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G1431F2U Datasheet(PDF) 11 Page - Global Mixed-mode Technology Inc |
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G1431F2U Datasheet(HTML) 11 Page - Global Mixed-mode Technology Inc |
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11 / 14 page  Ver: 1.5 Oct 25, 2006 TEL: 886-3-5788833 http://www.gmt.com.tw 11 G1431 Global Mixed-mode Technology Inc. Application Information Gain setting via GAIN0 and GAIN1 inputs The internal gain setting is determined by two input terminals, GAIN0 and GAIN1. The gains listed in Table 1 are realized by changing the taps on the input resis- tors inside the amplifier. This will cause the internal input impedance, ZI, to be dependent on the gain set- ting. Although the real input impedance will shift by 30% due to process variation from part-to-part, the actual gain settings are controlled by the ratios of the resistors and the actual gain distribution from part-to- part is quite good. Table 1 GAIN0 GAIN1 AV (dB) 0 0 6 0 1 10 1 0 15.6 1 1 21.6 Input Resistance The typical input impedance at each gain setting is given in the Table 2. Each gain setting is achieved by varying the input resistance of the amplifier, which can be over 6 times from its minimum value to the maxi- mum value. As a result, if a single capacitor is used in the input high pass filter, the –3dB or cut-off frequency will be also change over 3.5 times. To reduce the variation of the cut-off frequency, an additional resistor can be connected from the input pin of the amplifier to the ground, as shown in the figure below. With the extra resistor, the cut-off frequency can be re-calculated using equation : f-3dB= 1/ 2πC(R||RI). Using small external R can reduce the variation of the cut-off frequency. But the side effect is small external R will also let (R||RI) become small, the cut-off fre- quency will be larger and degraded the bass-band performance. The other side effect is with extra power dissipation through the external resistor R to the ground. So using the external resistor R to flatting the variation of the cut-off frequency, the user must also consider the bass-band performance and the extra power dissipation to choose the accepted external resistor R value. Table 2 Zi (k Ω) AV (dB) 30 21.6 45 15.6 70 10 90 6 Input Capacitor In the typical application, an input capacitor Ci is re- quired to allow the amplifier to bias the input signal to the proper dc level for optimum operation. In this case, Ci and the input impedance of the amplifier, Zi, form a high-pass filter with the –3dB determined by the equa- tion: f-3dB= 1/ 2πRI Ci The value of Ci is important to consider as it directly affects the bass performance of the application circuit. For example, if the input resistor is 15k Ω, the input capacitor is 1µF, the flat bass response will be down to 10.6Hz. Because the small leakage current of the input ca- pacitors will cause the dc offset voltage at the input to the amplifier that reduces the operation headroom, especially at the high gain applications. The low- leakage tantalum or ceramic capacitors are suggested to be used as the input coupling capacitors. When using the polarized capacitors, it is important to let the positive side connecting to the higher dc level of the application. Power Supply Decoupling The G1431 is a high-performance CMOS audio ampli- fier that requires adequate power supply decoupling to make sure the output total harmonic distortion (THD) as low as possible. The optimum decoupling is using two capacitors with different types that target different types of noise on the power supply leads. For high frequency transients, spikes, a good low ESR ceramic capacitor works best, typically 0.1µF/1µF used and placed as close as possible to the G1431 VDD lead. A larger aluminum electrolytic capacitor of 10µF or greater placed near the device power is recommended for filtering low-frequency noise. Optimizing DEPOP Operation Circuitry has been implemented in G1431 to minimize the amount of popping heard at power-up and when coming out of shutdown mode. Popping occurs when- ever a voltage step is applied to the speaker and making the differential voltage generated at the two ends of the speaker. To avoid the popping heard, the bypass capacitor should be chosen promptly, 1/(CBx170kΩ) ≦ 1/(CI*(RI+RF)). Where 170kΩ is the output impedance of the mid-rail generator, CB is the mid-rail bypass capacitor, CI is the input coupling ca- pacitor, RI is the input impedance, RF is the gain set- Input Signal C R IN Zi Zf Input Signal C R IN Zi Zf |
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