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SPX2950ACN-3.3 Datasheet(PDF) 9 Page - Sipex Corporation |
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SPX2950ACN-3.3 Datasheet(HTML) 9 Page - Sipex Corporation |
9 / 13 page 9 Rev. 3/6/03 SPX2950/SPX2951 150mA Low Dropout Voltage Regulator ©Copyright 2003 Sipex Corporation PROGRAMMING THE OUTPUT VOLTAGE OF SPX2951 The SPX2951 may be pin-strapped for 5V using its internal voltage divider by tying Pin 1 (out- put) to Pin 2 (sense) and Pin 7 (feedback) to Pin 6 (5V Tap). Also, it may be programmed for any output voltage between its 1.235V reference and its 30V maximum rating . As seen in Figure 2, an external pair of resistors is required. Refer to the below equation for the programming of the output voltage: VOUT = VREF × (1 + R1/R2)+ IFBR1 The VREF is 1.235 and IFB is the feedback bias current, nominally -20nA. The minimum rec- ommended load current of 1 µA forces an upper limit of 1.2M Ω on value of R2. If no load is presented the IFB produces an error of typically 2% in VOUT which may be eliminated at room temperature by trimming R1. To improve the accuracy choose the value of R2 = 100k this reduces the error by 0.17% and increases the resistor program current by 12 µA. Since the SPX2951 typically draws 60 µA at no load with Pin 2 open-circuited this is a small price to pay. REDUCING OUTPUT NOISE It may be an advantage to reduce the AC noise present at the output. One way is to reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only way that noise can be reduced on the 3 lead SPX2950 but is relatively inefficient, as increasing the ca- pacitor from 1 µF to 220µF only decreases the noise from 430 µV to 160µVRMS for a 100kHz bandwidth at 5V output. Noise could also be reduced fourfold by a bypass capacitor across R1, since it reduces the high frequency gain from 4 to unity. Pick C BYPASS ≅ (1 / 2πR1 × 200 Hz) or choose 0.01 µF. When doing this, the output capacitor must be increased to 3.3 µF to main- tain stability. These changes reduce the output noise from 430 µV to 100µV RMS for a 100kHz bandwidth at 5V output. With the bypass ca- pacitor added, noise no longer scales with out- put voltage so that improvements are more dra- matic at higher output voltages. 100K OUTPUT ERROR ERROR ______ SD V OUT GND FB +VIN +VIN 1.23V 1.2 to 30V R1 .01µF 3.3µF R2 VREF 3 8 5 1 4 7 SHUTDOWN INPUT + SPX2951 Figure 2. Adjustable Regulator TYPICAL APPLICATIONS |
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