DESIGN IDEAS A Linear Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers by Jim Williams 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 eliminates inductors and saves space. A trade off in the linear approach is heat dissipation, which makes it useful in applications where space is premium, and the extra heat is manageable. Figure 1 shows the linear controller. The LTC2053 chopper stabilized instrumentation amplifier extracts an error signal from a bridge network. One bridge leg is a thermistor temperature sensor located within the laser module. The amplifier provides 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 different approach to temperature control using the LT1970 in a linear circuit instead of a switching regulator circuit. The most distinguishing requirement of laser temperature control is that the controller must be able to extract heat as well as supply it. This necessitates Peltier-effect based thermoelectric heater coolers (TEC) located within the laser module—a feature +3.3V 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. Current limiting is provided by the LT1970 sensing across the 0.05Ω shunt, protecting the laser. Here, the current limit points, set by the voltages at VCSINK and VCSRC pins, are identical 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 saturation capability, drives the TEC. The TEC’s thermal feedback to the bridge located thermistor closes a control loop, stabilizing the laser module temperature. The bipolar power supply continued on page 37 300Ω 34k +3.3V –5V D1 15k 0.1µF 240Ω 1µF 51Ω 0.1µF D2 V– +IN LTC2053 22µF LT1970 COM 24.9k VEE VEE VEE –IN VEE 22µF VCSINK + SENSE+ V+ 10M SENSE– 12.5k** REF EN + RTHERMISTOR RG 51Ω LASER TEC OUT 18Ω V– – Q1 VCSRC + V+ VCC 10k* EN 10k* 0.05Ω Q2 51Ω 10k 5.1k * 1%, 10PPM/˚C ** TYPICAL VALUE, SELECTED FOR DESIRED LASER TEMPERATURE 10PPM/˚C D1: D2: Q1: Q2: LT1004-2.5 1N5222-2.5 DH45VH10 DH44VH10 0.01µF –5V 1µF THERMAL FEEDBACK PATH 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. 28 Linear Technology Magazine • March 2002 DESIGN IDEAS 0.005˚C LT1970, continued from page 28 10mV/DIV AC COUPLED 0.5s/DIV 1 HOUR Figure 2: Optimized gain-bandwidth results in nearly critically damped response with settling in 1.5 seconds Figure 3. 10-hour cooling mode stability measured in an environment that steps 20˚C above ambient every hour. Data shows the resulting 0.0035˚C peak-to-peak variation, indicating a thermal gain of 5700. permits positive or negative TEC bias, allowing either heating or cooling in response to feedback loop demands. The temperature control set point is fixed by the bridge resistor value adjacent to the thermistor, in this case 12.5kΩ. Alternately, a DAC controlled potential, supplied to the LTC2053 – input, can establish the set point. The considerable feedback lag due to thermal time constants requires loop compensation. Loop gain-bandwidth is set at the LTC2053 by the 10M–24.9k ratio and the associated feedback capacitor. Various laser modules have different thermal characteristics, mandating care in setting loop gain-bandwidth values. Optimal performance is determined by observing loop response to 0.10°C temperature setpoint step changes while adjusting gain-bandwidth values. Figure 2, the amplified thermistor bridge difference, shows a nearly critically damped response to such step inputs, indicating proper loop compensation. 2 Once optimized, this controller can easily maintain 0.01°C stability under widely varying ambient temperature conditions. Figure 3’s strip-chart recording measures cooling mode stability against an environment that steps 20°C above ambient every hour over 10 hours.3 The data shows .0035°C resulting variation, indicating a thermal gain of 5700. LTC3423/LTC3424, continued from page 29 fixed frequency switching is enabled when driving the MODE/SYNC pin low. The operating frequency can be set externally by connecting a clock to the MODE/SYNC pin. The part can also be shut down, drawing less than 1µA of quiescent current. Single Cell to 1.8V at 700mA Application Linear Technology Magazine • March 2002 100 1.5V 90 BURST MODE OPERATION 80 EFFICIENCY (%) The LTC3423 is optimized for applications requiring 0.6 watts or less of output power, whereas the LTC3424 is optimized for applications requiring 1.2 watts or less. Both converters offer programmable operating frequencies from 100kHz to 3MHz via a resistor from the RT pin to ground. Output voltage is adjustable from 1.5V to 5.5V with a simple resistor voltage divider to the FB (feedback) pin. The transient response can be optimized over a wide range of loads and output capacitors via current mode control architecture, OPTI-LOOP® compensation and adaptive slope compensation. To further improve efficiency for light loads, the user can take advantage of high efficiency Burst Mode, where the IC consumes only 38µA of quiescent current. Burst Mode operation is enabled by driving the MODE/SYNC pin high, otherwise 1.2V 70 0.9V 60 50 Notes: 1. “A Thermoelectric Cooler Temperature Controller for Fiber Optic Lasers” Linear Technology Magazine, pg. 10–13, September 2001. See also LTC Application Note 89, with the same title but considerably more detail. 2. This forum must suffer brevity. Those finding this discussion intolerably brief are commended to LTC Application Note 89, where thermal loop optimization techniques are treated in an appropriately studious manner. 3. That’s right, a strip-chart recording. Stubborn, locally based aberrants persist in their use of such archaic devices, forsaking more modern alternatives. Technical advantage could account for this choice, although deeply seated cultural bias may be a factor. Figure 2 shows a circuit for generating 1.8V directly from the single cell at 700ma load current. Figure 3 shows the efficiency curves over the operating voltage range of the battery. The efficiency peaks at 95% with a fresh battery, and drops to 90% when the battery terminal voltage is 0.9V. If Burst Mode is enabled at light loads, the efficiency stays above 70% down to a 500µA load. Conclusion 40 30 20 10 0 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 Figure 3. Efficiency curve for the circuit in Figure 2 at various battery input voltages Linear Technology’s Low Output Voltage Synchronous Boost converters allow designers of handheld electronics to efficiently regulate a voltage as low as 1.5V directly from a single cell. They provide solutions for prolonged battery life, small circuit board size and easy programmability all in a small MSOP-10 Package. 37