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TPS54327DDA Datasheet(PDF) 11 Page - Texas Instruments |
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TPS54327DDA Datasheet(HTML) 11 Page - Texas Instruments |
11 / 29 page 2 2 = IPP Lo(RMS) O 1 + 12 I I I lpp = + Ipeak O 2 I I I V V V IN(max) OUT OUT = x IPP V L x IN(max) O SW - I f F = P OUT OUT 1 2 L x C p V = 0.765 x OUT R1 1 + R2 æ ö÷ ç ÷ ç ÷÷ çè ø TPS54327 www.ti.com SLVSAG1C – DECEMBER 2010 – REVISED DECEMBER 2015 8.2.2.1 Output Voltage Resistors Selection The output voltage is set with a resistor divider from the output node to the VFB pin. TI recommends using 1% tolerance or better divider resistors. Start by using Equation 2 to calculate VOUT. To improve efficiency at very light loads consider using larger value resistors, too high of resistance will be more susceptible to noise and voltage errors from the VFB input current will be more noticeable. (2) 8.2.2.2 Output Filter Selection The output filter used with the TPS54327 is an LC circuit. This LC filter has double pole at: (3) At low frequencies, the overall loop gain is set by the output set-point resistor divider network and the internal gain of the TPS54327. The low-frequency phase is 180 degrees. At the output filter pole frequency, the gain rolls off at a -40 dB per decade rate and the phase drops rapidly. D-CAP2™ introduces a high-frequency zero that reduces the gain roll off to -20 dB per decade and increases the phase to 90 degrees one decade above the zero frequency. The inductor and capacitor selected for the output filter must be selected so that the double pole of Equation 3 is located below the high-frequency zero but close enough that the phase boost provided be the high-frequency zero provides adequate phase margin for a stable circuit. To meet this requirement use the values recommended in Table 2 Table 2. Recommended Component Values OUTPUT VOLTAGE (V) R1 (k Ω) R2 (k Ω) C4 (pF) L1 (µH) C8 + C9 (µF) 1 6.81 22.1 1.5 22 to 68 1.05 8.25 22.1 1.5 22 to 68 1.2 12.7 22.1 1.5 22 to 68 1.8 30.1 22.1 5 - 22 2.2 22 to 68 2.5 49.9 22.1 5 - 22 2.2 22 to 68 3.3 73.2 22.1 5 - 22 2.2 22 to 68 5 124 22.1 5 - 22 3.3 22 to 68 6.5 165 22.1 5 - 22 3.3 22 to 68 Because the DC gain is dependent on the output voltage, the required inductor value will increase as the output voltage increases. For higher output voltages above 1.8 V, additional phase boost can be achieved by adding a feed forward capacitor (C4) in parallel with R1 The inductor peak-to-peak ripple current, peak current and RMS current are calculated using Equation 4, Equation 5 and Equation 6. The inductor saturation current rating must be greater than the calculated peak current and the RMS or heating current rating must be greater than the calculated RMS current. Use 700 kHz for fSW. Use 700 kHz for fSW. Make sure the chosen inductor is rated for the peak current of Equation 5 and the RMS current of Equation 6. (4) (5) (6) For this design example, the calculated peak current is 3.47 A and the calculated RMS current is 3.01 A. The inductor used is a TDK SPM6530-1R5M100 with a peak current rating of 11.5 A and an RMS current rating of 11 A. Copyright © 2010–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: TPS54327 |
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