JAJSMM7 データシート

JAJSMM7A August 2022 – September 2023 TPSM8D6B24

PRODUCTION DATA

パッケージ・オプション

メカニカル・データ（パッケージ｜ピン）

MOW|59

サーマルパッド・メカニカル・データ

発注情報

8.2.2.3.2 Output Voltage Ripple

The output-voltage ripple is the second criterion for output capacitor selection. Use Equation 17 to calculate the minimum output capacitance required to meet the output-voltage ripple specification.

Equation 17.

C_{O U T (m i n)} = \frac{I_{R I P P L E}}{8 \times f_{s w} \times V_{O U T (R I P P L E)}} = \frac{9.62 A}{8 \times 550 k H z \times 20 m V} = 110 µ F

In this case, the target maximum output-voltage ripple is 20 mV. Under this requirement, the minimum output capacitance for ripple is 110 µF. This capacitance value is smaller than the output capacitance required for the transient response, so select the output capacitance value based on the transient requirement. Considering the variation and derating of capacitance, in this design, two 470-µF low-ESR tantalum polymer bulk capacitors and four 47-µF ceramic capacitors were selected to meet the transient specification with sufficient margin. Therefore, the selected nominal C_OUT is equal to 1128 µF.

With the output capacitance value selected the ESR must be considered. This is an important consideration in this example because it uses mixed output capacitor types. First use Equation 18 to calculate the maximum allowable impedance for the output capacitor bank at the switching frequency to meet the output voltage ripple specification. Equation 18 indicates the output capacitor bank impedance should be less than 2.1 mΩ. The impedance of the ceramic capacitors is calculated with Equation 19 and the impedance of the bulk capacitor is calculated with Equation 20. The result from Equation 20 shows the impedance of the bulk capacitor at the switching frequency is dominated by its ESR. Equation 21 calculates the total output impedance of the output capacitor bank at the switching frequency to be 1.2 mΩ, which meets the 2.1-mΩ requirement.

Equation 18.

Equation 19.

Z_{C E R_f_{s w}} = \frac{1}{2 π \times f_{s w} \times C_{C E R}} = \frac{1}{2 π \times 550 k H z \times (4 \times 47 μ F)} = 1.5 m Ω

Equation 20.

Z_{B U L K_f_{s w}} = \sqrt{{E S R}_{B U L K}^{2} + {(\frac{1}{2 π \times f_{S W} \times C_{B U L K}})}^{2}} = \sqrt{{(\frac{10 m Ω}{2})}^{2} + {(\frac{1}{2 π \times 550 k H z \times (2 \times 470 μ F)})}^{2}} = 5.3 m Ω

Equation 21.

Z_{C O U T_f_{s w}} = \frac{Z_{C E R_f_{s w}} \times Z_{B U L K_f_{s w}}}{Z_{C E R_f_{s w}} + Z_{B U L K_f_{s w}}} = \frac{1.5 m Ω \times 5.3 m Ω}{1.5 m Ω + 5.3 m Ω} = 1.2 m Ω