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LP2951CD Datasheet(PDF) 8 Page - ON Semiconductor |
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LP2951CD Datasheet(HTML) 8 Page - ON Semiconductor |
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8 / 16 page  LP2950 http://onsemi.com 8 APPLICATIONS INFORMATION Introduction The LP2950/LP2951 regulators are designed with internal current limiting and thermal shutdown making them user–friendly. Typical application circuits for the LP2950 and LP2951 are shown in Figures 17 through 25. These regulators are not internally compensated and thus require a 1.0 µF (or greater) capacitance between the LP2950/LP2951 output terminal and ground for stability. Most types of aluminum, tantalum or multilayer ceramic will perform adequately. Solid tantalums or appropriate multilayer ceramic capacitors are recommended for operation below 25 °C. At lower values of output current, less output capacitance is required for output stability. The capacitor can be reduced to 0.33 µF for currents less than 10 mA, or 0.1 µF for currents below 1.0 mA. Using the 8–pin versions at voltages less than 5.0 V operates the error amplifier at lower values of gain, so that more output capacitance is needed for stability. For the worst case operating condition of a 100 mA load at 1.23 V output (Output Pin 1 connected to the feedback Pin 7) a minimum capacitance of 3.3 µF is recommended. The LP2950 will remain stable and in regulation when operated with no output load. When setting the output voltage of the LP2951 with external resistors, the resistance values should be chosen to draw a minimum of 1.0 µA. A bypass capacitor is recommended across the LP2950/LP2951 input to ground if more than 4 inches of wire connects the input to either a battery or power supply filter capacitor. Input capacitance at the LP2951 Feedback Pin 7 can create a pole, causing instability if high value external resistors are used to set the output voltage. Adding a 100 pF capacitor between the Output Pin 1 and the Feedback Pin 7 and increasing the output filter capacitor to at least 3.3 µF will stabilize the feedback loop. Error Detection Comparator The comparator switches to a positive logic low whenever the LP2951 output voltage falls more than approximately 5.0% out of regulation. This value is the comparator’s designed–in offset voltage of 60 mV divided by the 1.235 V internal reference. As shown in the representative block diagram. This trip level remains 5.0% below normal regardless of the value of regulated output voltage. For example, the error flag trip level is 4.75 V for a normal 5.0 V regulated output, or 9.50 V for a 10 V output voltage. Figure 1 is a timing diagram which shows the ERROR signal and the regulated output voltage as the input voltage to the LP2951 is ramped up and down. The ERROR signal becomes valid (low) at about 1.3 V input. It goes high when the input reaches about 5.0 V (Vout exceeds about 4.75 V). Since the LP2951’s dropout voltage is dependent upon the load current (refer to the curve in the Typical Performance Characteristics), the input voltage trip point will vary with load current. The output voltage trip point does not vary with load. The error comparator output is an open collector which requires an external pull–up resistor. This resistor may be returned to the output or some other voltage within the system. The resistance value should be chosen to be consistent with the 400 µA sink capability of the error comparator. A value between 100 k and 1.0 M Ω is suggested. No pull–up resistance is required if this output is unused. When operated in the shutdown mode, the error comparator output will go high if it has been pulled up to an external supply. To avoid this invalid response, the error comparator output should be pulled up to Vout (see Figure 15). Figure 15. ERROR Output Timing 5.0 V 4.75 V 4.70 V 4.75 V + Vdropout 4.70 V + Vdropout 1.3 V 1.3 V Not Valid Pull–Up to Vout Pull–Up to Ext Output Voltage ERROR Input Voltage Not Valid Programming the Output Voltage (LP2951) The LP2951CX may be pin–strapped for 5.0 V using its internal voltage divider by tying Pin 1 (output) to Pin 2 (sense) and Pin 7 (feedback) to Pin 6 (5.0 V tap). Alternatively, it may be programmed for any output voltage between its 1.235 reference voltage and its 30 V maximum rating. An external pair of resistors is required, as shown in Figure 16. |
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