Mar 2001 A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers LT Journal Lin

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Specifications	Mar 2001 A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers LT Journal Linear Technology Corporation
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Linear Technology Magazine • March 200228 DESIGN IDEAS An article in an earlier issue of this magazine described a swit

Specifications	Mar 2001 A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers LT Journal Linear Technology Corporation
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Content	Linear Technology Magazine • March 200228 DESIGN IDEAS An article in an earlier issue of this magazine described a switched mode temperature controller for ﬁber optic lasers.1 This previous effort combined high efﬁciency switching regulator characteristics with precision, closed loop control. This article offers a dif- ferent approach to temperature control using the LT1970 in a linear circuit instead of a switching regula- tor circuit. The most distinguishing require- ment of laser temperature control is that the controller must be able to extract heat as well as supply it. This necessitates Peltier-effect based ther- moelectric heater coolers (TEC) located within the laser module—a feature usually included in the laser module by the laser manufacturer. In most cases the switching based approach is preferable to maintain high efﬁciency. Occasionally a linear controller is desirable because it elimi- nates inductors and saves space. A trade off in the linear approach is heat dissipation, which makes it use- ful in applications where space is premium, and the extra heat is man- ageable. Figure 1 shows the linear control- ler. The LTC2053 chopper stabilized instrumentation ampliﬁer extracts an error signal from a bridge network. One bridge leg is a thermistor tem- perature sensor located within the laser module. The ampliﬁer provides A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers gain, deﬁned by the 10M/24.9K ratio, and feeds a power output stage. The LT1970, augmented by Q1 and Q2, forms a 2A driver for the TEC. Cur- rent limiting is provided by the LT1970 sensing across the 0.05W shunt, pro- tecting the laser. Here, the current limit points, set by the voltages at VCSINK and VCSRC pins, are identi- cal for sourcing and sinking current. Different programming voltages would permit asymmetric limits. The power stage, operating at a gain of three to ensure Q1–Q2 satura- tion capability, drives the TEC. The TEC’s thermal feedback to the bridge located thermistor closes a control loop, stabilizing the laser module tem- perature. The bipolar power supply by Jim Williams – + + +LASER TEC10M 10k10k 12.5k*RTHERMISTOR 240W 34k 300W 15k RG EN REF V–V+ 24.9k 5.1k 51W 51W 18W 0.05W 51W 10k 0.1µF 0.1µF 1µF 1µF 22µF 22µF 0.01µF +3.3V –5V –5V +3.3V 1%, 10PPM/˚C **TYPICAL VALUE, SELECTED FOR DESIRED LASER TEMPERATURE 10PPM/˚C D1:LT1004-2.5 D2:1N5222-2.5 Q1:DH45VH10 Q2:DH44VH10 D1 D2 THERMAL FEEDBACK PATH Q2 Q1 –IN OUT +IN V EE V CC V EE EN V EE V EE V CSINK V CSRC V – V + SENSE – SENSE + LTC2053 LT1970COM Figure 1. Two ampliﬁers form a thermoelectric temperature cooler for a ﬁber optic laser module. The linear approach eliminates inductors. As low as 0.01˚C control stability over a wide ambient temperature swing is achievable. continued on page 37
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Mar 2001 A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers LT Journal Linear Technology Corporation

Mar 2001 A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers LT Journal Linear Technology Corporation