Dot Type (Display Mode 1: Dspm05 = 0) - NEC mPD780208 Subseries User Manual

15.10.2 Dot type (display mode 1: DSPM05 = 0)

The calculation method for the total power dissipation in the case of the display example in Figure 15-24 is
described below.
Example Assume the following conditions:
= 5 V ±10%, 5.0 MHz oscillation
V
DD
Supply current (I
Display output: 16 grids × 35 segments (cut width = 1/16: when DIMS1 to DIMS3 = 000B)
Display output voltage: grid
Fluorescent display control voltage (V
Mask option pull-down resistor = 25 kΩ
By placing the above conditions in calculation <1> to <3>, the total dissipation can be worked out.
<1> CPU power dissipation: 5.5 V × 21.6 mA = 118.8 mW
<2> Output pin power dissipation:
Grid
Segment (V
<3> Pull-down resistor power dissipation:
Grid
Segment
Total power dissipation = <1> + <2> + <3> = 118.8 + 26.5 + 11.1 + 52.3 + 595.9 = 804.6 mW
In this example, the total power dissipation exceeds the allowable total power dissipation rating shown in Figure
15-22. In this case, the power dissipation can be lowered by reducing the number of enabled on-chip pull-down
resistors.
Next, calculation expressions are shown for the display example where on-chip pull-down resistors are enabled
for S0 through S24 only.
CHAPTER 15 VFD CONTROLLER/DRIVER
) = 21.6 mA
DD
Maximum current at the grid pin is 15 mA.
Maximum current at the segment pin is 3 mA.
At the key scan timing, display output pin is OFF.
V
OD
segments V
OD
LOAD
Total current value of each grid
) ×
(V – V
DD OD
No. of grids + 1
15 mA × 16 grids
2 V ×
16 grids + 1
Total segment current value of illuminated dots
) ×
– V
DD OD
3 mA × 168 dots
0.4 V ×
16 grids + 1
2
(V – V )
OD LOAD
×
Pull-down resistor value No. of grids + 1
2
(5.5 V – 2 V – (–35 V))
×
25 kΩ 16 grids + 1
2
(V – V ) No. of illuminated dots
OD LOAD
×
Pull-down resistor value
2
(5.5 V – 0.4 V – (–35 V))
×
25 kΩ
User's Manual U11302EJ4V0UM
= V – 2 V (voltage drop of 2 V)
DD
= V – 0.4 V (voltage drop of 0.4 V)
DD
) = –35 V
× Digit width (1 – Cut width) =
1
× (1 –
)
16
No. of grids + 1
1
× (1 –
)
16
No. of grids
× Digit width (1 – Cut width) =
16 grids 1
× (1 –
)
16
× Digit width (1 – Cut width) =
No. of grids + 1
168 dots 1
× (1 –
)
16 grids + 1 16
= 26.5 mW
× Digit width (1 – Cut width) =
= 11.1 mW
= 52.3 mW
= 595.9 mW
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