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LTC2315-12

231512f

For more information www.linear.com/2315-12

Figure 8: LTC2315-12 Entering/Exiting Nap Mode

Figure 9: LTC2315-12 Entering/Exiting Sleep Mode

SCK

SDO

HI-Z STATE

231512 F08

NAP MODE

HOLD STATIC HIGH or LOW

START tACQ

SCK

SDO

HI-Z STATE

231512 F09

NAP MODE

SLEEP MODE

HOLD STATIC HIGH or LOW

START tACQ

VREF RECOVERY

tWAIT

applicaTions inForMaTion

Entering Nap/Sleep Mode

Pulsing CS two times and holding SCK static places the

LTC2315-12 into nap mode. Pulsing CS four times and

holding SCK static places the LTC2315-12 into sleep

mode. In sleep mode, all bias circuitry is shut down,

including the internal bandgap and reference buffer, and

only leakage currents remain (0.8µA typical). Because

the reference buffer is externally bypassed with a large

capacitor (2.2µF), the LTC2315-12 requires a significant

wait time (1.1ms) to recharge this capacitance before an

accurate conversion can be made. In contrast, nap mode

does not power down the internal bandgap or reference

buffer allowing for a fast wake-up and accurate conversion

within one conversion clock cycle. Supply current during

nap mode is nominally 1.8mA.

Exiting Nap/Sleep Mode

WakinguptheLTC2315-12fromeithernaporsleepmode,

as shown in Figures 8 and 9, requires SCK to be pulsed

one time. A conversion may be started immediately fol-

lowing nap mode as shown in Figure 8. A period of time

allowing the reference voltage to recover must follow

waking up from sleep mode as shown in Figure 9. The

wait period required before initiating a conversion for the

recommended value of CREF of 2.2µF is 1.1ms.

Power Supply Sequencing

The LTC2315-12 does not have any specific power sup-

ply sequencing requirements. Care should be taken to

observe the maximum voltage relationships described in

the Absolute Maximum Ratings section.

Single-Ended Analog Input Drive

The analog input of the LTC2315-12 is easy to drive. The

input draws only one small current spike while charging

the sample-and-hold capacitor at the end of conversion.

During the conversion, the analog input draws only a small

leakage current. If the source impedance of the driving

circuit is low, then the input of the LTC2315-12 can be

driven directly. As the source impedance increases, so

will the acquisition time. For minimum acquisition time

with high source impedance, a buffer amplifier should be

used. The main requirement is that the amplifier driving

the analog input must settle after the small current spike

beforethenextconversionstarts.Settlingtimemustbeless

than tACQ-MIN (40ns) for full performance at the maximum

throughput rate. While choosing an input amplifier, also

keep in mind the amount of noise and harmonic distortion

the amplifier contributes.

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