100

1k

10k

100k

1M

10M

0

5

10

15

20

25

30

35

40

FREQUENCY (Hz)

-ADJ

-5.0

LM22675, LM22675-Q1

SNVS591L – SEPTEMBER 2008 – REVISED NOVEMBER 2014

www.ti.com

Feature Description (continued)

7.3.4 Internal Loop Compensation

The LM22675 has internal loop compensation designed to provide a stable regulator over a wide range of

external power stage components.

The internal compensation of the -ADJ option is optimized for output voltages below 5 V. If an output voltage of

5 V or greater is needed, the -5.0 option with an external resistor divider can be used.

Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The

LM22675 stability can be verified using the WEBENCH Designer online circuit simulation tool. A quick start

spreadsheet can also be downloaded from the online product folder.

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

transfer functions. The LM22675 has internal type III loop compensation, as detailed in Figure 11. 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 dc gain and a second order pole created by the inductor and output capacitor(s). Due to

the input voltage feedforward employed in the LM22675, the power stage dc gain is fixed at 20 dB. 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 Equation 3.

(3)

Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is given by Equation 4.

(4)

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 and Implementation for more details.

Figure 11. 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. AN-1889 How to Measure the

Loop Transfer Function of Power Supplies (SNVA364) shows how to perform a loop transfer function

measurement with only an oscilloscope and function generator.

10

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