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UC3172ADWP датащи(PDF) 9 Page - Texas Instruments |
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UC3172ADWP датащи(HTML) 9 Page - Texas Instruments |
9 / 12 page whichever is greater than unity. For a typical case, where VREF has been set at VIN/2, the required gain for a 5 volt 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 amplifier will saturate before the A amplifier on one polarity of the voltage swing. During the time when the B amplifier is saturated and the A amplifier is not, the small signal bandwidth of the loop will be re- duced by a factor of (AVB + 1). E. The normal configuration for compensation of the power amplifier is shown in Figure 1. A simple RC network, RCCC, around the A amplifier is all that is re- quired. In the closed loop transconductance amplifier, the A amplifier 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 Figure 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 ampli- fier 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 re- sult for the maximum 3dB bandwidth that can be achieved for a given configuration. (5) f3dBmax = fGBWA • (1+AVB) • AVCS • RS • RFA 2 πL • (RFA+RFB) 1 2 In the UC3172A, to accommodate wider power am- plifier bandwidths, the fGBWA has been extended to 3.5MHz. The bandwidth of the closed loop amplifier can be set by choosing the value of RC. Calculate RC according to: (6) RC = 2 πL • f3dB • RFB (1 + AVB)AVCS • RS 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 calculated by: (7) CC = L RC (RS + RL) F. When the range change feature of the UC3172A is used, the closed loop bandwidth of the power ampli- fier will change according to (6). In other words, the bandwidth 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, un- less the value of RC is adjusted to compensate. The COMPA pin on the UC3172A can be used to do this. The COMPA pin acts as a simple switch that allows a parallel 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 independent of the range condition. To maintain the same 3dB bandwidth in both the high and low range modes set RCA and CCA to: (8) RCA = RC 3 , CCA = 3CC Head Parking In Figure 2, the UC3172A is shown configured to force a programmed voltage at the A amplifier output upon the activation of a park condition. A pair of feedback resistors R1 and R2 set this voltage as defined by: (9) R1 = R2 VPARK 1.3 − 1 R2 is typically chosen in the range of 10k Ω to 100kΩ. The B amplifier output is tri-stated during park, this side of the load is driven low by the PRKDRV pin. A series re- sistor, RP in the figure, can be inserted in series with the load to limit the peak current if required. The UV thresholds for the supply monitors are set by picking R4 and R6 values in the 10k Ω to 100kΩ range and then calculating R3 and R5 according to: UC3172A APPLICATION INFORMATION (cont.) The COMPA pin switches in a parallel compensation net- work to stabilize the small signal bandwidth with range changes. UDG-94041 9 |
Аналогичный номер детали - UC3172ADWP |
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Аналогичное описание - UC3172ADWP |
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