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## LT1959 Datasheet(PDF) 19 Page - Linear Technology

 Part No. LT1959 Description 4.5A, 500kHz Step-Down Switching Regulator Download 24 Pages Scroll/Zoom 100% Maker LINER [Linear Technology] Homepage http://www.linear.com Logo

## LT1959 Datasheet(HTML) 19 Page - Linear Technology

 19 / 24 page 19LT1959APPLICATIONS INFORMATIONFigure 10. Response from VC Pin to OutputFREQUENCY (Hz)30002500200015001000500200150100500–5010010k100k10M1595 F111k1MGAINPHASEROUT200kCOUT12pFVCERROR AMPLIFIER EQUIVALENT CIRCUITRLOAD = 50ΩVFB 2 × 10–3)(Figure 11. Error Amplifier Gain and PhaseFigure 12. Overall Loop CharacteristicsWhat About a Resistor in the Compensation Network?It is common practice in switching regulator design to adda “zero” to the error amplifier compensation to increaseloop phase margin. This zero is created in the externalnetwork in the form of a resistor (RC) in series with thecompensation capacitor. Increasing the size of this resis-tor generally creates better and better loop stability, butthere are two limitations on its value. First, the combina-tion of output capacitor ESR and a large value for RC maycause loop gain to stop rolling off altogether, creating again margin problem. An approximate formula for RCwhere gain margin falls to zero is:R LoopVGGESRCOUTMPMAGain = 1()=()()( )( )121.FREQUENCY (Hz)40200–20–40400–40–80–120101k10k1M1959 F10100100kGAINPHASEVIN = 10VVOUT = 5VIOUT = 2AFREQUENCY (Hz)806040200–20200150100500–50101k10k1M1595 F12100100kGAINPHASEVIN = 10VVOUT = 5V, IOUT = 2ACOUT = 100µF, 10V, AVX TPSCC = 1.5nF, RC = 0, L = 10µHError amplifier transconductance phase and gain are shownin Figure 11. The error amplifier can be modeled as atransconductance of 2000µMho, with an output imped-ance of 200kΩ in parallel with 12pF. In all practicalapplications, the compensation network from VC pin toground has a much lower impedance than the outputimpedance of the amplifier at frequencies above 500Hz.This means that the error amplifier characteristics them-selves do not contribute excess phase shift to the loop, andthe phase/gain characteristics of the error amplifier sec-tion are completely controlled by the external compensa-tion network.In Figure 12, full loop phase/gain characteristics areshown with a compensation capacitor of 1.5nF, giving theerror amplifier a pole at 530Hz, with phase rolling off to 90°and staying there. The overall loop has a gain of 74dB atlow frequency, rolling off to unity-gain at 100kHz. Phaseshows a two-pole characteristic until the ESR of the outputcapacitor brings it back above 10kHz. Phase margin isabout 60° at unity-gain.Analog experts will note that around 4.4kHz, phase dipsvery close to the zero phase margin line. This is typical ofswitching regulators, especially those that operate over awide range of loads. This region of low phase is not aproblem as long as it does not occur near unity-gain. Inpractice, the variability of output capacitor ESR tends todominate all other effects with respect to loop response.Variations in ESRwill cause unity-gain to move around,but at the same time phase moves with it so that adequatephase margin is maintained over a very wide range of ESR(≥ ±3:1).