LTC2445 datasheet(24/28 Pages) LINER

24 / 28 page

LTC2444/LTC2445/

LTC2448/LTC2449

sn2444589 2444589fs

APPLICATIO S I FOR ATIO

SLEEP

CONVERT

SLEEP

CONVERT

SLEEP

8µA

2440 F15

8mA

8µA

8mA

8µA

DATA

OUT

DATA

OUT

CONVERTER

STATE

SUPPLY

CURRENT

Figure 14. Reduced Power Timing Mode

VREF+

VIN+

VCC

RSW (TYP)

500Ω

ILEAK

VCC

ILEAK

VCC

RSW (TYP)

500Ω

CEQ

5pF

(TYP)

(CEQ = 2pF

SAMPLE CAP

+ PARASITICS)

RSW (TYP)

500Ω

ILEAK

IIN+

VIN–

IIN–

IREF+

IREF–

2440 F16

ILEAK

VCC

ILEAK

SWITCHING FREQUENCY

fSW = 1.8MHz INTERNAL OSCILLATOR

fSW = fEOSC/5 EXTERNAL OSCILLATOR

VREF–

RSW (TYP)

500Ω

MUX

Figure 15. LTC2444/LTC2448 Input Structure

Reduced Power Operation

In addition to adjusting the speed/resolution of the

LTC2444/LTC2445/LTC2448/LTC2449, the speed/reso-

lution/power dissipation may also be adjusted using the

automatic sleep mode. During the conversion cycle, the

LTC2444/LTC2445/LTC2448/LTC2449 draw 8mA supply

current independent of the programmed speed. Once the

conversion cycle is completed, the device automatically

enters a low power sleep state drawing 8µA. The device

remains in this state as long as CS is HIGH and data is not

shifted out. By adjusting the duration of the sleep state

(hold CS HIGH longer) and the duration of the conversion

cycle (programming OSR) the DC power dissipation can

be reduced, see Figure 14.

Average Input Current

The LTC2444/LTC2448 switch the input and reference to

a 2pF capacitor at a frequency of 1.8MHz. A simplified

equivalent circuit is shown in Figure 15. The sample

capacitor for the LTC2445/LTC2449 is 4pF, and its aver-

age input current is externally buffered from the input

source.

The average input and reference currents can be ex-

pressed in terms of the equivalent input resistance of the

sample capacitor, where: Req = 1/(fSW • Ceq)

When using the internal oscillator, fSW is 1.8MHz and the

equivalent resistance is approximately 110kΩ.

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