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UCC3750DWTRG4 Datasheet(PDF) 8 Page - Texas Instruments |
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UCC3750DWTRG4 Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 18 page  8 UCC2750 UCC3750 Sine Reference Generator The IC has a versatile low frequency sinewave reference generator with low harmonic distortion and good fre- quency accuracy. In its intended mode as shown in Fig. 5, the reference generator will take an input from a 32kHz crystal (connected between XTAL1 and XTAL2) and generate a sine-wave at 20Hz, 25Hz or 50Hz based on the programming of pins FS0 and FS1 (See Table 2). If the crystal frequency is changed, the output frequen- cies will be appropriately shifted. C-2 type Quartz crystals (Epson makes available through DigiKey) are recom- mended for this application. If the frequency accuracy is not a major concern, the more common and less expen- sive clock crystal (C-type) at 32.768kHz can be used with a minor output frequency offset (20.5Hz instead of 20Hz). Additionally, the XTAL1 input can be clocked at a desired frequency to get a different set of output frequencies at the sine-wave output (with divide ratios of 1600, 1280 and 640). The sine-wave output is centered around an in- ternal reference of 3V. A capacitor from SINREF to GND helps provide smoothing of the sine wave reference. Recommended value is at least 0.01 µF and maximum of 0.1 µF. When FS0 and FS1 are both 1 (high), the sine ref- erence is disabled and external sine-wave can be fed into the SINREF pin. This signal should have the same DC offset as the internal sine-wave (3V). Reference and Error Amplifier The recommended circuit connections for these circuits are shown in Fig. 6. The sine-wave is added to a DC off- set to create the composite reference signal for the error amplifier. The DC reference can vary over a wide range. For pure AC outputs it is zero, while in many common applications, it is the talk battery voltage (–48V). The UCC3750 accomplishes this task by summing the two signals weighted by resisting R14 and R15. The output of AMP1 also helps determine the mode of the circuit. Referring to Fig. 6, the output of AMP1 is given by : VOUT1 = 1+ R13 R14 + R13 R26 •VCM – R13 R26 •REF (1) – R13 R14 •VB – R13 R15 • VAC In order to nullify the effect of VCM on this value, the ratio of R26 to R14 should be made 1.5. With this ratio, the equation becomes: VOUT1 = VCM – R13 R14 •VB – R13 R15 • VAC (2) VOUT1 is the reference voltage that the second amplifier (AMP2) uses to program the output voltage. Assuming that Z4 is high DC impedance, the output voltage is de- rived by summing the currents into pin 18. The output is given as: VO = 1+ R10 R27 + R10 R12 •VCM (3) – R10 R27 •REF – R10 R12 •VOUT1 Again, if the ratio of R27 to R10 is made 1.5, the effect of VCM is nullified and the output voltage becomes (after substituting for VOUT1): VO = R10•R13 R12•R14 •VB + R10•R13 R12•R15 • VAC (4) From equation 4, it can be seen that if the output voltage DC value has to track VB directly, the following condition should be forced: R10 • R13 = R12 • R14 (5) However, in some cases, this becomes impractical due to large AC gain required form VAC to VO. Only a small part of the gain can be accommodated in the first ampli- fier stage due to its output voltage limitations. As a result, the required resistance values become very high. This APPLICATION INFORMATION (cont.) Figure 5. Sine-wave generator. Figure 6. Error amplifier setup. UDG-96166-1 UDG-96167-1 |
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