SC2544 datasheet(16/24 Pages) SEMTECH

16 / 24 page

 2005 Semtech Corp.

www.semtech.com

SC2544

POWER MANAGEMENT

The compensator in Figure 10 includes an error am-

plifier and impedance networks Zf and Zs. It is imple-

mented by the circuit in Figure 12. The compensator

provides an integrator, double poles, and double ze-

ros.

As shown in Figure 11, the integrator is used to

boost the gain at low frequency. Two zeros are intro-

duced to compensate excessive phase lag at the

loop gain crossover due to the integrator (-90deg)

and the complex pole pair (-180deg). Two high fre-

quency poles are designed to compensate the ESR

zero and to attenuate high frequency noise.

Fig. 11. Bode plots for control loop design.

where 0.75V is the internal reference voltage of the

SC2544.

The other components of the compensator can be

calculated using following design procedure:

(1). Plot the converter gain, including LC filter and

PWM modulator.

Rtop

Rbot

∗

−

Fig. 12. Compensation network.

A resistive divider is used to program the output volt-

age. The top resistor Rtop of the divider in Fig. 12

can be chosen from 20k Ω to 30k Ω . Then the bot-

tom resistor Rbot is found from:

Fz1

Fz2

Fp 1

F p 2

FR E Q U E N C Y (H z)

C O M PEN SA T O R GA IN

E R

T E

R G

A IN

L OOP

Fz1

Fz2

Fp 1

F p 2

FR E Q U E N C Y (H z)

C O M PEN SA T O R GA IN

E R

T E

R G

A IN

L OOP

Rbot

E/A

Rtop

0.75V

Out

(2). Select the open loop crossover frequency Fc lo-

cated at 10% to 20% of the switching frequency. At

Fc, find the required DC gain.

(3). Use the first compensator pole Fp1 to cancel the

ESR zero Fz.

(4). Have the second compensator pole Fp2 at half

the switching frequency to attenuate the switching

ripple and high frequency noise.

(5). Place the first compensator zero Fz1 at or below

50% of the power stage resonant frequency Fo.

(6). Place the second compensator zero Fz2 at or

below the power stage resonant frequency Fo.

A MathCAD program is available upon request for the

calculation of the compensation parameters.

Applications Information (Cont.)

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