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NCP1508 데이터시트(PDF) 12 Page - ON Semiconductor |
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NCP1508 데이터시트(HTML) 12 Page - ON Semiconductor |
12 / 15 page NCP1508 http://onsemi.com 12 APPLICATIONS INFORMATION Component Selection 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 RMS capacitor current is calculated as: IRMS [ IO D @ D (eq. 1) The maximum RMS current occurs at 50% duty cycle with maximum output current, which is IO,max/2. For NCP1508, a low profile ceramic capacitor of 10 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 VCC Pin. Inductor Value Selection Selecting the proper inductor value is based on the desired ripple current. The relationship between the inductance and the inductor ripple current is given by the equation in below. DiL + Vout Lfs 1 * Vout Vin (eq. 2) Large value inductors will have small ripple current and low value inductor will have high ripple current. For NCP1508, the compensation is internally fixed and a fixed 6.8 mH inductor is needed for most of the applications. Output Capacitor Selection Selecting the proper output capacitor is based on the desired output ripple voltage. Ceramic capacitors with low ESR values will have the lowest output ripple voltage and are strongly recommended. The output ripple voltage is given by: DVc + DiL @ ESR ) 1 4fsCout (eq. 3) The RMS output capacitor current is given by: IRMS(Cout) + VO @ (1 * D) 23 @ L @ fs (eq. 4) Where fs is the switching frequency and ESR is the effective series resistance of the output capacitor. A low ESR, 22 mF ceramic capacitor is recommended for NCP1508 in most of applications. For example, with TDK C2012X5R0J226 output capacitor, the output ripple is less than 10 mV at 300 mA. Design Example As a design example, assume that the NCP1508 is used in a single lithium−ion battery application. The input voltage, Vin, is 3.0 V to 4.2 V. Output condition is Vout at 1.5 V with a typical load current of 120 mA and a maximum of 300 mA. For NCP1508, the inductor has a predetermined value, 6.8 mH. The inductor ESR will factor into the overall efficiency of the converter. The inductor needs to be selected by the required peak current. Equation 5 is the basic equation for an inductor and describes the voltage across the inductor. The inductance value determines the slope of the current of the inductor. VL L + diL dt (eq. 5) Equation 5 is rearranged to solve for the change in current for the on−time of the converter in Continuous Conduction Mode. (eq. 6) iL, pk−pk + (Vin * Vout) L @ DTs + (Vin * Vout) L @ Vin Vout @ 1 fs iL, max + IO, max ) DiL, pk−pk 2 Utilizing Equations 6, the peak−to−peak inductor current is calculated using the following worst−case conditions. Vin, max + 4.2 V, Vout + 1.5 V, fs + 1MHz−20%, L + 6.8 mH−10%, iL, pk−pk + 197 mA, iL, max + 399 mA Therefore, the inductor must have a maximum current exceeding 405 mA. Since the compensation is fixed internally in the IC, the input and output capacitors as well as the inductor have a predetermined value too: Cin = 10 mF and Cout = 22 mF. Low ESR capacitors are needed for best performance. Therefore, ceramic capacitors are recommended. Please see Table 3 for recommended inductors and capacitors. PCB Layout Recommendations Good PCB layout plays an important role in switching mode power conversion. Careful PCB layout can help to minimize ground bounce, EMI noise and unwanted feedbacks that can affect the performance of the converter. Hints suggested below can be used as a guideline in most situations. 1. Use star−ground connection to connect the IC ground nodes and capacitor GND nodes together at one point. Keep them as close as possible. And then connect this to the ground plane (if it is used) through several vias. This will reduce noise in ground plane by preventing the switching currents from flowing through the ground plane. 2. Place the power components (i.e., input capacitor, inductor and output capacitor) as close together as possible for best performance. All connecting traces must be short, direct, and thick to reduce voltage errors caused by resistive losses across these traces. 3. Separate the feedback path of the output voltage from the power path. Keep this path close to the NCP1508 circuit. And also route it away from noisy components. |
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