Vishay Semiconductors

SiP12202

Document Number: 73542

S-52083–Rev. A, 10-Oct-05

www.vishay.com

9

New Product

Compensation:

The SiP12201 uses voltage mode control in conjunction

with a high frequency Transconductance error amplifier.

The voltage feedback loop is compensated at the Comp/

SD pin, which is the output node of the error amplifier.

The feedback loop is generally compensated with an RC

+ C (one pole, one zero) network from comp to GND.

Loop stability is affected by the values of the inductor,

the output capacitor, the output capacitor ESR, and the

error amplifier compensation network.

The ideal Bode plot for a compensated system would be

gain that rolls off at a slope of -20 dB/decade, crossing

0dB at the desired bandwidth and a phase margin

greater than 90° for all frequencies below the 0dB cros-

sing.

The compensation network used with the error amplifier

must provide enough phase margin at the 0dB cross-

over frequency for the overall open-loop transfer func-

tion to be stable. The following guidelines will calculate

the compensation pole and zero to stabilize the

SiP12201.

The inductor and output capacitor values are usually

determined by efficiency, voltage and current ripple

requirements. The inductor and the output capacitor cre-

ate a double pole at the frequency and a -180° phase

change:

The ESR of the output capacitor and the output capaci-

tor value form a zero at the frequency:

give a 90° phase boost. R3 and C1 will establish the sec-

ond zero of the system. The frequency of the zero

should be 2X lower than the double pole frequency of

the inductor and the output capacitor.

Choose a value for R3 usually between 1 k

Ω and 10 kΩ.

This second zero will provide the second 90° phase

boost and will stabilize the closed loop system.

The second pole should be placed at ½ the switching

frequency.

Although a mathematical approach to frequency com-

pensation can be used, the added complication of input

and/or output filters, unknown capacitor ESR, and gross

operating point changes with input voltage, load current

variations, all suggest a more practical empirical me-

thod. This can be done by injecting at the load a variable

frequency small signal voltage between the output and

the feedback network and using an RC network box to

iterate toward the final values; or by obtaining the opti-

mum loop response using a network analyzer to mea-

sure the loop Gain and Phase.

Layout:

As in the design of any switching dc-to-dc converter,

driver careful layout will ensure that there is a successful

transition from design to production. One of the few

drawbacks of switching dc-to-dc converters is the noise

induced by their high-frequency switching. Parasitic

inductance and capacitance may become significant

when a converter is switching at 500 kHz. However,

GM

∆Vosc

0.6 V

PWM Comp

ESR

R1

R2

R3

L

C2

COUT

C1

VIN

VOUT

VOUT

OSC

500KHz

FB

Compensation

OUT

p(LC)

C

*

L

2

1

f

π

=

)

ESR)(C

(

2

1

f

OUT

Z(ESR)

π

=

R3C1

2

1

π

=

1

-

SW

F

C1

R1

2

C1

C2

∗

∗

∗

=

π