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LP2950-03BZ Datasheet(PDF) 9 Page - Micrel Semiconductor |
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LP2950-03BZ Datasheet(HTML) 9 Page - Micrel Semiconductor |
9 / 14 page LP2950/2951 Micrel 3 February 1999 3-43 which V OUT = 4.75V). Since the LP2951’s dropout voltage is load-dependent (see curve in Typical Performance Characteristics), the input voltage trip point (about 5V) will vary with the load current. The output voltage trip point (approximately 4.75V) does not vary with load. The error comparator has an open-collector output which requires an external pull-up resistor. Depending on system requirements, this resistor may be returned to the 5V output or some other supply voltage. In determining a value for this resistor, note that while the output is rated to sink 400 µA, this sink current adds to battery drain in a low battery condition. Suggested values range from 100k to 1M Ω. The resistor is not required if this output is unused. Programming the Output Voltage (LP2951) The LP2951 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). Alternatively, it may be programmed for any output voltage between its 1.235V reference and its 30V maximum rating. An external pair of resistors is required, as shown in Figure 2. The complete equation for the output voltage is V OUT = V REF x { 1 + R 1/R2 } + I FB R 2 where V REF is the nominal 1.235 reference voltage and I FB is the feedback pin bias current, nominally 20 nA. The minimum recommended load current of 1 µA forces an upper limit of 1.2 M Ω on the value of R 2 , if the regulator must work with no load (a condition often found in CMOS in standby), I FB will produce a 2% typical error in V OUT which may be eliminated at room temperature by trimming R 1. For better accuracy, choosing R 2 = 100k Ω reduces this error to 0.17% while increasing the resistor program current to 12 µA. Since the LP2951 typically draws 60 µA at no load with Pin 2 open-circuited, this is a small price to pay. Reducing Output Noise In reference applications it may be advantageous to reduce the AC noise present at the output. One method is to reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only method by which noise can be reduced on the 3 lead LP2950 and is relatively inefficient, as increasing the capacitor from 1 µF to 220µF only decreases the noise from 430 µV to 160µV rms for a 100kHz bandwidth at 5V output. Noise can be reduced fourfold by a bypass capacitor across R 1 , since it reduces the high frequency gain from 4 to unity. Pick or about 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 100µV rms for a 100kHz 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. Applications Information External Capacitors A 1.0 µF (or greater) capacitor is required between the LP2950/ LP2951 output and ground to prevent oscillations due to instability. Most types of tantalum or aluminum electrolytics will be adequate; film types will work, but are costly and therefore not recommended. Many aluminum electrolytics have electrolytes that freeze at about –30 °C, so solid tantalum capacitors are recommended for operation below –25 °C. The important parameters of the capacitor are an effective series resistance of about 5 Ω or less and a resonant frequency above 500kHz. The value of this capacitor may be increased without limit. At lower values of output current, less output capacitance is required for output stability. The capacitor can be reduced to 0.33 µF for current below 10mA or 0.1µF for currents below 1mA. Using the 8-Pin versions at voltages below 5V runs the error amplifier at lower gains so that more output capacitance is needed. For the worst-case situation of a 100mA load at 1.23V output (Output shorted to Feedback) a 3.3 µF (or greater) capacitor should be used. The LP2950 will remain stable and in regulation with no load in addition to the internal voltage divider, unlike many other voltage regulators. This is especially important in CMOS RAM keep-alive applications. When setting the output voltage of the LP2951 version with external resistors, a minimum load of 1 µA is recommended. A 0.1 µF capacitor should be placed from the LP2950/LP2951 input to ground 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. Stray capacitance to the LP2951 Feedback terminal (pin 7) can cause instability. This may especially be a problem when using high value external resistors to set the output voltage. Adding a 100pF capacitor between Output and Feedback and increasing the output capacitor to at least 3.3 µF will remedy this. Error Detection Comparator Output A logic low output will be produced by the comparator whenever the LP2951 output falls out of regulation by more than approximately 5%. This figure is the comparator’s built-in offset of about 60mV divided by the 1.235V reference voltage. (Refer to the block diagram on Page 1). This trip level remains “5% below normal” regardless of the programmed output voltage of the LP2951. For example, the error flag trip level is typically 4.75V for a 5V output or 11.4V for a 12V output. The out of regulation condition may be due either to low input voltage, current limiting, or thermal limiting. Figure 1 is a timing diagram depicting the ERROR signal and the regulated output voltage as the LP2951 input is ramped up and down. The ERROR signal becomes valid (low) at about 1.3V input. It goes high at about 5V input (the input voltage at CBYPASS ≅ 1 2R1 • 200 Hz π |
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