MIC2124YMM datasheet(15/24 Pages) MICREL

15 / 24 page

Micrel, Inc.

MIC2124

June 2010

M9999-060810-D

Feedback Loop Compensation

The MIC2124 controller comes with an internal error

amplifier used for optimizing control loop stability by

placing a capacitor C1 in series with a resistor R1 and

another capacitor C2 in parallel from the COMP pin to

ground.

Figure 6. Loop Compensation

a. Power Stage

The adaptive on-time current mode control applied in

MIC2124 controller eliminates the double-pole in the

power stage, which is caused by the output inductor and

output capacitor. At the frequency range which is far

below the switching frequency (f < fSW/6), the transfer

function from the output of the error amplifier to the buck

converter output can be approximated by the following

equation:

OUT

con

ESR

G(s)

OUT

≈

(29)

where:

LOAD

OUT

LOAD

OUT

COUT = total output capacitors

ESRCOUT = electrical series resistance of the output

capacitor

RLOAD = load resistance

Ri = 2.4 x Rds(on)_bottom (low-side MOSFET Rds(on))

fSW = switching frequency

L = inductance of the output inductor

D = duty cycle

According to equation (29), there is a pole and zero pair

set by the load resistance RLOAD, the output capacitor,

and the output inductor in the power stage:

OUT

)

con

(

ESR

(30)

)

(

OUT

LOAD

OUT

)

con

(

(31)

Therefore, type II compensation, which is comprised by

C1, R1 and C2, is able to achieve a stabilized loop for

MIC2124 in most applications.

b. gm Error Amplifier

It is undesirable to have high error amplifier gain at high

frequencies because high frequency noise spikes would

be picked up and transmitted at large amplitude to the

output; thus, gain should be permitted to fall off at high

frequencies. At low frequency, it is desired to have high

open-loop gain to attenuate the power line ripple. Thus,

the error amplifier gain should be allowed to increase

rapidly at low frequencies.

The transfer function with R1, C1, and C2 for the internal

gm error amplifier can be expressed as:

()

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

G(s)

err

(32)

One pole and one zero can be seen from the above

transfer function at the following frequencies:

)

err

(

(33)

)

err

(

(34)

c. Total Open-Loop Response

The open-loop response for the MIC2124 controller is

easily obtained by combining the power stage, the

feedback resistor divider, and the error amplifier gains

together.

err

con

total

)

(

G(s)

)

(

(35)

where RFB1 and RFB2 are the voltage divider resistors, as

shown in the typical application schematic on Page 1.

It is desirable to have the gain curve intersect zero dB at

tens of kilohertz, this is commonly called crossover

frequency; the phase margin at crossover frequency

should be at least 45°.

12V to 1.8V @ 10A application is applied as an example

to demonstrate the loop compensation for MIC2124. In

this application:

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