Datasheet

100 1k 10k 100k 1M 10M

COMPENSATOR GAIN (dB)

FREQUENCY (Hz)

-ADJ

-5.0

LM22679/LM22679Q

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SNVS581K –FEBRUARY 2013–REVISED NOVEMBER 2012

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

transfer functions. The LM22679 has internal type III loop compensation, as detailed in Internal Loop

Compensation section. 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 LM22679, 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:

(7)

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

(8)

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 Application Information for more details.

Figure 14. 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 shows

how to perform a loop transfer function measurement with only an oscilloscope and function generator.

Product Folder Links: LM22679/LM22679Q