Datasheet

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SBAS417B − JUNE 2007 − REVISED JANUARY 2008
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19
POWER DISSIPATION
There are two major power modes for the TSC2046E:
full-power (PD0 = 1) and auto power-down (PD0 = 0).
When operating at full speed and 16
clocks-per-conversion (see Figure 11), the TSC2046E
spends most of the time acquiring or converting. There is
little time for auto power-down, assuming that this mode is
active. Therefore, the difference between full-power mode
and auto power-down is negligible. If the conversion rate
is decreased by slowing the frequency of the DCLK input,
the two modes remain approximately equal. However, if
the DCLK frequency is kept at the maximum rate during a
conversion but conversions are done less often, the
difference between the two modes is dramatic.
Figure 15 shows the difference between reducing the
DCLK frequency (scaling DCLK to match the conversion
rate) or maintaining DCLK at the highest frequency and
reducing the number of conversions per second. In the
latter case, the converter spends an increasing
percentage of time in power-down mode (assuming the
auto power-down mode is active).
1k 1M10k 100k
f
SAMPLE
(Hz)
Supply Current (
µ
A)
1000
100
10
1
f
CLK
=2MHz
Supply Current from
+V
CC
and IOVDD
T
A
=25¡C
+V
CC
=2.7V
IOVDD = 1.8V
f
CLK
= 16 ⋅ f
SAMPLE
Figure 15. Supply Current versus Directly Scaling
the Frequency of DCLK with Sample Rate or
Maintaining DCLK at the Maximum Possible
Frequency
Another important consideration for power dissipation is
the reference mode of the converter. In the single-ended
reference mode, the touch panel drivers are ON only when
the analog input voltage is being acquired (see Figure 9
and Table 1). The external device (for example, a resistive
touch screen), therefore, is only powered during the
acquisition period. In the differential reference mode, the
external device must be powered throughout the
acquisition and conversion periods (see Figure 9). If the
conversion rate is high, it could substantially increase
power dissipation.
CS
also puts the TSC2046E into power-down mode.
When CS
goes high, the TSC2046E immediately goes into
power-down mode and does not complete the current
conversion. The internal reference, however, does not turn
off with CS
going high. To turn the reference off, an
additional write is required before CS
goes high (PD1 = 0).
When the TSC2046E first powers up, the device draws
about 20µA of current until a control byte is written to it with
PD0 = 0 to put it into power-down mode. This current draw
can be avoided if the TSC2046E is powered up with CS
=
0 and DCLK = IOVDD.