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SPX2700T5-3.3 Datasheet(PDF) 4 Page - Sipex Corporation |
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SPX2700T5-3.3 Datasheet(HTML) 4 Page - Sipex Corporation |
4 / 11 page Rev. 10/25/00 SPX2700/01/02 APPLICATION HINTS EXTERNAL CAPACITORS The stability of the SPX2700/01/02 requires a 10 µF or greater capacitor between output and ground. Oscillation could occur without this capacitor. Most types of tantalum or aluminum electrolytic works fine here. For operations below -25 °C solid tantalum is recommended since the many aluminum types have electrolytes that freeze at about -30 °C. The ESR of about 5Ω or less and resonant frequency above 500 kHz are the most important parameters in the value of the capacitor. The capacitor value may be increased without limit. At lower values of output current, less output capacitance is required for stability. For the currents below 10mA the value of the capacitor can be reduced to 22 µF and 0.15µF for 1mA. More output capacitance is needed for the 8-pin version at voltages below 5V since it runs the error amplifier at lower gain. At worst case 22 µF or greater must be used for the condition of 750mA load at 1.23V output. The SPX2700/01/02, unlike other low dropout regulators will remain stable and in regulation with no load in addition to the internal voltage divider. This feature is especially important in applications like CMOS RAM keep-alive. When setting the output voltage of the SPX2700/01/02 version with external resistors, a minimum load of 5mA is recommended. If there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery is used as the input then a 1 µA tantalum or aluminum electrolytic capacitor should be placed from the input to the ground. ERROR DETECTION COMPARATOR OUTPUT The Comparator produces a logic low output whenever the SPX2700/01/02 output falls out of regulation by more than around 5%. This is around 75mV 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 SPX2700/01/02 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 Vout = 4.75 ). Since the SPX2700/01/02’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 output, which requires an external pull-up resistor. Depending on the system requirements the resistor 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 250 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 SPX2700/01/02 The SPX2700/01/02 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 -20 nA. The minimum recommended load current of 1 µA forces an upper limit of 1.2 M 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 SPX2700/01/02 typically draws 100 µA at no load with SHUTDOWN 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 SPX2700/01/02 but is relatively inefficient, as increasing the capacitor from 1 µF to 220µF only decreases the noise from 430 µV to 160µVRMS for a 100 kHz 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 CBYPASS ≅ 1 / 2πR1 × 200 Hz or choose 0.01 µF. When doing this, the output capacitor must be increased to 3.3 µF to maintain stability. These changes reduce the output noise from 430 µV to 100µVRMS for a 100 kHz bandwidth at 5V output. With the bypass capacitor added, noise no longer scales with output voltage so that improvements are more dramatic at higher output voltages. + + + + 4.75V OUTPUT VOLTAGE ERROR* _______ INPUT VOLTAGE 1.3V 5.0V Figure 1. ERROR Output Timing _______ * See Application Info. SPX2703 Fig.2 Adjustable Voltage Regulator V IN V OUT GND ADJ SHUTDOWN |
Similar Part No. - SPX2700T5-3.3 |
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Similar Description - SPX2700T5-3.3 |
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