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ADP2114ACPZ-R7 数据表(PDF) 27 Page - Analog Devices |
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ADP2114ACPZ-R7 数据表(HTML) 27 Page - Analog Devices |
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27 / 40 page ![]() ADP2114 Rev. 0 | Page 27 of 40 EXTERNAL COMPONENTS SELECTION INPUT CAPACITOR SELECTION The input current to a buck converter is pulsating in nature. The current is zero when the high-side switch is off and approximately equal to the load current when it is on. Because this occurs at reasonably high frequencies (300 kHz to 1.2 MHz), the input bypass capacitor ends up supplying most of the high frequency current (ripple current), allowing the input power source to supply only the average (dc) current. The input capacitor needs a sufficient ripple current rating to handle the input ripple as well as an ESR that is low enough to mitigate the input voltage ripple. For the ADP2114, place a 22 μF, 6.3 V, X5R ceramic capacitor close to the VINx pin for each channel. X5R or X7R dielectrics are recommended with a voltage rating of 6.3 V or 10 V. Y5V and Z5U dielectrics are not recommended due to their poor temperature and dc bias characteristics. VDD RC FILTER It is recommended to apply the input power, VIN, to the VDD pin through a low-pass RC filter, as shown on Figure 76. Connecting a 10 Ω resistor in series with VIN and a 1 μF, 6.3 V, X5R (or X7R) ceramic capacitor between VDD and GND creates a 16 kHz (−3 dB) low-pass filter that effectively attenuates voltage glitches on the input power rail caused by the switching regulator. This provides a clean power supply to the internal, sensitive, analog and digital circuits in the ADP2114, ensuring robust operation. VDD GND ADP2114 1µF 10Ω VIN Figure 76. Low-Pass Filter at VDD INDUCTOR SELECTION The high switching frequency of ADP2114 allows for minimal output voltage ripple even with small inductors. The sizing of the inductor is a trade-off between efficiency and transient response. A small inductor leads to larger inductor current ripple that provides excellent transient response but degrades efficiency. Due to the high switching frequency of ADP2114, shielded ferrite core inductors are recommended for their low core losses and low EMI. As a guideline, the inductor peak-to-peak current ripple, ΔIL, is typically set to 1/3 of the maximum load current for optimal transient response and efficiency. 3 ) ( ) (MAX LOAD SW IN OUT IN OUT L I L f V V V V I ≈ × × − × = Δ ) ( ) ( 3 MAX LOAD IN SW OUT IN OUT IDEAL I V f V V V L × × − × × = ⇒ (5) where: VIN is the input voltage on the VINx terminal. VOUT is the desired output voltage. fSW is the converter switching frequency. The internal slope compensation introduces additional limitations on the optimal inductor value for stable operation because the internal ramp is scaled for each VOUT setting. The limits for different VIN, VOUT, and fSW combinations are listed in Table 8. Table 8. Minimum and Maximum Inductor Values fSW (kHz) VIN (V) VOUT (V) Min L (μH) Max L (μH) 300 5 3.3 6.8 10 300 5 2.5 5.6 15 300 3.3 2.5 5.6 6.8 300 5 1.8 4.7 12 300 3.3 1.8 4.7 8.2 300 5 1.5 2.2 12 300 3.3 1.5 2.2 8.2 300 5 1.2 2.2 10 300 3.3 1.2 2.2 8.2 300 5 0.8 1.5 6.8 300 3.3 0.8 1.5 6.8 600 5 3.3 3.3 4.7 600 5 2.5 3.3 6.8 600 3.3 2.5 3.3 3.3 600 5 1.8 2.2 6.8 600 3.3 1.8 2.2 3.3 600 5 1.5 1.5 5.6 600 3.3 1.5 1.5 4.7 600 5 1.2 1.5 4.7 600 3.3 1.2 1.5 3.3 600 5 0.8 1.0 3.3 600 3.3 0.8 1.0 3.3 1200 5 2.5 1.0 3.3 1200 5 1.8 1.0 3.3 1200 3.3 1.8 1.0 2.2 1200 5 1.5 0.8 2.2 1200 3.3 1.5 0.8 2.2 1200 5 1.2 0.8 2.2 1200 3.3 1.2 0.8 2.2 1200 5 0.8 0.47 1.5 1200 3.3 0.8 0.47 1.5 To avoid saturation, the rated current of the inductor has to be larger than the maximum peak inductor IL_PEAK current given by 2 _ _ L MAX LOAD PEAK L I I I Δ + = (6) where: ILOAD_MAX is the maximum dc load current. ΔIL is the inductor ripple current (peak to peak). |
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