Datasheet

Memory LCD Programming
LCD Application Note 7
POWER CONSIDERATIONS
One of the prime attributes of the memory LCD is
low power operation. The actual power usage is
directly related to how often data is written to the panel,
and how often VCOM is toggled. We’ll look at two sce-
narios and calculate power draw.
Power Scenario 1
Here all of the pixels will be written to the panel once
per second. For the 1.35” panel, the quiescent current
for the panel is 2.1 µA. Each write to the panel (assum-
ing that the entire panel is written), takes 360 µA; and
lasts for about 5.38 ms (assuming a 2 MHz SPI clock
speed). See Figure 9.
If averaged over a second, this amounts to an “aver-
age” currant draw of approximately 2 µa. Therefore, the
panel would have an average power draw of 20.5 µW
(5 V operation). Note that a VCOM toggle can be done
whenever the panel is written to including a data write.
Power Scenario 2
Here, we write all the pixels to the panel only every
30 seconds. The current to write to the panel “aver-
aged” over that time is approximately 0.1 µA. If VCOM
is toggled once a second (using the Change VCOM
command), the additional power draw for this action
(assuming that it is done 29 times) is an average of
0.1 µA. Therefore the average power draw over the 30-
second period becomes 11.5 µW. See Figure 10.
Applying these conditions to the 2.7” panel: writing
all pixels to the whole panel once a second takes an
average of 485 µW. Writing all pixels once every 30
seconds (with a 1-second VCOM toggle) takes
135 µW.
SUMMARY
Sharp’s Memory LCDs have a serial interface that
makes them simple to program. The most challenging
tasks for the programmer will be to ensure that VCOM
is toggled periodically to maintain the lack of DC bias
on the display; and that data is sent to the panel in the
correct order.
Figure 9. Writing All Pixels, Once per Second
Figure 10. Writing All Pixels, Once Every 30 Seconds, Toggling VCOM Once Per Second
1.5 mA × 50 ms = average of
75 µA over 1 second
Quiescent current = 22 µA
1 Second
LCD45-9
Average current for VCOM switch = 3 µA
1.5 mA × 50 ms = average of
75 µA over 1 second
Quiescent current = 22 µA
LCD45-10