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LTC1290

1290fe

APPLICATI

TYPICAL

Power Shutdown

For battery-powered applications it is desirable to keep

power dissipation at a minimum. The LTC1290 can be

powered down when not in use reducing the supply

current from a nominal value of 5mA to typically 5

µA (with

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

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

LTC1290 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

LTC1290 power shutdown routine

The actual subroutine is:

SHUTDOWN: LDA #$3D

Load DIN word for

LTC1290 into ACC

JSR TRANSFER Read LTC1290 routine

RTS

Return

To place the device in power shutdown the word length

bits are set to WL1 = 0 and WL0 = 1. The LTC1290 is

powered up on the next request for a conversion and it’s

ready to digitize an input signal immediately.

Power Shutdown Timing Considerations

After power shutdown has been requested, the LTC1290

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 LTC1290 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

LTC1290 waits for the next request for conversion. If the

SCLKs have not finished once the LTC1290 has finished its

dummy conversion, it will recognize the next remaining

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

LTC1290 (see Figure 23). To prevent this, bring either CS

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

SCLKs (Figure 25) when power shutdown is requested.

110

SCLK

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

POWER UP

1290 TAF23

Figure 23. Power Shutdown Timing Problem

110

SCLK

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

POWER UP

1290 TAF24

Figure 24. Power Shutdown Timing

110

SCLK

POWER SHUTDOWN STARTS

DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS

POWER UP

1290 TAF25

Figure 25. Power Shutdown Timing

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