LTC1289BCSW datasheet(25/28 Pages) LINER

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LTC1289

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APPLICATI

TYPICAL

Power Shutdown

For battery-powered applications it is desirable to keep

power dissipation at a minimum. The LTC1289 can be

powered down when not in use reducing the supply

current from a nominal value of 1mA to typically 1

µA (with

ACLK turned off). See the Curve for Supply Current (Power

Shutdown) vs ACLK if ACLK cannot be turned off when the

LTC1289 is powered down. In this case the supply current

is proportional to the ACLK frequency and is independent

of temperature until it reaches the magnitude of the supply

current attained with ACLK turned off.

As an example of how to use this feature let’s add this to

the previous application, SNEAK-A-BIT. After the CHK

SIGN subroutine call insert the following:

•

JSR CHK SIGN

Determines which reading has valid

data, converts to 2’s complement

and stores in RAM

JSR SHUTDOWN

LTC1289 power shutdown routine

The actual subroutine is:

SHUTDOWN: LDA #$3D

Load DIN word for

LTC1289 into ACC

JSR TRANSFER Read LTC1289 routine

RTS

Return

To place the device in power shutdown the word length

bits are set to WL1 = 0 and WL0 = 1. The LTC1289 is

powered up on the next request for conversion and it's

ready to digitize an input signal immediately.

Power Shutdown Timing Considerations

After power shutdown has been requested, the LTC1289

is powered up on the next request for a conversion. This

request can be initiated either by bringing CS low or by

starting the next cycle of SCLKs if CS is kept low (see

Figures 3 and 4). When the SCLK frequency is much

slower than the ACLK frequency a situation can arise

where the LTC1289 could power down and then prema-

turely power back up. Power shutdown begins at the

negative going edge of the 10th SCLK once it has been

requested. A dummy conversion is executed and the

LTC1289 waits for the next request for conversion. If the

SCLKs have not finished once the LTC1289 has finished its

dummy conversion, it will recognize the next remaining

SCLKs as a request to start a conversion and power up the

LTC1289 (see Figure 23). To prevent this, bring either CS

high at the 19th SCLK (Figure 24) or clock out only 10

SCLKs (Figure 25) when power shutdown is requested.

Figure 23. Power Shutdown Timing Problem

Figure 24. Power Shutdown Timing

110

SCLK

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

POWER UP

LTC1289 TAF23

110

SCLK

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

POWER UP

LTC1289 TAF24

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