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SPX2920U-3.3 Datasheet(PDF) 5 Page - Sipex Corporation |
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SPX2920U-3.3 Datasheet(HTML) 5 Page - Sipex Corporation |
5 / 10 page 5 Rev: A Date: 03/30/04 SPX2920 400mA Low Drop Out Voltage Regulator with Shutdown ©Copyright 2004 Sipex Corporation Error Detection Comparator Output The Comparator produces a logic low output whenever the SPX2920 output falls out of regu- lation by more than around 5%. This is around 60mV offset divided by the 1.235 reference voltage. This trip level remains 5% below normal regardless of the programmed output voltage of the regulator. Figure 1 shows the timing diagram depicting the ERROR signal and the regulator output voltage as the SPX2920 input is ramped up and down. The ERROR signal becomes low at around 1.3V input, and goes high around 5V input (input voltage at which V OUT = 4.75). Since the SPX2920’s dropout voltage is load dependent, the input voltage trip point (around 5V) will vary with the load current. The output voltage trip point (approx. 4.75V) does not vary with load. The error comparator has an open-collector out- put, which requires an external pull-up resistor. Depending on the system requirements the re- sistor may be returned to 5V output or other supply voltage. In determining the value of this resistor, note that the output is rated to sink 400 µA; this value adds to battery drain in a low battery condition. Suggested values range from 100K to 1M Ω. If the output is unused this resistor is not required. Programming the Output Voltage of SPX2920 The SPX2920 may be pin-strapped for 5V or 3.3V using its internal voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feed- back) to Pin 6 (5V/3.3V Tap). Also, it may be programmed for any output voltage between its 1.235V reference and its 20V maximum rating. As seen on the front page, an external pair of resistors is required. Refer to the below equation for the program- ming of the output voltage:: V OUT = VREF x ( 1 + R1/ R2 )+ IFBR1 The V REF 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 R 2. If no load is presented the I FB produces an error of typically 2% in V OUT, which may be eliminated at room temperature by trimming R 1. To improve the accuracy choose the value of R 2 = 100k; this reduces the error by 0.17% and increases the resistor program current by 12 µA. Since the SPX2920 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 SPX2920 but is relatively inefficient, as increasing the ca- pacitor from 1 µF to 220µF only decreases the noise from 430 µV to 160µV Vrms for a 100kHz bandwidth at 5V output. Noise could also be reduced fourfold by a by- pass capacitor across R 1, since it reduces the high frequency gain from 4 to unity. Pick C BYPASS ≅1 / 2πR1 x 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 Vrms 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. + + + + 4.75V OUTPUT VOLTAGE ERROR* _______ INPUT VOLTAGE 1.3V 5.0V * See Application Info. Figure 1. ERROR Output Timing |
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