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ADP2118ACPZ-R7 数据表(PDF) 16 Page - Analog Devices |
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ADP2118ACPZ-R7 数据表(HTML) 16 Page - Analog Devices |
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16 / 24 page  ADP2118 Rev. 0 | Page 16 of 24 APPLICATIONS INFORMATION This section describes the external components selection for the ADP2118. The typical application circuit is shown in Figure 41. RTOP 10k Ω RBOT 10k Ω R2 10k Ω R1 10 Ω SYNC/MODE FREQ TRK FB PVIN ADP2118 SW SW SW 1 2 3 4 12 11 10 9 CIN 100µF X5R, 6.3V COUT 100µF X5R, 6.3V C1 0.1µF VIN 5V VOUT 1.2V 3A L 1µH 16 15 14 13 5 6 7 8 Figure 41. Application Circuit OUTPUT VOLTAGE SELECTION The output voltage of the adjustable version of the ADP2118 can be set by an external resistive voltage divider by using the following equation to set the voltage: + × = BOT TOP OUT R R V 1 6 . 0 To limit output voltage accuracy degradation due to FB bias current (0.1 µA maximum) to less than 0.5% (maximum), ensure that RBOT is less than 30 kΩ. INDUCTOR SELECTION The inductor value is determined by the operating frequency, input voltage, output voltage, and ripple current. Using a small inductor leads to larger inductor current ripple and provides fast transient response but degrades efficiency, whereas a large inductor value leads to small current ripple and good efficiency but slow transient response. As a guideline, the inductor current ripple, ΔIL, is typically set to 1/3 of the maximum load current trade-off between the transient response and efficiency. The inductor can be calculated using the following equation: ( ) S L OUT IN f I D V V L × ∆ × − = where: VIN is the input voltage. VOUT is the output voltage. ΔIL is the inductor current ripple. D is the duty cyle. IN OUT V V D = The ADP2118 uses slope compensation in the current loop to prevent subharmonic oscillations when the duty cycle is larger than 50%. The internal slope compensation limits the minimum inductor value. The negative current limit (−0.9 A) also limits the minimum inductor value. The inductor current ripple (ΔIL) calculated by the selected inductor should not exceed 1.8 A. The peak inductor current should be kept below the peak current limit threshold value and can be calculated as 2 L O PEAK I I I ∆ + = Ensure that the rms current of the selected inductor is greater than the maximum load current and that its saturation current is greater than the peak current limit of the converter. OUTPUT CAPACITOR SELECTION The output voltage ripple, load step transient, and loop stability determine the output capacitor selection. The output ripple is determined by the ESR and the capacitance. × × + × ∆ = ∆ S OUT L OUT f C ESR I V 8 1 The load transient response depends on the inductor, output capacitor, and the control loop. The ADP2118 has integrated loop compensation for simple power design. Table 5 and Table 6 show the typical recom- mended inductors and capacitors for the ADP2118. X5R or X7R ceramic capacitors are highly recommended. Table 5. Recommended L and COUT Value at fS = 1.2 MHz VIN (V) VOUT (V) L (µH) COUT (µF) 3.3 1.0 1 100 + 47 3.3 1.2 1 100 3.3 1.5 1 100 3.3 1.8 1 100 3.3 2.5 1 100 5 1.0 1 100 + 47 5 1.2 1 100 5 1.5 1 100 5 1.8 1 100 5 2.5 1 100 5 3.3 1 100 |
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