MIC2156 datasheet(29/35 Pages) MICREL

29 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

The Modulator Gain is proportional to the input voltage

and inversely proportional to the internal ramp voltage

generated by the oscillator. The MIC2155/6 peak-peak

ramp voltage is 1V:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

RAMP

MOD

The output voltage divider attenuates VOUT and feeds it

back to the error amplifier. The divider gain is:

OUT

REF

The modulator, filter and voltage divider gains can be

multiplied together to show the open loop gain of these

parts:

MOD

FILTER

)

(

)

(

This transfer function is plotted in Figure 25. At low

frequency, the transfer function gain equals the

modulator gain times the voltage divider gain. As the

frequency increases toward the LC filter resonant

frequency, the gain starts to peak. The increase in the

gain’s amplitude equals Q. Just above the resonant

frequency, the gain drops at a -40db/decade rate. The

phase quickly drops from 0° to almost 180° before the

phase boost of the zero brings it back up to -90°. Higher

values of Q will cause the phase to drop quickly. In a

well damped, low Q system the phase will change more

slowly.

-45

-90

-135

-180

FREQUENCY

100

10k 100k

Gain

Phase

Figure 25. GVD Transfer Function

As the frequency approaches the zero frequency (Fz),

formed by Co and it’s ESR, the slope of the gain curve

changes from -40db/dec. to -20db/dec and the phase

increases. The zero causes a 90° phase boost. Ceramic

capacitors, with their smaller values of capacitance and

ESR, push the zero and its phase boost out to higher

frequencies, which allow the phase lag from the LC filter

to drop closer to -180°. The system will be close to being

unstable if the overall open loop gain crosses 0dB while

the phase is close to -180°. If the output capacitance

and/or ESR is high, the zero moves lower in frequency

and helps to boost the phase, leading to a more stable

system.

The error amplifier is a type III which has two zeros, two

poles and a pole at the origin. This type of error amplifier

works well when Ceramic output capacitors make up the

majority of COUT because it introduces an extra zero that

helps improve phase margin:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

)

(

Gea

Where:

)

(

⋅

Figure 26 shows the bode plot of the error amplifier

transfer function.

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