Light Leakage Of Vfd - NEC mPD780208 Subseries User Manual

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.
T0
T1
S0
(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
15-12, as there is capacitance between the grid and segment of the VFD, the timing signal pin voltage will
be increased via C
As shown in Figure 15-13, when this voltage exceeds the cut-off voltage (E
This spike noise voltage depends on the size of C
the value of C or the R
SG
This C value differs according to the area of the data displayed on the VFD. The greater the area, the
SG
greater C . Consequently, pull-down resistor values with which light will not leak also depend on the size
SG
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
VFD, or attach a 10 kΩ pull-down resistor externally to the timing signal pin.
CHAPTER 15 VFD CONTROLLER/DRIVER
Figure 15-11. Light Leakage due to Short Blanking Time
when the segment signal turns on.
SG
value, the greater the voltage, making it easy for light to leak.
L
User's Manual U11302EJ4V0UM
Blanking time
Light leakage
and the on-chip pull-down resistor (R
SG
in Figure
SG
), light will leak.
K
). The greater
L
), place a filter over the
K
315