SC339 datasheet(10/15 Pages) SEMTECH

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SC339

PRELIMINARY

POWER MANAGEMENT

Applications Information (Cont.)

Using Ceramic Capacitors

SC339 is capable of operation using an all-ceramic so-

lution, needing only an external R-C compensation. The

Typical Application schematic (R3, C3) from page 1 is re-

produced here:

1.05 V @3A

SC339

DRV

ADJ

PGD

GND

5V IN

1.05V

Enable

1.05 V

Power Good

1.2 V +/-5 % IN

IRF7311

or similar

Vout = (1+R1/R2)*0.5

Notes :

(1 ) Optional Components to use with ceramic output

capacitors

(1)

()

Typical Applications Circuit

100

10K

100K

1MEG

10MEG

-40

-20

0,0

Gain (dB)

FREQ (Hz)

Low frequency pole

generated internally

Low frequency zero

generated internally

Zin

Rout / Cout pole, moves with

the load current

Prout

High frequency pole

generated by ESR

and Output Capacitor

Ceramic Capacitor operation

with very low ESR for the

output capacitor . We get a

three pole 1 zero system as

shown above and is unstable

without external compensation

Pcout

-60

-90

-180

180

270

-270

PHASE

PHASE (deg)

Typical Frequency Response without Compensation and

Ceramic Output Capacitors:

SC339 Frequency Response with

Ceramic Output Capacitors and no Compensation

From this response we see that the system is not stable

as it has a phase margin of approximately 0 degrees.

For stable operation we introduce a low frequency pole

and a zero. The low frequency pole is used to roll off the

gain quicker and the zero is used to increase the band-

width of the system.

The Pole-Zero location:

100

10K

100K

1MEG

10MEG

-40

-20

0,0

Gain (dB)

FREQ (Hz)

Pin + Pcomp

Zin + Zcomp

Prout

Ceramic Capacitor

Operation with

Compensation on DRV pin

Pcout

-60

-90

-180

180

270

-270

PHASE

PHASE (deg)

SC339 Frequency Response with

Ceramic Output Capacitors and R-C Compensation

From the above ﬁgure we can see that the overall re-

sponse of the system is stable with a decent phase

margin

It is important to select the external compensation zero

to be between 1 kHz and 5 kHz for optimum bandwidth

and phase margin. In this example we have selected

zero at approximately 3 kHz.

The compensation values are calculated by the following

empirical equation:

We chose a low R3 compensation value to roll off gain.

R3 = 100 Ω.

½ • •

R3 • C3 = 3kHz

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