LINEAR TECHNOLOGY LT3525A Technical data

Application Note 5

December 1984

Thermal Techniques in Measurement and Control Circuitry

Jim Williams

Designers spend much time combating thermal effects in
circuitry. The close relationship between temperature and
electronic devices is the source of more design headaches
than any other consideration.

In fact, instead of eliminating or compensating for thermal
parasitics in circuits, it is possible to utilize them. In par­ticular, applying thermal techniques to measurement and
control circuits allows novel solutions to diffi cult problems.
The most obvious example is temperature control. Famil­iarity with thermal considerations in temperature control
loops permits less obvious, but very useful, thermally­based circuits to be built.

Temperature Controller

Figure 1 shows a precision temperature controller for
a small components oven. When power is applied, the
thermistor, a negative TC device, is at a high value. A1

®

saturates positive. This forces the LT

15V

100k*

100k*

0.05

100k*

R

T

3525A switching

0.02

100M

–

+

A1

LT1012

1N914

1N914

10k

THERMAL FEEDBACK

regulator’s output low, biasing Q1. As the heater warms,
the thermistor ’s value decreases. When its inputs fi nally
balance, A1 comes out of saturation and the LT3525A pulse
width modulates the heater via Q1, completing a feedback
path. A1 provides gain and the LT3523A furnishes high
effi ciency. The 2kHz pulse width modulated heater power
is much faster than the thermal loop’s response and the
oven sees an even, continuous heat fl ow.

The key to high performance control is matching the gain
bandwidth of A1 to the thermal feedback path. Theoreti­cally, it is a simple matter to do this using conventional
servo-feedback techniques. Practically, the long time
constants and uncertain delays inherent in thermal systems
present a challenge. The unfortunate relationship between
servo systems and oscillators is very apparent in thermal
control systems.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.

15V

141116

LT3525A

12 6 5

5k

139

0.015
≈2kHz

1k

2k

+

47

Q1
2N5023

20Ω HEATER

50Ω

STEP TEST
50Ω ≈ 0.01°C

*TRW MAR-6 RESISTOR

= YSI #44014 RT = 300k AT 25°C

R

T

Figure 1. Precision Temperature Controller

AN05 F01

an5f

AN5-1

Application Note 5

The thermal control loop can be very simply modeled as
a network of resistors and capacitors. The resistors are
equivalent to the thermal resistance and the capacitors
equivalent to thermal capacity. In Figure 2 the heater,
heater-sensor interface, and sensor all have RC factors
that contribute to a lumped delay in the ability of a ther­mal system to respond. To prevent oscillation, A1’s gain
bandwidth must be limited to account for this delay. Since
high gain bandwidth is desirable for good control, the
delays must be minimized. The physical size and electri­cal resistivity of the heater selected give some element of
control over the heater’s time constant. The heater-sensor
interface time constant can be minimized by placing the
sensor in intimate contact with the heater.

The sensor ’s RC product can be minimized by selecting a
sensor of small size relative to the capacity of its thermal
environment. Clearly, if the wall of an oven is 6" thick
aluminum, the tiniest sensor available is not an absolute

necessity. Conversely, if one is controlling the temperature
of 1/16" thick glass microscope slide, a very small sensor
(i.e., fast) is in order.

After the thermal time constants relating to the heater and
sensor have been minimized, some form of insulation for
the system must be chosen. The function of insulation
is to keep the loss rate down so the temperature control
device can keep up with the losses. For any given sys­tem, the higher the ratio between the heater-sensor time
constants and the insulation time constants, the better
the performance of the control loop.

After these thermal considerations have been attended
to, the control loop’s gain bandwidth can be optimized.
Figures 3A, 3B and 3C show the effects of different com­pensation values at A1. Compensation is trimmed by ap­plying small steps in temperature setpoint and observing
the loop response at A1’s output. The 50Ω resistor and

2V/DIV

HEATER

OR CURRENT CORRESPONDING TO TEMPERATURE)

Figure 2. Thermal Control Loop Model

0.5V/DIV

5 SECONDS/DIV

ABC

AN05 F03a

HEATER-SENSOR INTERFACE

SENSOR

TEMPERATURE REFERENCE

(CAN BE A RESISTANCE, VOLTAGE

2 SECONDS/DIV

AN05 F02

AN05 F03b

0.5V/DIV

HORIZONTAL = 0.5 SECONDS/DIV

AN05 F03c

AN5-2

Figure 3. Loop Response for Various Gain Bandwidths

an5f

Application Note 5

switch in the thermistor leg of the bridge furnish a 0.01°C
step generator. Figure 3A shows the effects of too much
gain bandwidth. The step change forces a damped, ringing
response over 50 seconds in duration! The loop is margin­ally stable. Increasing A1’s gain bandwidth (GBW) will force
oscillation. Figure 3B shows what happens when GBW is
reduced. Settling is much quicker and more controlled. The
waveform is overdamped, indicating that higher GBW is
achievable without stability compromises. Figure 3C shows
the response for the compensation values given and is a
nearly ideal critically damped recovery. Settling occurs
within 4 seconds. An oven optimized in this fashion will
easily attenuate external temperature shifts by a factor of
thousands without overshoots or excessive lags.

Thermally Stabilized PIN Photodiode Signal
Conditioner

PIN photodiodes are frequently employed in wide range
photometric measurements. The photodiode specifi ed in
Figure 4 responds linearly to light intensity over a 100dB
range. Digitizing the diode’s linearly amplifi ed output

would require an A/D converter with 17 bits of range.
This requirement can be eliminated by logarithmically
compressing the diode’s output in the signal condition­ing circuity. Logarithmic amplifi ers utilize the logarithmic
relationship between V

and collector current in transis-

BE

tors. This characteristic is very temperature sensitive and
requires special components and layout considerations
to achieve good results. Figure 4’s circuit logarithmically
signal conditions the photodiode’s output with no special
components or layout.

A1 and Q4 convert the diode’s photocurrent to a voltage
output with a logarithmic transfer function. A2 provides
offsetting and additional gain. A3 and its associated com­ponents form a temperature control loop which maintains
Q4 at constant temperature (all transistors in this circuit
are part of a CA3096 monolithic array). The 0.033μF value
at A3’s compensation pins gives good loop damping if the
circuit is built using the array’s transistors in the locations
shown. These locations have been selected for optimal
control at Q4, the logging transistor. Because of the array

15V

LT1021-10V

OUT

IN

Q4

500pF

50k*

1M

750k*

1M

FULL-SCALE

TRIM

0.01

46

10

5

11

Q5

2k

12

50k
DARK
TRIM

–

A1

10k*

LT1012

+

10k*

2k

15

14

Q1 Q3

13

–

A3

LM301A

+

0.033

3k

33Ω

7

8

9

15V

I

P

–

A2

LM107

+

1

2

3

Q2

AN05 F04

= HP-5082-4204 PIN PHOTODIODE

Q1 TO Q5 = CA3096
CONNECT SUBSTRATE OF CA3096
ARRAY TO Q4’s EMITTER
*1% RESISTOR

E

OUT

LIGHT

(900 NANOMETERS)

1mW

100μW

10μW

1μW

100nW

10nW

RESPONSE DATA

DIODE CURRENT

350μA

35μA

3.5μA

350nA

35nA

3.5nA

CIRCUIT OUTPUT

10.0V

7.85V

5.70V

3.55V

1.40V

–0.75V

Figure 4. 100dB Range Logarithmic Photodiode Amplifi er

an5f

AN5-3