SC1104B datasheet(8/14 Pages) SEMTECH

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POWER MANAGEMENT

SC1104A/B

∏

•

NET

Value for the resistor R

N should

be 1/10 of the output

divider upper resistor R

Example.

Switching frequency f

SW = 300kHz

Output capacitance C

OUT = 3 x 330µF

Output capacitor ESR = 45m

Ω/each

Output inductance L

OUT = 4.7µH

Input voltage V

IN = 12V

Output voltage V

OUT = 3.3V

Let’s choose crossover frequency

CO = 1/20 • fSW = 15kHz

The compensation values used in this example are based

on the following criteria:

Z = fLC; fNET = 1/10 • fLC; fP = 10 • fCO = 150kHz

Therefore,

kHz

990

4.7µ

∏

•

kHz

990

015

ESR

∏

•

Since, the EA can sink/source about 1mA, let’s choose

Rc = 680

Ω, then

∏

•

1500

∏

•

Assuming the output divider lower resistor R

B = 1k, then

for V

OUT = 3.3V the RA = 2.32k.

NET

∏

•

At the closed-loop crossover frequency f

CO, the

Theory of Operation (Cont.)

attenuation due to the L

O, CO filter and the output

resistor divider R

A, RB is compensated by the gain of

the PWM modulator and the gain of the

transconductance error amplifier (Gm

• Z

COMP).

Shown below is a typical Bode plot of the open-loop

frequency response of SC1104 based buck converter.

100

10k

100k

1Meg

frequency in hertz

-450

-350

-250

-150

-50.0

-80.0

-40.0

40.0

80.0

1 vdberr

2 vpherr

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