ADCS7477AIMF datasheet(20/22 Pages) NSC

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Applications Information (Continued)

10.0 EXITING SHUTDOWN MODE

To exit shutdown mode, bring CS back low. Upon bringing

CS low, the ADCS7476/77/78 will begin powering up. Power

up typically takes 1 µs. This microsecond of power-up delay

results in the first conversion result being unusable. The

second conversion performed after power-up, however, is

valid, as shown in Figure 11.

If CS is brought back high before the 10th falling edge of

SCLK, the device will return to shutdown mode. This is done

to avoid accidentally entering normal mode as a result of

noise on the CS line. To exit shutdown mode and remain in

normal mode, CS must be kept low until after the 10th falling

edge

SCLK.

The

ADCS7476/77/78

will

fully

powered-up after 16 SCLK cycles.

11.0 POWER-UP TIMING

The ADCS7476/77/78 typically requires 1 µs to power up,

either after first applying V

DD, or after returning to normal

mode from shutdown mode. This corresponds to one

"dummy" conversion for any SCLK frequency within the

specifications in this document. After this first dummy con-

version, the ADCS7476/77/78 will perform conversions prop-

erly. Note that the t

QUIET time must still be included between

the first dummy conversion and the second valid conversion.

12.0 STARTUP MODE

When the V

DD supply is first applied, the ADCS7476/77/78

may power up in either of the two modes: normal or shut-

down. As such, one dummy conversion should be performed

after start-up, exactly as described in Section 11.0. The part

may then be placed into either normal mode or the shutdown

mode, as described in Sections 8.0 and 9.0.

13.0 POWER MANAGEMENT

When the ADCS7476/77/78 is operated continuously in nor-

mal mode, throughput up to 1 MSPS can be achieved. The

user may trade throughput for power consumption by simply

performing fewer conversions per unit time, and putting the

ADCS7476/77/78 into shutdown mode between conver-

sions. This method is not advantageous beyond 350 kSPS

throughput.

A plot of maximum power consumption versus throughput is

shown in Figure 12 below. To calculate the power consump-

tion for a given throughput, remember that each time the part

exits shutdown mode and enters normal mode, one dummy

conversion is required. Generally, the user will put the part

into normal mode, execute one dummy conversion followed

by one valid conversion, and then put the part back into

shutdown mode. When this is done, the fraction of time

spent in normal mode may be calculated by multiplying the

throughput (in samples per second) by 2 µs, the time taken

to perform one dummy and one valid conversion. The power

consumption can then be found by multiplying the fraction of

time spent in normal mode by the normal mode power

consumption figure. The power dissipated while the part is in

shutdown mode is negligible.

For example, to calculate the power consumption at 300

kSPS with V

DD = 5V, begin by calculating the fraction of time

spent in normal mode: 300,000 samples/second·2µs= 0.6,

or 60%. The power consumption at 300 kSPS is then 60% of

17.5 mW (the maximum power consumption at V

DD = 5V) or

10.5 mW.

20057716

FIGURE 10. Entering Shutdown Mode

20057717

FIGURE 11. Entering Normal Mode

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