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FAB3103UCX Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FAB3103UCX Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 13 page © 2011 Fairchild Semiconductor www.fairchildsemi.com FAB3103 • Rev. 1.0.2 9 Line Color AGCT Configuration AGC Trip Point (V) Red Float 3.25 Green Ground 3.55 Blue VBATT 3.75 Figure 10. Target Gain vs. Battery Voltage Figure 11 is similar to Figure 10 except that the target gain is expressed in dB rather than V/V. Line Color AGCT Configuration AGC Trip Point (V) Red Float 3.25 Green Ground 3.55 Blue VBATT 3.75 Figure 11. Target Gain vs. Battery Voltage Figure 12 shows examples of peak output voltage vs. battery voltage. Line Color AGCT Configuration AGC Trip Point (V) Input Voltage (Vpk) Magenta Float 3.25 0.3 Cyan Ground 3.55 0.3 Black VBATT 3.75 0.3 Figure 12. Output Voltage vs. Battery Voltage Figure 13 shows examples of output power vs. battery voltage with a 0.4Vpk sinusoidal input signal. Line Color AGCT Configuration AGC Trip Point (V) Input Voltage (Vpk) Magenta Float 3.25 0.3 Cyan Ground 3.55 0.3 Black VBATT 3.75 0.3 Figure 13. Output Power vs. Battery Voltage Examples (VIN=0.4Vpk Sine) The speed at which gain can change is limited (see Electrical Characteristics) ; therefore, the actual gain may lag the target gain if VBATT voltage changes quickly. Figure 14 and Figure 15 show examples of AGC changes over time. In these examples, AGCT is grounded, so the AGC trip point is 3.55V. 1. Initially, VBATT is 3.6V and gain is 10V/V (20dB). 2. A narrow VBATT drop of less than 2µs is ignored by the AGC. 3. The next VBATT drop lasts longer and the AGC is tripped. The initial 0.5dB gain reduction occurs 3.9µs after VBATT crosses below the 3.55V trip point. 4. VBATT is now 3.1V, so target gain is 10V/V – 3V/V × 10V/V × [(3.55V – 3.1V) / 5.2V]=7.40V/V=17.4dB. 5. Gain continues to drop by 0.5dB every 10 µs until it is below the target gain, where it settles at 17.0dB. 6. When VBATT rises above the trip point, gain increases by 0.5dB. If more than 800ms has passed since the last gain change, gain rises immediately, as shown in Figure 14. Otherwise, gain does not rise until after 800ms has passed, as shown in Figure 15. 7. While VBATT remains above the trip point, gain continues to increase by 0.5dB every 800ms until it returns to 20dB. The intent of the AGC circuitry is to limit current draw from the battery to extend runtime. This is particularly important for handsets that incorporate advanced shutdown algorithms to measure battery voltage. The AGC circuit dynamically adjusts the amplifier gain based on the trip point used. Even though the amplifier gain is reduced in response to lower battery voltages, two conditions result in continued higher current draw: 1) the handset volume is turned up in an attempt to maintain the same loudness, or 2) the input signal is increased. If 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 2.5 3.0 3.5 4.0 4.5 5.0 VBATT (V) 8 10 12 14 16 18 20 2.5 3.0 3.5 4.0 4.5 5.0 VBATT (V) 0.5 1 1.5 2 2.5 3 3.5 2.5 3.0 3.5 4.0 4.5 5.0 VBATT (V) VIN = 0.3Vpk 0.0 0.2 0.4 0.6 0.8 1.0 2.5 3.0 3.5 4.0 4.5 5.0 VBATT (V) RL = 8Ω + 33µH |
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