MCP19035-AAAAESLASHMF datasheet(23/44 Pages) MICROCHIP

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DS22326B-page 23

MCP19035

5.2.8

FEEDBACK LOOP COMPENSATION

Since the MCP19035 implements a Voltage-Mode

PWM control, a Type-III compensation network is

recommended. Correct placing of poles and zeros

require analysis of the Bode plots for the buck

converter power train.

FIGURE 5-1:

Bode Plots for Buck

Converter Power Train (Representation Using

Asymptotes).

The power train of a buck converter that uses voltage

mode control is a second order system. At the LC

resonance frequency, a double pole occurs; this pole

will “push” the gain down with a slope of -40db/decade.

This double pole also introduces a phase lag of -180°.

The compensation network must counteract the effects

of this double pole in order to achieve the stability of the

system.

The Equivalent Series Resistance (ESR) of the output

capacitor introduces a zero that “pushes” the gain and

phase up again. This zero helps the stability of the

system if it occurs before the phase reaches the critical

point of -180°. However, due to the performance criteria

(output voltage ripple, efficiency), the application

requires the use of low ESR capacitors. For capacitors

that have very low ESR (ceramic capacitors), this zero

occurs at high frequency, where the phase reaches the

critical point.

The frequencies for pole and zero are determined

using Equation 5-20:

EQUATION 5-20:

POLE AND ZERO

FREQUENCIES

EQUATION 5-21:

PWM MODULATOR GAIN

The Type-III compensation network is represented in

Figure 5-2:

FIGURE 5-2:

Type-III Compensation

Network.

Magnitude

(dB)

Frequency

(log scale)

AMOD

0dB

-40 dB/decade

-20 db/decade

fLC

fESR

Phase

(deg)

0°

-90°

-180°

Frequency

(log scale)

 LC

OUT



-------------------------------------

ESR

 ESR C

OUT



-----------------------------------------------

MOD

RAMP



---------------------

log



log







---------------------------------



--------------------







 C



--------------------------------------











---------------------------------









--------------------







---------------------------------------------------------------------------------------------------



Where:

VREF

VIN

COMP

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