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LMR23630AFDDA Datasheet(PDF) 23 Page - Texas Instruments |
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LMR23630AFDDA Datasheet(HTML) 23 Page - Texas Instruments |
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23 / 42 page  FF X FBT 1 C 4 f R S u u X OUT OUT 8.32 f V C u 2 2 OH OL OUT 2 2 OUT OS OUT I I C L V V V ! u OH OL OUT SW US 4 I I C f V u ! u 23 LMR23630 www.ti.com SNVSAH2C – DECEMBER 2015 – REVISED JUNE 2017 Product Folder Links: LMR23630 Submit Documentation Feedback Copyright © 2015–2017, Texas Instruments Incorporated (16) where • IOL = Low level output current during load transient • IOH = High level output current during load transient • VUS = Target output voltage undershoot • VOS = Target output voltage overshoot (17) For this design example, the target output ripple is 50 mV. Presuppose ΔVOUT_ESR = ΔVOUT_C = 50 mV, and chose KIND = 0.4. Equation 14 yields ESR no larger than 41.7 mΩ and Equation 15 yields COUT no smaller than 7.5 μF. For the target over/undershoot range of this design, VUS = VOS = 5% × VOUT = 250 mV. The COUT can be calculated to be no smaller than 108 μF and 28.5 μF by Equation 16 and Equation 17, respectively. Consider of derating, one 47-μF, 16-V and one 100-μF, 10-V ceramic capacitor with 5-mΩ ESR are used in parallel. 9.2.2.6 Feed-Forward Capacitor The LMR23630 is internally compensated. Depending on the VOUT and frequency fSW, if the output capacitor COUT is dominated by low-ESR (ceramic types) capacitors, it could result in low phase margin. To improve the phase boost an external feed-forward capacitor CFF can be added in parallel with RFBT. CFF is chosen such that phase margin is boosted at the crossover frequency without CFF. A simple estimation for the crossover frequency (fX) without CFF is shown in Equation 18, assuming COUT has very small ESR, and COUT value is after derating. (18) Equation 19 for CFF was tested: (19) For designs with higher ESR, CFF is not needed when COUT has very high ESR and CFF calculated from Equation 19 must reduced with medium ESR. Table 2 can be used as a quick starting point. For the application in this design example, a 47-pF, 50-V, COG capacitor is selected. 9.2.2.7 Input Capacitor Selection The LMR23630 device requires high-frequency input decoupling capacitor(s) and a bulk input capacitor, depending on the application. The typical recommended value for the high-frequency decoupling capacitor is 4.7 μF to 10 μF. TI recommends a high-quality ceramic capacitor type X5R or X7R with sufficiency voltage rating. To compensate the derating of ceramic capacitors, a voltage rating twice the maximum input voltage is recommended. Additionally, some bulk capacitance can be required, especially if the LMR23630 circuit is not located within approximately 5 cm from the input voltage source. This capacitor is used to provide damping to the voltage spike due to the lead inductance of the cable or the trace. For this design, two 4.7-μF, 50-V, X7R ceramic capacitors are used. A 0.1-μF for high-frequency filtering and place it as close as possible to the device pins. 9.2.2.8 Bootstrap Capacitor Selection Every LMR23630 design requires a bootstrap capacitor (CBOOT). TI recommends a capacitor of 0.47 μF, ated 16 V or higher. The bootstrap capacitor is located between the SW pin and the BOOT pin. The bootstrap capacitor must be a high-quality ceramic type with an X7R or X5R grade dielectric for temperature stability. 9.2.2.9 VCC Capacitor Selection The VCC pin is the output of an internal LDO for LMR23630. To insure stability of the device, place a minimum of 2.2-μF, 16V, X7R capacitor from this pin to ground. |
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