SC1403ITSTRT datasheet(14/31 Pages) SEMTECH

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 2004 Semtech Corp.

SC1403

PRELIMINARY

POWER MANAGEMENT

United States Patent No. 6,377,032

www.semtech.com

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 those obtained from the simulation model.

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

help speed up the design process, a list of recommended output

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.

-50

-40

-30

-20

-10

100

1000

10000

100000

f (Hz)

-200

-150

-100

-50

100

150

200

100

1000

10000

100000

f (Hz)

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

100kHz) is plotted below.

-50

-40

-30

-20

-10

100

1000

10000

100000

f (Hz)

-200

-150

-100

-50

100

150

200

100

1000

10000

100000

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.

Applications Information (Cont.)

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