LM2641MTC-ADJ Datasheet, PDF (20/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
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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 Loop Compensation 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.
C_OUT: Increasing the capacitance of C_OUT moves the frequency of the pole f_p(C_OUT) to a lower value and reduces
loop bandwidth. Increasing C_OUT can be beneficial (increasing the phase margin) if the loop bandwidth is too
wide (>F_OSC/5) which places the high-frequency poles too close to the unity-gain crossover frequency.
ESR of C_OUT: The ESR forms a zero f_z(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 f_z(R11), but does not significantly shift the
C10 pole (since the value of R11 is much less than the 160kΩ 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 f_z(R11) is in the
frequency range of f_c/4 to f_c (where f_c 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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