Datasheet

100 1k 10k 100k 1M 10M

COMPENSATOR GAIN (dB)

FREQUENCY (Hz)

-ADJ

-5.0

LM22672

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SNVS588L –SEPTEMBER 2008–REVISED APRIL 2013

The complete transfer function for the regulator loop is found by combining the compensation and power stage

transfer functions. The LM22672 has internal type III loop compensation, as detailed in Figure 20. This is the

approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer

function consists of a D.C. gain and a second order pole created by the inductor and output capacitor(s). Due to

the input voltage feedforward employed in the LM22672, the power stage D.C. gain is fixed at 20dB. The second

order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the

product of inductance and output capacitance should conform to the following equation:

(9)

Alternatively, this pole should be placed between 1.5kHz and 15kHz and is given by the equation shown below:

(10)

The Q factor depends on the parasitic resistance of the power stage components and is not typically in the

control of the designer. Of course, loop compensation is only one consideration when selecting power stage

components; see the Application Information section for more details.

Figure 20. Compensator Gain

In general, hand calculations or simulations can only aid in selecting good power stage components. Good

design practice dictates that load and line transient testing should be done to verify the stability of the application.

Also, Bode plot measurements should be made to determine stability margins. Application note AN-1889

SNVA364 shows how to perform a loop transfer function measurement with only an oscilloscope and function

generator.

Application Information

Typical Buck Regulator Application

Figure 21 shows an example of converting an input voltage range of 5.5V to 35V, to an output of 3.3v at 1 Amp.

See AN-1896 SNVA369 for more information.

Product Folder Links: LM22672