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LP2951CN3.0 Datasheet(PDF) 10 Page - ON Semiconductor |
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LP2951CN3.0 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 22 page LP2950, LP2951, NCV2951 http://onsemi.com 10 Figure 19. Adjustable Regulator Error Output Shutdown Input Vin Vout 1.23 to 30 V 3.3 mF 0.01 mF NC NC R2 R1 100 k 5 3 Error SD GND FB 47 6 VO T SNS 2 Vout Vin 8 1 The complete equation for the output voltage is: Vout + Vref (1 ) R1 R2) ) IFB R1 where Vref is the nominal 1.235 V reference voltage and IFB is the feedback pin bias current, nominally −20 nA. The minimum recommended load current of 1.0 mA forces an upper limit of 1.2 M W on the value of R2, if the regulator must work with no load. IFB will produce a 2% typical error in Vout which may be eliminated at room temperature by adjusting R1. For better accuracy, choosing R2 = 100 k reduces this error to 0.17% while increasing the resistor program current to 12 mA. Since the LP2951 typically draws 75 mA at no load with Pin 2 open circuited, the extra 12 mA of current drawn is often a worthwhile tradeoff for eliminating the need to set output voltage in test. Output Noise In many applications it is desirable to reduce the noise present at the output. Reducing the regulator bandwidth by increasing the size of the output capacitor is the only method for reducing noise on the 3 lead LP2950. However, increasing the capacitor from 1.0 mF to 220 mF only decreases the noise from 430 mV to 160 mVrms for a 100 kHz bandwidth at the 5.0 V output. Noise can be reduced fourfold by a bypass capacitor across R1, since it reduces the high frequency gain from 4 to unity. Pick CBypass [ 1 2pR1 x 200 Hz or about 0.01 mF. When doing this, the output capacitor must be increased to 3.3 mF to maintain stability. These changes reduce the output noise from 430 mV to 126 mVrms for a 100 kHz bandwidth at 5.0 V output. With bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic at higher output voltages. Figure 20. 1.0 A Regulator with 1.2 V Dropout 0.01 mF 10 k MTB23P06E 1.0 mF Unregulated Input Error Output Shutdown Input Vout 5.0 V 1.0% 0 to 1.0 A 220 mF 2.0 k 5 3 Error SD GND FB 47 6 VO T SNS 2 Vout Vin 8 1 0.002 mF 1.0 M LP2951CN |
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