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MAX8520 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX8520 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 18 page Smallest TEC Power Drivers for Optical Modules 10 ______________________________________________________________________________________ Detailed Description The MAX8520/MAX8521 TEC drivers consist of two switching buck regulators that operate together to directly control the TEC current. This configuration cre- ates a differential voltage across the TEC, allowing bi- directional TEC current for controlled cooling and heating. Controlled cooling and heating allow accurate TEC temperature control to within 0.01°C. The voltage at CTLI directly sets the TEC current. An external thermal- control loop is typically used to drive CTLI. Figures 1 and 2 show examples of the thermal-control-loop circuit. Ripple Cancellation Switching regulators like those used in the MAX8520/ MAX8521 inherently create ripple voltage on the output. The dual regulators in the MAX8520/MAX8521 switch in-phase and provide complementary in-phase duty cycles so ripple waveforms at the TEC are greatly reduced. This feature suppresses ripple currents and electrical noise at the TEC to prevent interference with the laser diode. Switching Frequency For the MAX8521, FREQ sets the switching frequency of the internal oscillator. With FREQ = GND, the oscilla- tor frequency is set to 500kHz. The oscillator frequency is 1MHz when FREQ = VDD. For the MAX8520, connect a resistor (REXT in Figure 2) from FREQ to GND. Choose REXT = 60kΩ for 1MHz operation, and REXT = 150kΩ for 500KHz operation. For any intermediary frequency between 500kHz and 1MHz, use the following equation to find the value of REXT value needed for VDD = 5V: where REXT is the resistance given in kΩ, and fs is the desired frequency given in MHz. Note that for VDD < 5V, the frequency is reduced slightly, to the extent of about 7% when VDD reaches 3V. This should be taken into consideration when selecting the value for REXT at known supply voltage. Voltage and Current-Limit Setting Both the MAX8520 and MAX8521 provide control of the maximum differential TEC voltage. Applying a voltage to MAXV limits the maximum voltage across the TEC. The voltage at MAXIP and MAXIN sets the maximum positive and negative current through the TEC. These current limits can be independently controlled. Current Monitor Output ITEC provides a voltage output proportional to the TEC current (ITEC). See the Functional Diagram for more detail: VITEC = 1.5V +(8 (VOS1-VCS)) Reference Output The MAX8520/MAX8521 include an on-chip voltage ref- erence. The 1.50V reference is accurate to 1% over temperature. Bypass REF with 0.1µF to GND. REF can be used to bias an external thermistor for temperature sensing as shown in Figures 1 and 2. Thermal and Fault-Current Protection The MAX8520/MAX8521 provide fault-current protec- tion in either FETs by turning off both high-side and low-side FETs when the peak current exceeds 3A in either FETs. In addition, thermal-overload protection limits the total power dissipation in the chip. When the device’s die junction temperature exceeds +165°C, an on-chip thermal sensor shuts down the device. The thermal sensor turns the device on again after the junc- tion temperature cools down by +15°C. Design Procedures Duty-Cycle Range Selection By design, the MAX8520/MAX8521 are capable of operating from 0% to 100% duty cycle, allowing both LX outputs to enter dropout. However, as the LX pulse width narrows, accurate duty-cycle control becomes difficult. This can result in a low-frequency noise appearing at the TEC output (typically in the 20kHz to 50kHz range). While this noise is typically filtered out by the low thermal-loop bandwidth, for best result, operate the PWM with a pulse width greater than 200ns. For 500kHz application, the recommended duty-cycle range is from 10% to 90%. For 1MHz application, it is from 20% to 80%. R fs EXT =× − ⎛ ⎝⎜ ⎞ ⎠⎟ 90 11 3 TEC Connection Thermistor Heating Mode PTC Cooling Mode NTC Table 1. TEC Connection for Figure 1 TEC Connection Thermistor Heating Mode NTC Cooling Mode PTC Table 2. TEC Connection for Figure 2 |
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