
CHAPTER 11 FIP CONTROLLER/DRIVER
137
(2) Segment-grid capacitance of fluorescent indicator panel
Even if a sufficiently long blanking time is ensured as shown in Figure 11-9, leakage emission may still occur.
This is because the fluorescent indicator panel has a capacitance between the grid and segment, as indicated
by C
SG
in the figure, and the timing signal pin is raised via C
SG
. If the voltage of the timing signal rises beyond
the cutoff voltage (E
K
) as shown in Figure 11-9, leakage emission occurs.
This whisker-like voltage changes with the values of C
SG
and internal pull-down resistor (R
L
). The greater the
value of C
SG
, and the greater the value of R
L
, the higher this voltage, increasing the possibility of the occurrence
of leakage emission.
The value of C
SG
differs depending on the display area of the fluorescent indicator panel. The larger the area,
the higher the C
SG
.
Therefore, the value of the pull-down resistor differs depending on the size of the fluorescent indicator panel,
in order to prevent leakage emission.
Because the value of the pull-down resistor that can be connected by mask option is relatively high, the leakage
emission may not be suppressed by the internal pull-down resistor alone.
In case sufficient display quality cannot be obtained, deepen the back bias (increase E
K
), attach a filter to the
fluorescent indicator panel, or connect an external pull-down resistor of several 10 k
W
to the timing signal pin.
The likelihood of leakage emission caused by C
SG
occuring changes depending on the duty cycle of the whisker
voltage vis-a-vis the total display cycle. The fewer the number of display digits, the less likelihood of occurrence
of leakage emission.
Lowering the display luminance also has an effect of suppressing the leakage emission.
Figure 11-8. Leakage Emission Caused by C
SG
Segment
grid filament
FIP
V
DD
E
K
C
SG
V
LOAD
R
L
R
L
S0–
T0–
–30 V
+5 V
E
K
: Cutoff voltage
R
L
: Internal pull-down resistor
PD780228
μ