hf/rf ZONE Products for the week of August 20, 2001
Linear Technology Says . . .
High Efficiency Thermoelectric Cooler Controller
Linear Technology Corporation announced the LTC1923, a high efficiency ThermoElectric Cooler (TEC) controller IC designed to be used in DWDM (Dense Wave Division Multiplexing) laser-based fiber optic systems. All of the necessary control circuitry and two sets of complementary outputs drivers are integrated into the LTC1923 to drive a full-bridge, providing an efficient means of bi-directional current flow to a TEC. An accurate temperature control loop to stabilize the laser diode system is easily achieved with the addition of just a few external components, allowing a set point stability of 0.1 degrees C to be easily attained. Modified versions achieve .01 degrees C stability. Other applications can include LAN based fiber-optic communications, medical instruments and CPU temperature regulators.
The LTC1923 includes independent adjustable heating and cooling pulse-by-pulse
current limit, current soft-start for controlled start-up, output slew rate
control to reduce system noise, differential current sense and voltage amplifiers
and a host of auxiliary circuits to protect the laser and provide redundant
system monitoring. Its adjustable/synchronizable 250kHz oscillator frequency
operation allows for low profile inductors and tiny ceramic capacitors.
This means that a circuit footprint of less than 1 square inch can be attained
on a double-sided PCB.
analogZONE Says . . .
This is one of those very few parts that come along where you just have to say to yourself, "I wish I had thought of this!" Just looking at a block diagram of the part without any documentation you would wonder if the product was a dc-dc converter controller combined with battery management functions. Some of those basic architectures are here but the purpose of the part is a unique spin-off.
The Peltier-Effect-derived TEC (thermoelectric cooler) used in DWDM systems is generally used to control the temperature of the laser within 0.1 degrees C, sometimes much tighter, to keep the laser wavelength at its peak operating point -- a resonant window of less than 0.1 nm for 20 dB reductions in intensity. In this application the TEC, which is tightly thermally bonded to the laser, is at the center point of an H-bridge of power MOSFETs allowing power to pass either way, to either heat or cool the thermal load. A thermistor on the TEC is used as the thermal feedback sensor and forms a bridge with an external DAC, the output of which is externally amplified and passed to the LTC1923. Both loop bandwidth and loop gain are set by single resistors at the input to the part. The LTC1923 is a pulse-width modulator which provides the correct modulated and phased drive to the power stages.
LTC has added many safety features in the form of monitoring, limiting and shutdown to protect what is a rather expensive load, which is also mission-critical in any DWDM system. The initial, optimal operating point for the system, of course, is set by the DAC, with a 2.5-V reference generated in the LTC1923. The oscillator frequency is a nominal 225 kHz, which can be varied with an external resistor/capacitor combination up to 1 MHz. Charge and discharge times through the capacitor are identical in what is a triangular wave oscillator design. The absolute value of the resistor will vary the dead time of switching between the sides of the power bridge (break-before-make time). The typical no-load operating current with a 5-V rail is 2 mA.
Care is needed in this type of application to avoid inductor ripple current through the inductor load, and noise pick up in the signal feed from the thermistor to the circuit point of the input bridge. Also, thermal management of any Peltier-Effect device is critical.
This will be a major part. Its use in DWDM systems will only be a first step -- any Peltier-Effect device would probably be a candidate for control in this way, maybe even down to those who want to overclock PCs!
The LTC1923 is in production in an SSOP-28. LTC is discussing the price
only with its major DWDM customers but said, "price (is) slightly below
the cost of a module and slightly above the cost of discretes."