MCP19035-AAAAESLASHMF datasheet(24/44 Pages) MICROCHIP

24 / 44 page

MCP19035

DS22326B-page 24

 2012-2013 Microchip Technology Inc.

FIGURE 5-3:

Bode Plots for Type III

Compensation Network (Representation Using

Asymptotes).

Assuming C3 «C2 and R3 «R1, the pole and zero

frequencies can be calculated using Equation 5-22:

EQUATION 5-22:

POLE AND ZERO

FREQUENCIES OF THE

COMPENSATION

NETWORK

EQUATION 5-23:

ZERO GAIN

EQUATION 5-24:

POLE GAIN

The Type-III compensation network provides two zeros

and three poles (including origin pole), pushing the

cross-over frequency as high as possible, and boosts

the phase margin of the system to greater than 45°. A

higher bandwidth yields a faster load transient

response. The faster transient response results in a

smaller output voltage overshoot.

The procedure for placing the poles and zeros to

achieve the optimum phase margin are presented

below:

Determine the frequency of the double pole

(LC pole) and ESR zero using Equation 5-20.

Choose resistor R1 (usually between 10 kΩ and

100 kΩ). This value is a compromise between

high values for additional capacitors (higher

cost) and possible noise induced problems.

Resistor R2 is calculated using Equation 5-25:

EQUATION 5-25:

FEEDBACK RESISTOR

DIVIDER

Choose the crossover frequency of the

compensated

system.

This

frequency

recommended to be between 1/10th and 1/5th of

the switching frequency (fSW). A higher

crossover frequency will improve the transient

response, but will decrease the phase margin.

For most of the applications, the crossover

frequency is set around 1/10th of switching

frequency. This is a reasonable compromise

between

simplifying

the

design

the

compensation loop and achieving a fast

transient response. Since the frequency of the

ESR zero is much higher than LC resonant

frequency, the gain of the power train can be

typically

approximated

the

crossover

frequency, using Equation 5-26:

EQUATION 5-26:

POWER TRAIN GAIN AT

CROSSOVER

FREQUENCY

The compensated error amplifier must have a

gain equal to APTco at crossover frequency

(fCO). Typically, this crossover frequency occurs

between FZ2 and FP1 (see Figure 5-3).

Magnitude

(dB)

Frequency

(log scale)

AOL

0dB

fZ1

fZ2

Phase

(deg)

0°

90°

-90°

Frequency

(log scale)

fP1

fP2

APOLE

AZERO



 C



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

 R



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



 R



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

 R



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







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

 R



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



 R



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

ZERO

------

log



POLE





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

log



REF



OUT

REF

–

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

0.6



OUT

0.6

–

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

MOD

–

--------





log



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