SC1403 datasheet(13/30 Pages) SEMTECH

13 / 30 page

 2002 Semtech Corp.

www.semtech.com

SC1403

POWER MANAGEMENT

PRELIMINARY

Applications Information

Applications Information (Cont.)

Designing the Loop

There are two aspects concerning the loop design. One is the

power train design and the other is the external compensation

design. A good loop design is a combination of the two. In the

SC1403, the control-to-output/power train response is dominated

by the load impedance, the effective current sense resistor, out-

put capacitance, and the ESR of the output caps. The low fre-

quency gain is dominated by the output load impedance and the

effective current sense resistor. Inherent to Virtual Current SenseTM,

there is one additional low frequency pole sitting between 100Hz

and 1KHz and a zero between 15KHz and 25KHz. To compensate

for the SC1403 is easy since the output of error amplifier COMP

pin is available for external compensation. A traditional pole-zero-

pole compensation is not necessary in the design using SC1403.

To ensure high phase margin at crossover frequency while mini-

mizing the component count, a simple high frequency pole is usu-

ally sufficient. In the reference design below, single-pole compen-

sation method is demonstrated. And the loop measurement re-

sults are compared to that obtained from the simulation model.

Transient response is also done to validate the model. Also, to

help speeding up the design process, a list of recommended out-

put caps vs. compensation caps value is given in table I.

Single-Pole compensation Method

Given parameters:

Vin = 19V, Vout = 3.3V @ 2.2A,

Output impedance, Ro = 3.3V/2.2A = 1.5 Ω ,

Panasonic SP cap, Co = 180uF, Resr = 15 Ω

m ,

Output inductor, Lo = 4.7uH

Switching frequency, Fs = 300KHz

Simulated Control-to-Output gain & phase response (up to

100KHz) is plotted below

Measured Control-to-Output gain & phase response (up to

100KHz) is plotted below.

-200

-150

-100

-50

100

150

200

1.00E+02

1.00E+03

1.00E+04

1.00E+05

f (Hz)

-50

-40

-30

-20

-10

1.00E+02

1.00E+03

1.00E+04

1.00E+05

f (Hz)

-200

-150

-100

-50

100

150

200

1.00E+02

1.00E+03

1.00E+04

1.00E+05

f (Hz)

Single-pole compensation of the error amplifier is achieved by

connecting a 100pF capacitor from the COMP pin of the SC1403

to ground. The simulated feedback gain & phase response (up to

100KHz) is plotted below.

-50

-40

-30

-20

-10

1.00E+02

1.00E+03

1.00E+04

1.00E+05

f (Hz)

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