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NCP1521 Datasheet(PDF) 10 Page  ON Semiconductor 

NCP1521 Datasheet(HTML) 10 Page  ON Semiconductor 
10 / 17 page NCP1521 http://onsemi.com 10 APPLICATION INFORMATION Output Voltage Selection The output voltage is programmed through an external resistor divider connected from VOUT to FB then to GND. For low power consumption and noise immunity, the resistor from FB to GND (R2) should be in the [100 k−600 k] range. If R2 is 200 k given the VFB is 0.6 V, the current through the divider will be 3.0 mA. The formula below gives the value of VOUT, given the desired R1 and the R1 value: VOUT + VFB (1 ) R1 R2 ) (eq. 2) • VOUT: Output Voltage (Volts) • VFB: Feedback Voltage = 0.6 V • R1: Feedback Resistor from VOUT to FB • R2: Feedback Resistor from FB to GND Input Capacitor Selection In PWM operating mode, the input current is pulsating with large switching noise. Using an input bypass capacitor can reduce the peak current transients drawn from the input supply source, thereby reducing switching noise significantly. The capacitance needed for the input bypass capacitor depends on the source impedance of the input supply. The maximum RMS current occurs at 50% duty cycle with maximum output current, which is IO, max/2. For NCP1521, a low profile, low ESR ceramic capacitor of 4.7 mF should be used for most of the cases. For effective bypass results, the input capacitor should be placed as close as possible to the VIN pin. Table 1. List of Input Capacitor Murata GRM188R60J475KE GRM21BR71C475KA Taiyo Yuden JMK212BY475MG TDK C2012X5ROJ475KB C1632X5ROJ475KT Output L−C Filter Design Considerations The NCP1521 is built in 1.5 MHz frequency and uses current mode architecture. The correct selection of the output filter ensures good stability and fast transient response. Due to the nature of the buck converter, the output L−C filter must be selected to work with internal compensation. For NCP1521, the internal compensation is internally fixed and it is optimized for an output filter of L = 2.2 mH and COUT = 10 mF. The corner frequency is given by: fc + 1 2 p L COUT (eq. 3) + 1 2 p 2.2 mH 10 mF + 34 kHz The device is intended to operate with inductance values between 1.0 mH and maximum of 4.7 mH. If the corner frequency is moved, it is recommended to check the loop stability depending on the output ripple voltage accepted and output current required. For lower frequency, the stability will be increased; a larger output capacitor value could be chosen without critical effect on the system. On the other hand, a smaller capacitor value increases the corner frequency and it should be critical for the system stability. Take care to check the loop stability. The phase margin is usually higher than 45 °. Table 2. L−C Filter Example Inductance (L) Output Capacitor (Cout) 1.0 mH 22 mF 2.2 mH 10 mF 4.7 mH 4.7 mF Inductor Selection The inductor parameters directly related to device performances are saturation current and DC resistance and inductance value. The inductor ripple current (ÄIL) decreases with higher inductance: DIL + VOUT L fSW 1− VOUT VIN (eq. 4) DIL peak to peak inductor ripple current L inductor value fSW switching frequency The saturation current of the inductor should be rated higher than the maximum load current plus half the ripple current: IL(MAX) + IO(MAX) ) DIL 2 (eq. 5) DIL(MAX) Maximum inductor current DIO(MAX) Maximum Output current The inductor’s resistance will factor into the overall efficiency of the converter. For best performances, the DC resistance should be less than 0.3 W for good efficiency. 
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