14
LTC1286/LTC1298
Figure 5. Shutdown current with CS high is 1nA typically,
regardless of the clock. Shutdown current with CS = ground
varies from 1µA at 1kHz to 35µA at 200kHz.
APPLICATION INFORMATION
WUU
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Clock Frequency
The maximum recommended clock frequency is 200kHz
for the LTC1286/LTC1298 running off a 5V supply. With
the supply voltage changing, the maximum clock fre-
quency for the devices also changes (see the typical curve
of Maximum Clock Rate vs Supply Voltage). If the maxi-
mum clock frequency is used, care must be taken to
ensure that the device converts correctly.
Mixed Supplies
It is possible to have a microprocessor running off a 5V
supply and communicate with the LTC1286 operating on
a 9V supply. The requirement to achieve this is that the
outputs of CS and CLK from the MPU have to be able to trip
the equivalent inputs of the LTC1286 and the output of
D
OUT
from the LTC1286 must be able to toggle the
equivalent input of the MPU (see typical curve of Digital
Input Logic Threshold vs Supply Voltage). With the
LTC1286 operating on a 9V supply, the output of D
OUT
may
go between 0V and 9V. The 9V output may damage the
MPU running off a 5V supply. The way to get around this
possibility is to have a resistor divider on D
OUT
(Figure 6)
and connect the center point to the MPU input. It should
be noted that to get full shutdown, the CS input of the
LTC1286 must be driven to the V
CC
voltage to keep the CS
input buffer from drawing current. An alternative is to
leave CS low after a conversion, clock data until D
OUT
outputs zeros, and then stop the clock low.
D
OUT
Loading
Capacitive loading on the digital output can increase power
consumption. A 100pF capacitor on the D
OUT
pin can add
more than 50µA to the supply current at a 200kHz clock
frequency. An extra 50µA or so of current goes into
charging and discharging the load capacitor. The same
goes for digital lines driven at a high frequency by any logic.
The C × V × f currents must be evaluated and the trouble-
some ones minimized.
OPERATING ON OTHER THAN 5V SUPPLIES (LTC1286)
The LTC1286 operates from 4.5V to 9V supplies and the
LTC1298 operates from a 5V supply. To operate the LTC1286
on other than 5V supplies a few things must be kept in
mind.
Input Logic Levels
The input logic levels of CS, CLK and D
IN
are made to meet
TTL on a 5V supply. When the supply voltage varies, the
input logic levels also change. For the LTC1286 to sample
and convert correctly, the digital inputs have to be in the
proper logical low and high levels relative to the operating
supply voltage (see typical curve of Digital Input Logic
Threshold vs Supply Voltage). If achieving micropower
consumption is desirable, the digital inputs must go rail-to-
rail between supply voltage and ground (see ACHIEVING
MICROPOWER PERFORMANCE section).
FREQUENCY (kHz)
1
0.002
SUPPLY CURRENT (µA)
5
1
0
15
20
25
35
20
100
140
LT1286/98 G01
10
30
80
180
200
40
60
120 160
CS = 0
(AFTER CONVERSION)
T
A
= 25°C
V
CC
= V
REF
= 5V
CS = V
CC
+IN
–IN
GND
V
CC
CLK
D
OUT
V
REF
50k
50k
5V
4.7µF
MPU
(e.g. 8051)
5V
P1.4
P1.3
P1.2
LTC1286/98 • F06
DIFFERENTIAL INPUTS
COMMON-MODE RANGE
0V TO 5V
9V
LTC1286
CS
Figure 6. Interfacing a 9V Powered LTC1286 to a 5V System
