LM2633 datasheet(35/40 Pages) NSC

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Control Loop Design (Continued)

When Q is higher than 0.5, there will be a double-pole at half

the switching frequency f

n. 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 fre-

quency region. When Q is too high (such as Q = 5 or higher),

there will be significant peaking at half the switching fre-

quency 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 (m

c). See Equation (34). The larger the duty

cycle, the higher the Q value. The deeper the ramp compen-

sation, 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 f

p. That makes it a voltage-mode control.

The ramp compensation becomes deeper when inductance

is increased, or input voltage is decreased, or sense resis-

tance is decreased.

In the case of Channel 1 of LM2633, if L = 1 to 3µH, V

in =5

to 24V, V

o = 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 pos-

sible so that noise appearing at the input has little effect on

the output. The open-loop audio susceptibility given by the

model in

Figure 7 is:

(47)

The closed-loop audio susceptibility is simply:

(48)

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

(49)

It can be seen from

Equation (47) that if m

c 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 m

c around the critical value and thus this phenom-

enon is of little value.

The open-loop and closed-loop audio susceptibility of the

previous example is shown in

Figure 16. It can be told, both

from the model and from

Equation (47), that open-loop gain

of audio susceptibility is just a level shift of the loop gain.

Closed-loop audio susceptibility starts to depart from its

open-loop counterpart when frequency drops below the

cross-over frequency.

Adjusting the Output Voltages of the Switching

Channels

Channel 1 output voltage is normally adjusted through the

VID pins. Channel 2 output voltage is adjusted through an

external voltage divider, as shown in

Figure 17.

The equation to find the value of R

2 when R1 has been

selected is:

(50)

where V

fb2 is equal to the internal reference voltage con-

nected to the non-inverting input of the Channel 2 error

20000878

FIGURE 15. How Control-Output Transfer Function

Changes with Q Values

20000882

FIGURE 16. Example Audio Susceptibility Gain

200008B7

FIGURE 17. Setting the Ch2 Output Voltage

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