Datasheet
16
LTC1325
FUNCTIONAL DESCRIPTIO
UU
U
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 V
DD
supply current is reduced to 30µA. In this mode the on-
chip 3V regulator and all circuits powered off it are shut
down. The only circuits that remain alive are D
IN
, 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 D
IN
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 C
REG
= 4.7µF. The microprocessor should
wait three seconds before applying a wake-up edge to the
CS pin to ensure proper power-up.
R
RTT
T
TO O
=−
exp β
11
(2)
RR
T
T
LTO
O
O
=
−
+
β
β
2
2
(3)
β=
−
T
T
TT
In
R
R
O
O
T
TO
(4)
α=
1
R
dR
dT
T
T
(5)
α
β
=
−
T
2
(6)
dV
dT
VT
T
T
DIV
DIV O
O
O
=
()
−
+
–
β
2
1
2
(7)
where,
V
DIV
(T) is the output of the divider,
V
REG
is the voltage at the REG pin (3.072V nominal),
R
T
is the thermistor resistance at some temperature T,
R
TO
is the thermistor resistance at some reference
temperature T
O
,
β is a constant dependent on thermistor material,
α is the temperature coefficient (in %/°C) of R
T
at
T
O
, 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 R
L
is small compared to that of the
thermistor.
Most thermistor data sheets specify R
TO
, β, R
T
/R
TO
ratios
for two temperatures, α, and tolerances for β and R
TO
.
Given β, and R
TO
, it is easy to calculate R
L
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
powered by the REG pin. This divider consists of a load
resistor R
L
in series with a thermistor R
T
as shown in
Figure 3. For a given thermistor, there is a value of R
L
which makes V
DIV
(T) linear over a narrow but adequate
temperature range. The easiest method (Inflection Point
Method) to calculate R
L
is to set the second temperature
derivative of the divider output to 0. The equations relevant
to this method are:
VT
V
R
R
fT
DIV
REG
L
T
()
=
+
=
()
1
1
(1)