LM2640

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SNVS019B – DECEMBER 1998 – REVISED APRIL 2013

Optimizing Stability

The best tool for measuring both bandwidth and phase margin is a network analyzer. If this is not available, a

simple method which gives a good measure of loop stability is to apply a minimum to maximum step of output

load current and observe the resulting output voltage transient. A design which has good phase margin (>50°)

will typically show no ringing after the output voltage transient returns to its nominal value.

It should be noted that the stability (phase margin) does not have to be optimal for the regulator to be stable. The

design analyzed in the previous section was re-compensated by changing R11 and C10 to intentionally reduce

the phase margin to about 35° and re-tested for step response. The output waveform displayed slight ringing

after the initial return to nominal, but was completely stable otherwise.

In most cases, the compensation components shown in the Typical Application Circuits will give good

performance. To assist in optimizing phase margin, the following guidelines show the effects of changing various

components.

COUT: Increasing the capacitance of COUT moves the frequency of the pole fp(COUT) to a lower value and reduces

loop bandwidth. Increasing COUT can be beneficial (increasing the phase margin) if the loop bandwidth is too

wide (>FOSC/5) which places the high-frequency poles too close to the unity-gain crossover frequency.

ESR of COUT: The ESR forms a zero fz(ESR), which is needed to cancel negative phase shift near the unity-gain

frequency. High-ESR capacitors can not be used, since the zero will be too low in frequency which will make the

loop bandwidth too wide.

R11/C10: These form a pole and a zero. Changing the value of C10 changes the frequency of both the pole and

zero. Note that since this causes the frequency of both the pole and zero to move up or down together, adjusting

the value of C10 does not significantly affect loop bandwidth.

Changing the value of R11 moves the frequency location of the zero fz(R11), but does not significantly shift the

C10 pole (since the value of R11 is much less than the 160 k

Ω output impedance of the Gm amplifier). Since

only the zero is moved, this affects both bandwidth and phase margin. This means adjusting R11 is an easy way

to maximize the positive phase shift provided by the zero. Best results are typically obtained if fz(R11) is in the

frequency range of fc/4 to fc (where fc is the unity-gain crossover frequency).

Design Procedure

This section presents guidelines for selecting external components.

INDUCTOR SELECTION

In selecting an inductor, the parameters which are most important are inductance, current rating, and DC

resistance.

Inductance

It is important to understand that all inductors are not created equal, as the method of specifying inductance

varies widely.

It must also be noted that the inductance of every inductor decreases with current. The core material, size, and

construction type all contribute the the inductor's dependence on current loading. Some inductors exhibit

inductance curves which are relatively flat, while others may vary more than 2:1 from minimum to maximum

current. In the latter case, the manufacturer's specified inductance value is usually the maximum value, which

means the actual inductance in your application will be much less.

An inductor with a flatter inductance curve is preferable, since the loop characteristics of any switching converter

are affected somewhat by inductance value. An inductor which has a more constant inductance value will give

more consistent loop bandwidth when the load current is varied.

The data sheet for the inductor must be reviewed carefully to verify that the selected component will have the

desired inductance at the frequency and current for the application.

Current Rating

This specification may be the most confusing of all when picking an inductor, as manufacturers use different

methods for specifying an inductor's current rating.

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