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SPX2951CS-5.0 Datasheet(PDF) 8 Page - Sipex Corporation |
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SPX2951CS-5.0 Datasheet(HTML) 8 Page - Sipex Corporation |
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8 / 13 page  8 Rev. 3/6/03 SPX2950/SPX2951 150mA Low Dropout Voltage Regulator ©Copyright 2003 Sipex Corporation APPLICATION HINTS EXTERNAL CAPACITORS The stability of the SPX2950/SPX2951 requires a 1 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 be- low -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 500kHz are the most important parameters in the value of the capacitor. The capacitors value may be in- creased 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 0.33 µF and 0.1µF 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 3.3 µF or greater must be used for the condition of 150mA load at 1.23V output. The SPX2950/51 unlike other low dropout regu- lators will remain stable and in regulation with no load in addition to the internal voltage di- vider. This feature is especially important in applications like CMOS RAM keep-alive. When setting the output voltage of the SPX2950/51 version with external resistors, a minimum load of 1 µA 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 µF tantalum or aluminum electrolytic capacitor should be placed from the input to the ground. Instability can occur if there is stray capacitance to the SPX2951 feedback terminal (pin 7). This could cause more problems when using a higher value of external resistors to set the output voltage. This problem can be fixed by adding a 100pF capacitor between output and feedback and increasing the output capacitor to at least 3.3 µF. + + + + 4.75V OUTPUT VOLTAGE ERROR* _______ INPUT VOLTAGE 1.3V 5.0V * See Application Info. Fig. 1 ERROR Output Timing ERROR DETECTION COMPARATOR OUTPUT The comparator produces a logic low output whenever the SPX2951 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 nor- mal regardless of the programmed output volt- age of the regulator. Figure 1 shows the timing diagram depicting the ERROR signal and the regulator output voltage as the SPX2951 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 SPX2951’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. |
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