MIC2156 datasheet(32/35 Pages) MICREL

32 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

Unlike the voltage output amplifier used for Channel 1

compensation, a transconductance amplifier is used for

the Channel 2 compensation since only a pole/zero

combination

required

for

compensation.

The

transconductance amplifier transfer function is:

)

(

Gea

Where:

Rz1 and Cz1 are the external components connected to

the COMP2 pin

gm is the transconductance of the internal amplifier.

The pole and zero frequencies are:

ZERO

POLE

The gain of the modulator is:

MOD

Where:

VM is the peak-to-peak amplitude of the internal

sawtooth.

The gain of the feedback circuit is output current divide

by VEA

The filter transfer function is the output current over the

applied voltage:

OUT

)

(

FILTER

−

The open loop transfer function is:

()

(

()

(

)

OUT

)

(

FILTER

MOD

)

(

)

(

−

The loop is inherently stable because the phase shift is

only 90 degrees. The error amplifier pole and zero is

selected to achieve a desired crossover frequency. In

this example, the desired crossover frequency is 50kHz.

The transfer function of the filter, modulator and

feedback is plotted in Figure 28.

-50

-100

FREQUENCY

100

10k 100k

Gain

Phase

VIN = 12V

VOUT = 1.8V

L = 1.5µH

Figure 28. Current Sharing Loop Gain/Phase

The gain boost required at 50kHz is 28dB which is a

gain of 25. The gain for frequencies above the zero is:

GMID

For a typical gm = 1.25mS, solving for Rz1:

MID

Set the zero frequency to be 1/5 of the crossover

frequency:

800

The compensated open loop gain/phase plot is shown in

Figure 29.

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