CHAPTER 15 VFD CONTROLLER/DRIVER
315
User’s Manual U11302EJ4V0UM
15.8 Light Leakage of VFD
Light may leak when a VFD is driven using the
µ
PD780208 Subseries. Two possible causes are as follows.
(1) Light leakage due to a short blanking time
Figure 15-11 shows the signal waveforms when only the first digit of two digits to be displayed is lit.
As shown in this figure, when the blanking time is short, the T1 signal rises before the segment signal
disappears, resulting in light leakage.
Generally, as approximately 20
µ
s is required for the blanking time, consider the values set to display mode
register 1 (DSPM1) carefully.
Figure 15-11. Light Leakage due to Short Blanking Time
(2) Light leakage due to capacitance between segment and grid of VFD
As shown in Figure 15-13, light may leak even if the blanking time is sufficient. As shown by C
SG
in Figure
15-12, as there is capacitance between the grid and segment of the VFD, the timing signal pin voltage will
be increased via C
SG
when the segment signal turns on.
As shown in Figure 15-13, when this voltage exceeds the cut-off voltage (E
K
), light will leak.
This spike noise voltage depends on the size of C
SG
and the on-chip pull-down resistor (R
L
). The greater
the value of C
SG
or the R
L
value, the greater the voltage, making it easy for light to leak.
This C
SG
value differs according to the area of the data displayed on the VFD. The greater the area, the
greater C
SG
. Consequently, pull-down resistor values with which light will not leak also depend on the size
of the VFD.
As the value of the pull-down resistor incorporated by the mask option is comparatively great, in some cases,
light leakage cannot be controlled with resistance only.
If the quality of the display is insufficient, increase the back bias (increase the E
K
), place a filter over the
VFD, or attach a 10 k
Ω
pull-down resistor externally to the timing signal pin.
T0
Blanking time
T1
S0
Light leakage