TSC2046IRGVT datasheet(19/34 Pages) TI2

19 / 34 page

TSC2046

SBAS265G − OCTOBER 2002 − REVISED JANUARY 2008

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POWER DISSIPATION

There are two major power modes for the TSC2046:

full-power (PD0 = 1) and auto power-down (PD0 = 0).

When

operating

full

speed

and

clocks-per-conversion (see Figure 11), the TSC2046

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

increasing

percentage of time in power-down mode (assuming the

auto power-down mode is active).

10k

100k

SAMPLE (Hz)

1000

100

CLK =2MHz

S upply Current from

CC and IO VD D

A =25¡C

CC =2.7V

IOVDD = 1.8V

fCLK =16⋅ fSAMPLE

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 (e.g., 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, this could substantially increase power dissipation.

CS also puts the TSC2046 into power-down mode. When

CS goes high, the TSC2046 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 TSC2046 first powers up, the device draws

about 20

µA of current until a control byte is written to it with

PD0 = 0 to put it into power-down mode. This can be

avoided if the TSC2046 is powered up with CS = 0 and

DCLK = IOVDD.

TSC2046IRGVT Datasheet(PDF) 19 Page - Texas Instruments

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