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TPS54719 Fiches technique(PDF) 18 Page - Texas Instruments |
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TPS54719 Fiches technique(HTML) 18 Page - Texas Instruments |
18 / 36 page ps L Adc = gm R ´ s 1+ 2 × z vo = Adc vc s 1+ 2 × p æ ö ç ÷ p ¦ è ø ´ æ ö ç ÷ p ¦ è ø VO RL VC fp fz Adc gmps RESR COUT VSENSE COMP VO R1 R3 C1 C2 R2 CO RO gm 250 µA/V Power Stage 25 A/V PH RESR COUT RL b a c 0.6 V TPS54719 SLVSB69 – JUNE 2012 www.ti.com Figure 30. Small Signal Model for Loop Response SIMPLE SMALL SIGNAL MODEL FOR PEAK CURRENT MODE CONTROL Figure 30 is a simple small signal model that can be used to understand how to design the frequency compensation. The TPS54719 power stage can be approximated to a voltage controlled current source (duty cycle modulator) supplying current to the output capacitor and load resistor. The control to output transfer function is shown in Equation 10 and consists of a dc gain, one dominant pole and one ESR zero. The quotient of the change in switch current and the change in COMP pin voltage (node c in Figure 30) is the power stage transconductance. The gm for the TPS54719 is 25 A/V. The low frequency gain of the power stage frequency response is the product of the transconductance and the load resistance as shown in Equation 11. As the load current increases and decreases, the low frequency gain decreases and increases, respectively. This variation with load may seem problematic at first glance, but the dominant pole moves with load current [see Equation 12]. The combined effect is highlighted by the dashed line in the right half of Figure 31. 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 it easier to design the frequency compensation. Figure 31. Simple Small Signal Model and Frequency Response for Peak Current Mode Control (10) (11) 18 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated Product Folder Link(s): TPS54719 |
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