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LTC1325

FUNCTIONAL DESCRIPTIO

communicate with the microprocessor. Data is transmit-

ted in both directions on a single wire. The processor pin

connected to this data line should be configurable as either

an input or an output. The LTC1325 will take control of the

data line and drive it low after the 23rd falling CLK edge

after the start bit is received. Therefore the processor port

must be switched to an input before this happens to avoid

a conflict.

Power-Up After Shutdown

When a control word with the PS bit set to one is written

to the LTC1325, it enters shutdown mode in which the VDD

supply current is reduced to 30

µA. In this mode the on-

chip 3V regulator and all circuits powered off it are shut

down. The only circuits that remain alive are DIN, CS and

CLK input buffers. To take the LTC1325 out from shut-

down mode, a high to low edge must be applied to the CS

pin. Either DIN or CLK must be low when CS is low to

prevent a false control word from being transmitted to the

LTC1325. The 3V output decays with a time constant of

300ms with CREG = 4.7µF. The microprocessor should

wait three seconds before applying a wake-up edge to the

CS pin to ensure proper power-up.

=−





















exp

(2)

LTO

−









(3)

β=

−





























(4)

α=









(5)

= −

(6)

DIV

() − +













–

(7)

where,

VDIV (T) is the output of the divider,

VREG is the voltage at the REG pin (3.072V nominal),

RT is the thermistor resistance at some temperature T,

RTO is the thermistor resistance at some reference

temperature TO,

β is a constant dependent on thermistor material,

α is the temperature coefficient (in %/°C) of RT at

TO, and

all temperatures are in

°K (i.e., T°C + 273)

There are two assumptions in the derivation of the above

equations.

β is assumed to be constant and the tempera-

ture coefficient of RL is small compared to that of the

thermistor.

Most thermistor data sheets specify RTO, β, RT/RTO ratios

for two temperatures,

α, and tolerances for β and RTO.

Given

β, and RTO, it is easy to calculate RL from equation

TEMPERATURE SENSING

NTC (Negative Temperature Coefficient) Thermistors

The simplest method to sense temperature (battery or

ambient) with an NTC thermistor is to use a voltage divider

resistor RL in series with a thermistor RT as shown in

Figure 3. For a given thermistor, there is a value of RL

which makes VDIV (T) linear over a narrow but adequate

temperature range. The easiest method (Inflection Point

Method) to calculate RL is to set the second temperature

derivative of the divider output to 0. The equations relevant

to this method are:

DIV

REG

()









()

(1)

LTC1325CSW Datasheet(PDF) 16 Page - Linear Technology

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