LTC1286CS8 datasheet(14/24 Pages) LINER

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LTC1286/LTC1298

Figure 5. Shutdown current with CS high is 1nA typically,

regardless of the clock. Shutdown current with CS = ground

varies from 1

µA at 1kHz to 35µA at 200kHz.

APPLICATION INFORMATION

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

DOUT 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 DOUT 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 DOUT (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 VCC voltage to keep the CS

input buffer from drawing current. An alternative is to

leave CS low after a conversion, clock data until DOUT

outputs zeros, and then stop the clock low.

DOUT Loading

Capacitive loading on the digital output can increase power

consumption. A 100pF capacitor on the DOUT pin can add

more than 50

µA to the supply current at a 200kHz clock

frequency. An extra 50

µA 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 DIN 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)

0.002

100

140

LT1286/98 G01

180 200

40 60

120

160

CS = 0

(AFTER CONVERSION)

TA = 25°C

VCC = VREF = 5V

CS = VCC

+IN

–IN

GND

VCC

CLK

DOUT

VREF

50k

4.7

µF

MPU

(e.g. 8051)

P1.4

P1.3

P1.2

LTC1286/98 • F06

DIFFERENTIAL INPUTS

COMMON-MODE RANGE

0V TO 5V

LTC1286

Figure 6. Interfacing a 9V Powered LTC1286 to a 5V System

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