20
LTC1923
1923f
TMP
CMD
–
+
–
+
REF
V
OUT
LTC1658
10k
10k
NTC
REF
R
C
R
A
R
B
FB
CNTRL
EAOU
LTC1923
ERROR
AMPLIFIER
C
A
6
5
1923 F1
4
A = 10
LTC2053
C
B
C
C
Figure 12. Alternative Compensation Method to Improve Transient Response
APPLICATIO S I FOR ATIO
WUUU
The linearized thermistor gain around 25°C is –25mV/°C.
For a minimum loop gain of 5000 as calculated above, the
combined gain of the instrumentation and error amplifiers
can be calculated:
K
IA
• K
EA
= T/(K
MOD
• K
PWR
• K
TEC
• K
THRM
)
K
IA
• K
EA
= 5000/(10 • 30 • 0.025) = 667
A combined gain of 1000 can be selected to provide
adequate margin. The instrumentation amplifier gain should
be set at typically 10, as this attenuates any errors by its
gain factor. The error amplifier gain would then be limited
to the remainder through the gain setting resistors, R
F
and
R
A
shown in Figure 11.
R
F
/R
A
= KEA – 1
The multiple poles associated with the TEC/thermistor
system makes it difficult to compensate. Compounding
this problem is that there will be significant variations in
thermal time constants for the same system, making
elaborate compensation schemes difficult to reliably imple-
ment. The most robust method (i.e., least prone to
oscillation) is to place a dominant pole well below the
thermal system time constant (τ) (anywhere from many
seconds to minutes). This time constant will set the
capacitor value by the following equation:
C
F
=
τ
/R
F
Please refer to Application Note 89 for more detailed
information on compensating the loop. Ceramic capaci-
tors are not recommended for use as the integrating
capacitor or anywhere in the signal path as they exhibit a
piezoelectric effect which can introduce noise into the
system. The component values shown on the front page of
this data sheet provide a good starting point, but some
adjustment may be required to optimize the response.
Dominant pole compensation does have its limitations. It
provides good loop response over a wide range of laser
module types. It does not provide the fastest transient
response to step changes in temperature. If this is a
necessity, a more complex compensation approach as
shown in Figure 12 may be required. This approach adds
an additional zero into the feedback loop to speed up the
transient response. First note that the LTC2053 inputs
have been swapped as the LTC1923 error amplifier is now
running in an inverting configuration. Capacitor C
A
is
needed to provide the lead term. Resistor R
C
is used to
buffer the LTC2053 from capacitive loading and limit the
error amplifier high frequency gain.
Since the system thermal pole locations are not known, a
qualitative compensation approach must be employed.
This entails looking at the transient response when the
TEC is heating (due to the inherent higher gain) for a small-
signal step change in temperature and modifying compen-
sation components to improve the response. A reasonable
starting point is to select components that mimic the
response that will be obtained from the front page of this
data sheet. Therefore R
A
, R
B
and C
B
would be selected to
be 1MΩ, 1MΩ and 0.47µF, respectively. R
C
should be
selected to be a factor of 100 smaller than R
A
, or on the
order of 10k. Make sure that the loop is stable prior to the
introduction of capacitor C
A
. The addition of C
A
will
provide some phase boost in the loop (in effect, offsetting
one of the poles associated with the thermal system). Start
