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SPX2955 Datasheet(PDF) 5 Page - Sipex Corporation |
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SPX2955 Datasheet(HTML) 5 Page - Sipex Corporation |
5 / 9 page Rev. 11/21/00 SPX2955/56/57 APPLICATION HINTS EXTERNAL CAPACITORS The stability of the SPX2955/56/57 requires a 2.2 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 of below -25 °C solid tantalum is recommended since the many aluminum types have electrolytes the 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 capacitor value can 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 0.5 µF and 0.15µF for 1mA. More output capacitance needed for the 8-pin version at voltages below 5V since it runs the error amplifier at lower gain. At worst case 5 µF or greater must be used for the condition of 250mA load at 1.23V output. The SPX2955/56/57, 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 application like CMOS RAM keep-alive. When setting the output voltage of the SPX2955/56/57, a minimum load of 10mA 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 0.1 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 SPX2955/56/57 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. ERROR DETECTION COMPARATOR OUTPUT The Comparator produces a logic low output whenever the SPX2955/56/57 output falls out of regulation 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 SPX2955/56/57 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 SPX2955/56/57’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 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 SPX2955/56/57 The SPX2956/57 may be pin-strapped for 5V using its internal voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback) to Pin 6 (5V Tap). + + + + 4.75V OU TPU T VO LT AG E E RRO R* __ ____ _ IN P U T VO LT AG E 1 .3 V 5.0V F igu r e 1. E R R O R O u tp u t T im in g ___ ___ _ * S ee A p p licatio n In fo . 100K OUTPUT ERROR ERROR ______ SD VOUT GND FB +VIN +VIN 1.23V 1.2 to 29V R1 .01uF 10uF R2 V REF 3 8 5 1 4 7 SHUTDOWN INPUT + Figure 2. Adjustable Regulator SPX2955/57 |
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Similar Description - SPX2955 |
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