOS type p)
H Scanning
Transistor
Vdc
Ly
T
Cr
R1
R2
Ri
Rfb
Cfb
Set
Reset
Q
Vref
TMOS
Zfb:
Vo
Vi
Vb
-
+
Rc
16
28
15
14
10nF
+
-
29
Rs
1nF
1k
24
HOut
FB
