CHAPTER 15 VFD CONTROLLER/DRIVER
329
User’s Manual U11302EJ4V0UM
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:
V
DD
= 5 V
±
10%, 5.0 MHz oscillation
Supply current (I
DD
) = 21.6 mA
Display output: 16 grids
×
35 segments (cut width = 1/16: when DIMS1 to DIMS3 = 000B)
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.
Display output voltage: grid
V
OD
= V
DD
– 2 V (voltage drop of 2 V)
segments V
OD
= V
DD
– 0.4 V (voltage drop of 0.4 V)
Fluorescent display control voltage (V
LOAD
) = –35 V
Mask option pull-down resistor = 25 k
Ω
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
(V
DD
– V
OD
)
×
Total current value of each grid
×
Digit width (1 – Cut width) =
No. of grids + 1
2 V
×
15 mA
×
16 grids
×
(1 –
1
)
= 26.5 mW
16 grids + 1
16
Segment (V
DD
– V
OD
)
×
Total segment current value of illuminated dots
×
Digit width (1 – Cut width) =
No. of grids + 1
0.4 V
×
3 mA
×
168 dots
×
(1 –
1
)
= 11.1 mW
16 grids + 1
16
<3> Pull-down resistor power dissipation:
Grid
(V
OD
– V
LOAD
)
2
×
No. of grids
×
Digit width (1 – Cut width) =
Pull-down resistor value
No. of grids + 1
(5.5 V – 2 V – (–35 V))
2
×
16 grids
×
(1 –
1
)
= 52.3 mW
25 k
Ω
16 grids + 1
16
Segment
(V
OD
– V
LOAD
)
2
×
No. of illuminated dots
×
Digit width (1 – Cut width) =
Pull-down resistor value No. of grids + 1
(5.5 V – 0.4 V – (–35 V))
2
×
168 dots
×
(1 –
1
)
= 595.9 mW
25 k
Ω
16 grids + 1
16
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.
Содержание mPD780208 Subseries
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