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TPS65321A-Q1 Datasheet(PDF) 17 Page - Texas Instruments |
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TPS65321A-Q1 Datasheet(HTML) 17 Page - Texas Instruments |
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17 / 43 page  VO Vref = 0.8 V COMP FB1 Error Amplifier gmea 310 µA/V + ± SW Power Stage gmps = 10.5 A/V c R3 RO_ea R1 R1 RESR RL a b C2 C1 CO_ea CO 17 TPS65321A-Q1 www.ti.com SLVSE55 – NOVEMBER 2017 Product Folder Links: TPS65321A-Q1 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated Feature Description (continued) Figure 11. Small-Signal Model for Loop Response 7.3.1.17 Simple Small-Signal Model for Peak-Current Mode Control Figure 12 shows a simple small-signal model that can be used to understand how to design the frequency compensation. A voltage-controlled current source (duty cycle modulator) supplying current to the output capacitor and load resistor can approximate the TPS65321A-Q1 buck regulator power stage. Equation 6 shows the control-to-output transfer function, which consists of a DC gain, one dominant pole, and one ESR zero. The quotient of the change in switch current divided by the change in COMP pin voltage (node c in Figure 11) is the power-stage transconductance. The gmps for the TPS65321A-Q1 buck regulator power-stage is 10.5 A/V. Use Equation 7 to calculate the low-frequency gain of the power stage which is the product of the transconductance and the load resistance. As the load current increases and decreases, the low-frequency gain decreases and increases, respectively. This variation with the load seems problematic at first, but the dominant pole moves with the load current (see Equation 8). The dashed line in the right half of Figure 12 highlights the combined effect. As the load current decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover frequency the same for the varying load conditions, which makes designing the frequency compensation easier. The type of output capacitor chosen determines whether the ESR zero has a profound effect on the frequency compensation design. Using high-ESR aluminum-electrolytic capacitors can reduce the number of frequency-compensation components required to stabilize the overall loop because the phase margin increases from the ESR zero at the lower frequencies (see Equation 9). |
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