LM2641 datasheet(14/18 Pages) NSC

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Application Information (Continued)

Looking at the plot, it can be seen that the unity-gain cross-

over frequency f

c is expected to be about 25kHz. Using this

value, the phase margin at the point is calculated to be about

84˚.

To verify the accuracy of these calculations, the circuit was

bench tested using a network analyzer. The measured gain

and phase are shown plotted in

Figure 7.

The measured gain plot agrees very closely to the predicted

values. The phase margin at 0dB is slightly less than pre-

dicted (71˚ vs. 84˚), which is to be expected due to the nega-

tive phase shift contributions of high frequency poles not in-

cluded in this simplified analysis.

It should be noted that 70˚ phase margin with 25kHz band-

width is excellent, and represents the optimal compensation

for this set of values for V

IN,VOUT, inductor and RL.

Optimizing Stability

The best tool for measuring both bandwidth and phase mar-

gin is a network analyzer. If this is not available, a simple

method which gives a good measure of loop stability is to ap-

ply 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 nomi-

nal 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 re-

turn 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.

OUT: Increasing the capacitance of COUT moves the fre-

quency of the pole f

p(COUT) to a lower value and reduces

loop bandwidth. Increasing C

OUT can be beneficial (increas-

ing 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 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 f

z(R11), but does not significantly shift the C10 pole

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

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 posi-

tive phase shift provided by the zero. Best results are typi-

cally obtained if f

z(R11) is in the frequency range of fc/4 to fc

(where f

c is the unity-gain crossover frequency).

Design Procedure

This section presents guidelines for selecting external com-

ponents.

INDUCTOR SELECTION

In selecting an inductor, the parameters which are most im-

portant 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 con-

struction 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 usu-

ally 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 consis-

tent loop bandwidth when the load current is varied.

DS100949-7

FIGURE 6. Calculated Gain Plot for 3.3V/4A Application

DS100949-8

FIGURE 7. Measured Gain/Phase Plot for 3.3V/4A

Application

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