MIC2155 datasheet(29/33 Pages) MICREL

29 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

Step 3: Determine the gain boost needed at the

crossover frequency (fc)

Typically, 50° of phase margin can be used for most

applications. This is a good tradeoff between an

overdamped system (slower response to transients) and

an underdamped system (overshoot or unstable

response to transients). It also allows some margin for

component tolerances and variations due to ambient

temperature changes. The phase margin at the

crossover frequency (fc) can be determined by plotting

the Gvd(s) phase on a bode plot or can be estimated

with the following formula:

⎥⎦

⎤

⎢⎣

⎡

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

−

tan

The additional phase boost required from the error

amplifier is:

Boost

−

Step 4: Determine the frequencies fz2 and fp1

The frequencies for the zero and pole (fz2 and fp1) are

calculated for the desired amount of phase boost at the

crossover frequency (fc).

[]

Boost

sin

−

Step 5: Determine the frequency for fz1

The low frequency zero, fz1, is initially set to one-fifth of

the LC resonant frequency. If it is set too low, it will force

the low frequency gain to be low and impact transient

response. If set too high, it will not add enough phase

boost at the LC resonant frequency. This could cause

conditional stability, which when the phase drops below -

180° before the gain crosses 0dB. If the DC gain should

drop in this situation, this may lead to an unstable

system.

Step 6: Determine the frequency for fp2

This is the high frequency pole, which is useful in

additional attenuation of the switching frequency. It

should initially be set at half of the switching frequency. If

it is set too low, it will lower the phase margin at the

crossover frequency, making it difficult to achieve the

proper phase margin. If set too high, it will not provide

attenuation of the switching frequency, which could lead

to jitter of the switching waveform or instability under

certain conditions.

Calculating Error Amplifier Component Values

Once the pole and zero frequencies have been fixed, the

error amplifier’s resistor and capacitor values are

calculated.

R1 This value is chosen first. All other component values

are calculated from R1. A value of 10K is suggested. If

R1 is chosen too high, R2 may be very large and the

high impedances could be sensitive to noise. If the

remote sense amplifier is used, R1 must be large

enough so than not more than 500µA of current is drawn

from the amplifier.

R2 The value of R2 is determined from the mid-band

gain of the error amplifier. This gain depends on the

frequencies of the poles, zeros and LC filter resonant

frequency. Based on the amount of gain necessary at

the crossover frequency the mid-band gain and R2 value

is calculated using the following formula.

Vin

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

The other component values are calculated as follows:

)

(

−

Compensation of the Current Sharing Loop

The control circuitry for Channel 2 forces the channel’s

output current to match the current in Channel 1. The

Channel 2 error amplifier compares the inductor currents

in the two channels and adjusts the duty cycle of

Channel 2 to control its output current. A block diagram

is shown in Figure 27.

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