Datasheet

19
LTC1923
1923f
APPLICATIO S I FOR ATIO
WUUU
V
IN
+
–
V
E
C
T
R
A
R
F
LTC1923LTC2053
C
F
V
REF
K
THRM
K
IA
K
EA
K
MOD
K
PWR
K
TEC
R1
10k
10k
NTC
–
+
–
+
–
+
POWER
STAGE
TEC
1923 F11
V
TECOOLER
+
ERROR
AMP
Figure 11. Simplified Loop Block Diagram
This voltage error translates back into a temperature
setpoint error.
Example:
R
THRM
= 10k
NTC with 4.4%/°C at 25°C
R1 = 10k
V
REF
= 2.5V
T = 25°C
For this thermistor with a 25°C temperature setpoint, the
change in thermistor voltage with temperature is given by
–25mV/°C. In order to maintain a 0.01°C temperature
accuracy, this translates into a 250µV error signal, V
E
. The
minimum loop gain can now be calculated from the above
equation:
V
E
= V
IN
/(1 + T)
A 25°C setpoint temperature requires V
IN
= 1.25V for
V
REF
= 2.5V. The required loop gain is 5000 or 74dB.
There are two handles to adjust the loop gain, K
IA
and K
EA
,
while the other handles are fixed and depend upon the TEC
and thermistor characteristics (K
TEC
and K
THRM
), V
SET
and
R1 (K
THRM
) and V
DD
(K
MOD
and K
PWR
). The modulator and
power gain product is given by:
K
MOD
• K
PWR
= 2 • V
DD
/V
CT
= 2 • V
DD
where V
CT
= the C
T
voltage which has a fixed 1V amplitude.
The TEC gain depends upon the TEC selected and corre-
sponds to the relationship between the voltage across the
device and what temperature differential is created. This
gain term changes with operating temperature, and whether
the TEC is heating or cooling. TECs are inherently more
efficient at heating (and therefore have a higher gain) as
compared to cooling. A worst-case rough estimation of
the gain can be obtained by taking the maximum TEC
voltage required to force a given change in temperature
from the TEC specifications:
K
TEC
= dT/V
TEC(MAX)
The thermistor gain should be linearized around tempera-
ture setpoint.
Example:
Setpoint T = 25°C
V
DD
= 5V
R
THRM
= 10k NTC with 4.4%/°C at 25°C
R1 = 10k
V
REF
= 2.5V
dT/V
TEC(MAX)
= 45°C/1.5V = 30°C/V