MIC2155 datasheet(31/35 Pages) MICREL

31 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

Based on the amount of gain necessary at the crossover

frequency the mid-band gain and R2 value is calculated

using the following formula:

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.

Vin

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

The other component values are calculated as follows:

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.

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.

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.

Channel 2

Transconductance E/A

Modulator

Filter

CZ1

RZ1

Driver

Ramp

VIN

RL2

Channel 1 IL

RL1

Figure 27. Current Sharing Loop and Transfer Functions

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