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LTC1923

1923f

APPLICATIO S I FOR ATIO

–

REF

VOUT

TMP

CMD

LTC1658

10k

NTC

LTC1923

ERROR

AMPLIFIER

1923 F13

4.7

µF

10M

10k

100k

CNTRL

EAOUT

Figure 13. Simplifed Temperature Control Loop Omitting

the LTC2053 Instrumentation Amplifier Front End

with CA on the order of CB and note its affect on system

response. Adjust the values based on observing whether

the transient response was improved or not with the goal

of reducing CB to improve settling time. As the system

thermal poles can vary between “identical” laser modules

(i.e., same manufacturer and model), care must be taken

to ensure that the values selected provide the desired

response even with these thermal term variations. Com-

pensation should also be tailored for each unique laser

module as thermal terms can vary significantly between

different brands. CC rolls off high frequency gain , mini-

mizing noise in the outputs. It is typically about 25 times

smaller than CB. CA, CB and CC should be film capacitors.

Temperature Stability

It is important to differentiate between temperature accu-

racy and stability. Since each laser’s output maximizes at

some temperature, temperature setpoint is typically

incremented until this peak is achieved. After this, only

temperature stability is required. The predominant param-

eters which affect temperature stability are the thermistor,

the thermistor biasing resistor and any offset drift of the

front-end electrical circuitry. Sufficient loop gain ensures

that any downstream variations do not contribute signifi-

cantly to temperature stability. The relatively mild operat-

ing conditions inside the laser module promote good long-

term thermistor stability. A high quality, low temperature

coefficient resistor should be selected to bias the ther-

mistor. If the 10k resistor has a 100ppm/

°C temperature

coefficient, this translates into a 0.18

°C setpoint tempera-

ture differential over a 0

°C to 70°C ambient for a desired

°C laser setpoint. Depending upon the temperature

stability requirements of the system, this is very signifi-

cant. A lower temperature coefficient resistor may there-

fore be desired. The LTC2053 has maximum offset drift to

50nV/

°C which translates into less than 0.001°C change

for a 0

°C to 70°C ambient.

The offset drift of the LTC1923 error amplifier divided by

the gain of the LTC2053 also affects temperature stability.

The offset drift of the LTC1923 (see characteristic curves)

is typically 1mV over a 0

°C to 70°C ambient. After attenu-

ation by the LTC2053 gain, this translates into a tempera-

ture setpoint variation of 0.004

°C. Neither of these offsets

drifts significantly with aging. Depending upon the setpoint

temperature stability requirements of the system, the

LTC2053 instrumentation amplifier may not be necessary.

Figure 13 shows a simplified schematic with the LTC2053

omitted.

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