MIC2155 datasheet(28/35 Pages) MICREL

28 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

The snubber capacitor, CS, is charged and discharged

each switching cycle. The energy stored in CS is

dissipated by the snubber resistor, RS, two times per

switching period. This power is calculated in the

equation below:

SNUBBER

Where:

fS is the switching frequency for each phase

VIN is the DC input voltage

Compensation of the Output Voltage Loop

The voltage regulation, filter and power stage section is

shown in Figure 24. The error amplifier for Channel 1 is

used to regulate the output voltage and compensate the

voltage regulation loop. It is a voltage output op amp that

is designed to use type III (PID) compensation. Type III

compensation has two compensating zeros, two poles

and a pole at the origin. The figure also shows the

transfer function for each section.

Compensation is necessary to insure the control loop

has adequate bandwidth and phase margin to properly

respond to input voltage and output current transients.

High gain at DC and low frequencies is needed for

accurate output voltage regulation. Attenuation near the

switching frequency prevents switching frequency noise

from interfering with the control loop.

For this analysis, the LC filter in the two phase design is

combined into one. The inductances are in parallel and

the output capacitance is the total sum of all COUT. The

ESR is the parallel combination of all ESRs. The output

load is represented by a resistor R.

The output filter contains a complex double pole formed

by the capacitor and inductor and a zero from the output

capacitor and it’s ESR. The transfer function of the filter

is:

)

(

Gfilter

⎟

⎠

⎞

⎜

⎝

⎛

Where:

E/A

Modulator

Filter

VREF = 0.7V

Driver

VIN

RESR

L1 || L2

Figure 24. Voltage Loop and Transfer Functions

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