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LM25122-Q1 Datasheet(PDF) 24 Page - Texas Instruments |
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LM25122-Q1 Datasheet(HTML) 24 Page - Texas Instruments |
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24 / 47 page  IN OUT V ' D V COMP CROSS S _ EQ FB2 S OUT R f D' [Hz] R R A C u S u u u u P _ EA COMP HF 1 & +LJK IUHTXHQF\ SROH R C u Z _ EA COMP COMP 1 & /RZ IUHTXHQF\ ]HUR R C u FB FB2 COMP HF 1 A (Feedback DC gain) R C C u Z _ EA COMP FB OUT P _ EA 1 Ö & V A ÖV s s 1 & s u § · u ¨ ¸ ¨ ¸ © ¹ LM25122-Q1 SNVSAF0A – DECEMBER 2015 – REVISED DECEMBER 2015 www.ti.com Application Information (continued) The feedback transfer function includes the feedback resistor divider and loop compensation of the error amplifier. RCOMP, CCOMP and optional CHF configure the error amplifier gain and phase characteristics, create a pole at origin, a low frequency zero and a high frequency pole. Feedback transfer function is defined as follows: where • • • (16) The pole at the origin minimizes the output steady state error. The low frequency zero should be placed to cancel the load pole of the modulator. The high frequency pole can be used to cancel the zero created by the output capacitor ESR or to decrease noise susceptibility of the error amplifier. By placing the low frequency zero an order of magnitude less than the crossover frequency, the maximum amount of phase boost can be achieved at the crossover frequency. The high frequency pole should be placed beyond the crossover frequency since the addition of CHF adds a pole in the feedback transfer function. The crossover frequency (open loop bandwidth) is usually selected between one twentieth and one fifth of the fSW. In a simplified formula, the estimated crossover frequency can be defined as: where • (17) For higher crossover frequency, RCOMP can be increased, while proportionally decreasing CCOMP. Conversely, decreasing RCOMP while proportionally increasing CCOMP, results in lower bandwidth while keeping the same zero frequency in the feedback transfer function. The modulator transfer function can be measured by a network analyzer and the feedback transfer function can be configured for the desired open loop transfer function. If the network analyzer is not available, step load transient tests can be performed to verify acceptable performance. The step load goal is minimum overshoot/undershoot with a damped response. 8.1.2 Sub-Harmonic Oscillation Peak current mode regulator can exhibit unstable behavior when operating above 50% duty cycle. This behavior is known as sub-harmonic oscillation and is characterized by alternating wide and narrow pulses at the SW pin. Sub-harmonic oscillation can be prevented by adding an additional slope voltage ramp (slope compensation) on top of the sensed inductor current. By choosing K ≥ 0.82~1.0, the sub-harmonic oscillation will be eliminated even with wide varying input voltage. 24 Submit Documentation Feedback Copyright © 2015, Texas Instruments Incorporated Product Folder Links: LM25122-Q1 |
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