SC2544TSTRT View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2544TSTRT

High Performance Wide Input Range Dual Synchronous Buck Controller

Semtech Corporation 

SC2544

POWER MANAGEMENT

Applications Information (Cont.)

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.

F z1

F p 1 C O M P E N S A T O R G A IN

Fp2

F z2

Fo

Fz

L O O P G A IN

C O N V E R T E R G A IN

Fc

F R E Q U E N C Y (H z)

Fig. 11. Bode plots for control loop design.

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:

Rbot

=

0.75V

Vo − 0.75V

∗ Rtop

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.

(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

C2

calculation of the compensation parameters.

C1

R2

C3

R3

Vc

-

Out

Rtop

Vo

+

E/A

Rbot

0.75V

Fig. 12. Compensation network.

 2005 Semtech Corp.

16

www.semtech.com

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