Datasheet

LTC2444/LTC2445/
LTC2448/LTC2449
24
2444589fb
APPLICATIO S I FOR ATIO
WUU
U
SLEEP CONVERT SLEEP CONVERT SLEEP
8µA
2440 F15
8mA8µA8mA8µA
DATA
OUT
DATA
OUT
CONVERTER
STATE
SUPPLY
CURRENT
CS
Figure 14. Reduced Power Timing Mode
V
REF
+
V
IN
+
V
CC
R
SW
(TYP)
500Ω
I
LEAK
I
LEAK
V
CC
I
LEAK
I
LEAK
V
CC
R
SW
(TYP)
500Ω
C
EQ
5pF
(TYP)
(C
EQ
= 2pF
SAMPLE CAP
+ PARASITICS)
R
SW
(TYP)
500Ω
I
LEAK
I
IN
+
V
IN
–
I
IN
–
I
REF
+
I
REF
–
2440 F16
I
LEAK
V
CC
I
LEAK
I
LEAK
SWITCHING FREQUENCY
f
SW
= 1.8MHz INTERNAL OSCILLATOR
f
SW
= f
EOSC
/5 EXTERNAL OSCILLATOR
V
REF
–
R
SW
(TYP)
500Ω
MUX
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/(f
SW
• Ceq)
When using the internal oscillator, f
SW
is 1.8MHz and the
equivalent resistance is approximately 110kΩ.