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UC3173A датащи(PDF) 9 Page - Texas Instruments |
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UC3173A датащи(HTML) 9 Page - Texas Instruments |
9 / 12 page 9 UC3173A () () V (optimum) = VIN - V + V 2 REFIN HS SAT LS SAT (3) Where:VHS(SAT) = high-side VSAT at maximum load VLS(SAT) = low-side VSAT at maximum load. A second approach is to raise the gain of the B amplifier to insure maximum swing. For a given REFIN voltage the gain of the B amplifier, set by the ratio of the feedback resistors, can be made greater than unity as given by: () () AB = VIN – V – V V– V V HS SAT REF REFIN LS SAT or, (4) () () V– V VIN – V – V REFIN LS SAT HS SAT REF whichever is greater than unity. For a typical case, where VREFIN has been set at VIN/2, the required gain fora5volt system will be about 1.5, and for a 12 volt system, 1.2. It is worth noting that when using this method the B am- plifier will saturate before the A amplifier on one polarity of the voltage swing. During the time when the B ampli- fier is saturated and the A amplifier is not, the small sig- nal bandwidth of the loop will be reduced by a factor of (AVB + 1). E. Loop Compensation The normal configuration for compensation of the power amplifier is shown in Fig. 1. A simple RC network, RCCC, around the A amplifier is all that is required. The value of the RCCC time constant is typically chosen to correspond to the electrical time constant of the load, given by RL/L. The bandwidth of the closed loop amplifier can be set by choosing the value of RC. Calculate RC according to: R= 2L f R (1 + A B)A CS R C 3dB FB VV S π• • • (5) Use AVCS = 0.5 if range changing is to be used, and AVCS = 2.0 if only the low range mode of operation is to be used. The compensation zero is typically set to coincide with the L/R time constant of the Load. CC can then be calcu- lated by: C= L R(R + R ) C CS L (6) In the closed loop transconductance amplifier, the A am- plifier operates at the highest noise gain. Noise gain is a measure of the feedback ratio at which the amplifier is operating. For the configuration of the A amplifier in Fig. 1, the noise gain is given by the impedance ratio of the RC-CC series network, to the parallel combination of RFA and RFB. For the A amplifier to operate at its expected closed loop gain, the noise gain at any frequency must not exceed its Gain Bandwidth Product (GBW) divided by that frequency. Applying this to the expression above will yield a result for the maximum 3dB bandwidth that can be achieved for a given configuration. () f= fA (1 + A B) A CS R R 2L (R + R ) 3dB MAX GBW V V S FA FA FB •• • • • π 1 2 (7) Where: fGBWA is the GBW of the A amplifier. In the UC3173A, to accommodate wider power amplifier bandwidths, the GBW Product of the A amplifier has been extended to 2MHz. Care should be taken that the A amplifier gain bandwidth product is not limiting the closed loop performance of the configured power ampli- fier. This is easily checked by making sure that RC is less than a critical value, RC(MAX), as given by: R= R fA 2 L R (A B + 1) A CS R (R + R ) C(MAX) FB GBW FA VV S FA FB •• •• π 1 2 (8) Again, use AVCS = 0.5 if range changing is used, and AVCS = 2.0 if only the low range mode of operation is used. F. Using The Comp Pin When the range change feature of the UC3173A is used, the closed loop bandwidth of the power amplifier will change according to (7). In other words, the band- width would be four times larger during the low range mode when AVCS is equal to 2, than during the high range mode when AVCS is equal to 0.5, unless the value of RC is adjusted to compensate. The COMP pin on the UC3173A can be used to do this. The COMP pin acts as a simple switch that allows a par- allel compensation network to be applied around the A amplifier during low range operation. A simple network as shown here will keep the loop response constant in- dependent of the range condition. To maintain the same 3dB bandwidth in both the high and low range modes set RCA and CCA to: R= R 3 ,C = 3C CA C CA C (9) APPLICATION INFORMATION (cont.) |
Аналогичный номер детали - UC3173A_08 |
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Аналогичное описание - UC3173A_08 |
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