LTC1289 Linear Technology, LTC1289 Datasheet - Page 26

• Manufacturer Part Number: LTC1289
• Description: 3 Volt Single Chip 12-Bit Data Acquisition System
• Manufacturer: Linear Technology

LTC1289
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 A (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
JSR SHUTDOWN
The actual subroutine is:
SHUTDOWN: LDA #$3D
26
TYPICAL
SCLK
SCLK
CS
CS
A
JSR TRANSFER Read LTC1289 routine
RTS
PPLICATI
Determines which reading has valid
data, converts to 2’s complement
and stores in RAM
LTC1289 power shutdown routine
1
1
•
•
O
Load D
LTC1289 into ACC
Return
U
S
Figure 23. Power Shutdown Timing Problem
IN
DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS
DUMMY CONVERSION FINISHES AFTER 52 ACLK PERIODS
word for
Figure 24. Power Shutdown Timing
POWER SHUTDOWN STARTS
POWER SHUTDOWN STARTS
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.
10
10
POWER UP
LTC1289 TAF24
LTC1289 TAF23
POWER UP