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NCP1117ST285T3 Datasheet(PDF) 8 Page - ON Semiconductor |
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NCP1117ST285T3 Datasheet(HTML) 8 Page - ON Semiconductor |
8 / 17 page NCP1117, NCV1117 http://onsemi.com 8 APPLICATIONS INFORMATION Introduction The NCP1117 features a significant reduction in dropout voltage along with enhanced output voltage accuracy and temperature stability when compared to older industry standard three−terminal adjustable regulators. These devices contain output current limiting, safe operating area compensation and thermal shutdown protection making them designer friendly for powering numerous consumer and industrial products. The NCP1117 series is pin compatible with the older LM317 and its derivative device types. Output Voltage The typical application circuits for the fixed and adjustable output regulators are shown in Figures 20 and 21. The adjustable devices are floating voltage regulators. They develop and maintain the nominal 1.25 V reference voltage between the output and adjust pins. The reference voltage is programmed to a constant current source by resistor R1, and this current flows through R2 to ground to set the output voltage. The programmed current level is usually selected to be greater than the specified 5.0 mA minimum that is required for regulation. Since the adjust pin current, Iadj, is significantly lower and constant with respect to the programmed load current, it generates a small output voltage error that can usually be ignored. For the fixed output devices R1 and R2 are included within the device and the ground current Ignd, ranges from 3.0 mA to 5.0 mA depending upon the output voltage. External Capacitors Input bypass capacitor Cin may be required for regulator stability if the device is located more than a few inches from the power source. This capacitor will reduce the circuit’s sensitivity when powered from a complex source impedance and significantly enhance the output transient response. The input bypass capacitor should be mounted with the shortest possible track length directly across the regulator’s input and ground terminals. A 10 mF ceramic or tantalum capacitor should be adequate for most applications. Figure 20. Fixed Output Regulator 1 2 Output 3 Input NCP1117 XTXX ++ Cout Cin Ignd Frequency compensation for the regulator is provided by capacitor Cout and its use is mandatory to ensure output stability. A minimum capacitance value of 4.7 mF with an equivalent series resistance (ESR) that is within the limits of 0.25 W to 2.2 W is required. The capacitor type can be ceramic, tantalum, or aluminum electrolytic as long as it meets the minimum capacitance value and ESR limits over the circuit’s entire operating temperature range. Higher values of output capacitance can be used to enhance loop stability and transient response with the additional benefit of reducing output noise. Figure 21. Adjustable Output Regulator 1 2 Output 3 Input NCP1117 XTA ++ Cout Cin Iadj R2 + Cadj Vref R1 Vout + Vref 1 ) R2 R1 ) Iadj R2 The output ripple will increase linearly for fixed and adjustable devices as the ratio of output voltage to the reference voltage increases. For example, with a 12 V regulator, the output ripple will increase by 12 V/1.25 V or 9.6 and the ripple rejection will decrease by 20 log of this ratio or 19.6 dB. The loss of ripple rejection can be restored to the values shown with the addition of bypass capacitor Cadj, shown in Figure 21. The reactance of Cadj at the ripple frequency must be less than the resistance of R1. The value of R1 can be selected to provide the minimum required load current to maintain regulation and is usually in the range of 100 W to 200 W. Cadj u 1 2 p fripple R1 The minimum required capacitance can be calculated from the above formula. When using the device in an application that is powered from the AC line via a transformer and a full wave bridge, the value for Cadj is: fripple + 120 Hz, R1 + 120 W, then Cadj u 11.1 mF The value for Cadj is significantly reduced in applications where the input ripple frequency is high. If used as a post regulator in a switching converter under the following conditions: fripple + 50 kHz, R1 + 120 W, then Cadj u 0.027 mF Figures 10 and 11 shows the level of ripple rejection that is obtainable with the adjust pin properly bypassed. |
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