DS1680

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PROGRAMMABLE DUTY CYCLE

multiplied by the ratio of the time the drivers are on, to the power of total sample time. In order to

minimize the average current, the on-time should be limited to the minimum time required for the tablet

RC delay.

Experimental data suggests that a typical RC time constant is between 4µs and 5

µs for a resistive touch

screen. In order to achieve 10-bit resolution, the settling time must be eight time constants. This creates a

requirement of a minimum of 80

µs on-time total, 40µs for each X and Y measurement.

To provide both low power and high sample rate, the on-time for the X- and Y- measurement duty cycle is

programmable. Bits 4 and 5 (SP0 and SP1) of the control register (0Bh) select the on-time of four

different frequency ranges. The frequencies given are the maximum frequency for that timing range,

which will not violate the 40

µs-per-measurement requirement.

SP1

SP0

FREQUENCY RANGE (MHz)

AVERAGE

CURRENT (A)

SAMPLES/SEC

NO. OF

CYCLES

0*

0*

2.0

870µ

543

5

0

1

2.8

1.217m

760

7

1

0

4.0

1.739m

1086

10

1

1

5.0

2.261m

1359

13

*This is the default setting

Average current is the current required for the measurement, averaged out over the entire sample. This

average current is only related to the measurement phase when the drivers are on. The average current

ADC, and the control logic.

The number of cycles indicated is the number of on-time state cycles. One state cycle is 16 main clock

cycles. If the frequency range is 2.0MHz, the state frequency is 2MHz/16 = 125kHz. There are 230 state

cycles in one complete sample. The number of cycles can be used to calculate the settling time and the

sample rate.

Example 1:

Frequency Range

:

2.0MHz

Input Clock Frequency

:

1.8432MHz

tsettle = (1 / 1.8432e6) x 16 x 5 = 43.4

µs

Iavg = (10 / 230) x 20mA = 870

µA

Sample Rate = 1.8432e6 / (16 x 230) = 501 samples/sec

Example 2:

Frequency Range

:

2.8MHz

Input Clock Frequency

:

1.8432MHz

tsettle = (1 / 1.8432e6) x 16 x 7 = 60.8

µs

Iavg = (14 / 230) / x 20mA = 1.217mA

Sample Rate = 1.8432e6 / (16 x 230) = 501 samples/sec