LM5000 datasheet(13/24 Pages) TI

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fZC =

2SCCRC

(in Hz)

fPC =

2S(RC + RO)CC

(in Hz)

RHPzero =

VOUT(D')

2S,LOADL

fZ1 =

2SRESRCOUT

(in Hz)

fP1 =

2S(RESR + RL)COUT

(in Hz)

LM5000

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SNVS176D – MAY 2004 – REVISED MARCH 2007

OUTPUT CAPACITOR SELECTION

The choice of output capacitors is somewhat more arbitrary. It is recommended that low ESR (Equivalent Series

Resistance, denoted RESR) capacitors be used such as ceramic, polymer electrolytic, or low ESR tantalum.

Higher ESR capacitors may be used but will require more compensation which will be explained later on in the

section. The ESR is also important because it determines the output voltage ripple according to the approximate

equation:

ΔVOUT ≊ 2ΔiLRESR (in Volts)

(7)

After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the

following equations:

(8)

where

•

RL is the minimum load resistance corresponding to the maximum load current

(9)

The zero created by the ESR of the output capacitor is generally very high frequency if the ESR is small. If low

ESR capacitors are used it can be neglected. If higher ESR capacitors are used see the HIGH OUTPUT

CAPACITOR ESR COMPENSATION section.

RIGHT HALF PLANE ZERO

A current mode control boost regulator has an inherent right half plane zero (RHP zero). This zero has the effect

of a zero in the gain plot, causing an imposed +20dB/decade on the rolloff, but has the effect of a pole in the

phase, subtracting another 90° in the phase plot. This can cause undesirable effects if the control loop is

influenced by this zero. To ensure the RHP zero does not cause instability issues, the control loop should be

designed to have a bandwidth of ½ the frequency of the RHP zero or less. This zero occurs at a frequency of:

where

•

ILOAD is the maximum load current

(10)

SELECTING THE COMPENSATION COMPONENTS

The first step in selecting the compensation components RC and CC is to set a dominant low frequency pole in

the control loop. Simply choose values for RC and CC within the ranges given in the INTRODUCTION TO

COMPENSATION section to set this pole in the area of 10Hz to 100Hz. The frequency of the pole created is

determined by the equation:

where

•

RO is the output impedance of the error amplifier, 850kΩ

(11)

Since RC is generally much less than RO, it does not have much effect on the above equation and can be

neglected until a value is chosen to set the zero fZC. fZC is created to cancel out the pole created by the output

capacitor, fP1. The output capacitor pole will shift with different load currents as shown by the equation, so setting

the zero is not exact. Determine the range of fP1 over the expected loads and then set the zero fZC to a point

approximately in the middle. The frequency of this zero is determined by:

(12)

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