LM2633 datasheet(39/47 Pages) TI1

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H(s) =

c =

1+G(s)H(s)

o =

Fp(s)Fh(s)

Vin

(mcD' - 0.5)

D[mcD' - (1-D/2)]

-60

-40

-20

-180

-135

-90

-45

100

10k

100k

FREQUENCY (Hz)

Q=0.05

Q=10

Q=0.05

Q=10

Curves change continuously with Q.

Plotted Q values: 0.05, 0.1, 0.2, 0.5, 1, 2, 10.

LM2633

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SNVS139C – MAY 2004 – REVISED APRIL 2005

The power stage component selection can be significantly different from the example values. Figure 20 shows

how the two high frequency poles of a current-mode-control buck regulator change with the Q value.

Figure 20. How Control-Output Transfer Function Changes with Q Values

When Q is higher than 0.5, there will be a double-pole at half the switching frequency fn. When Q is lower than

0.5, the double-pole is damped and becomes two separate poles. The lower the Q value is, the farther apart the

two poles are. When Q is too low (such as Q = 0.05 or lower), one of the two high frequency poles may move

well into the low frequency region. When Q is too high (such as Q = 5 or higher), there will be significant peaking

at half the switching frequency and the phase will rapidly go to

−180° near it. This typically results in a lower

cross-over frequency so that the peaking in the loop gain is well below the 0dB line.

Q is a function of duty cycle and the deepness of the ramp compensation (mc). See Equation 44. The larger the

duty cycle, the higher the Q value. The deeper the ramp compensation, the lower the Q value. When the inductor

current ramp is too much smaller than the compensation ramp, one of the two high frequency poles will move far

into the low frequency region and form a double-pole with the existing low frequency pole fp. That makes it a

voltage-mode control.

The ramp compensation becomes deeper when inductance is increased, or input voltage is decreased, or sense

resistance is decreased.

In the case of Channel 1 of LM2633, if L = 1 to 3µH, Vin = 5 to 24V, Vo = 0.925 to 2V, Rds = 5 to 20mΩ, the Q

value will be between 0.65 and 0.2.

AUDIO SUSCEPTIBILITY

Audio susceptibility is the transfer function from input to output. In a typical power supply design, it is desirable to

have as much attenuation in that transfer function as possible so that noise appearing at the input has little effect

on the output. The open-loop audio susceptibility given by the model in Figure 12 is:

(73)

The closed-loop audio susceptibility is simply:

(74)

where H(s) is the compensation transfer function defined by:

(75)

It can be seen from Equation 73 that if mc is equal to 1/(2D')+0.5, then the open-loop audio susceptibility is zero.

Unfortunately, the transfer function is rather sensitive to the value of mc around the critical value and thus this

phenomenon is of little value.

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