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XRP6658 Datasheet(PDF) 8 Page - Exar Corporation |
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XRP6658 Datasheet(HTML) 8 Page - Exar Corporation |
8 / 11 page X XR RP P6 66 65 58 8 1 1A A 1 1..5 5M MH Hz z P PF FM M/ /P PW WM M S Sy yn ncch hrro on no ou us s S Stte ep p--D Do ow wn n C Co on nv ve errtte err © 2011 Exar Corporation 8/11 Rev. 1.5.0 THEORY OF OPERATION The typical application circuit of adjustable version is shown in figure 22. Fig. 22: Typical Application INDUCTOR SELECTION Inductor ripple current and saturation current rating are two factors to be considered when selecting the inductor value. A low DCR inductor is preferred. The inductor value L can be calculated from the following equation: x 1 x 1 ∆ CIN AND COUT SELECTION A low ESR input capacitor can minimize the input voltage ripple. Voltage rating of the capacitor should be at least 50% higher than the input voltage. The RMS current of the input capacitor is required to be larger than the IRMS calculated by: ≅ - The ESR value is an important parameter to consider when selecting an output capacitor COUT. The output ripple VOUT is determined by: ∆ ≅ ∆IL ESR 1 8 f The output capacitor’s value can be optimized for very low output voltage ripple and small circuit size. Voltage rating of the capacitor should be at least 50% higher than the output voltage. Higher values, lower cost ceramic capacitors are now available in smaller sizes. These ceramic capacitors have high ripple currents, high voltage ratings and low ESR that make them ideal for switching regulator applications. It is recommended to use X5R or X7R ceramic capacitors as they have the best temperature and voltage characteristics. OUTPUT VOLTAGE SELECTION The output voltage is adjustable via the external resistor network R1 and R2 as per the following formula: 1 R2 R1 where, VREF is the reference voltage at 0.6V. The feedback resistors must be chosen such that power dissipation of the network is minimal. R1 must be selected within the range of 80kΩ≤ R1 ≤120kΩ. R2 is selected based on the above equation. THERMAL CONSIDERATIONS Although thermal shutdown is built-in in XRP6658 to protect the device from thermal damage, the total power dissipation that XRP6658 can sustain is based on the package thermal capability. The formula to ensure safe operation is shown in Note 1. To avoid XRP6658 from exceeding the maximum junction temperature, some thermal analysis is required. GUIDELINES FOR PCB LAYOUT To ensure proper operation of the XRP6658, please note the following PCB layout guidelines: 1. The GND, SW and VIN traces should be kept short, direct and wide. 2. VFB pin must be connected directly to the feedback resistors. Resistive divider R1/R2 must be connected in parallel to the output capacitor COUT. 3. The input capacitor CIN must be as close as possible to pin VIN. 4. Keep SW node away from the sensitive VFB node since SW signal experiences high frequency voltage swings. |
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