SC2441AEVB(2006) View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC2441AEVB
(Rev.:2006)

1.8V to 20V Input 2-Phase Synchronous Step-down Controllers with Step-up Converter

Semtech Corporation 

SC2441A

POWER MANAGEMENT

Applications Information

b) Select inductance value: Sometimes the calculated

inductance value is not available off-the-shelf. The

designer can choose the adjacent (larger) standard

inductance value. The inductance varies with

temperature and DC current. It is a good engineering

practice to re-evaluate the resultant current ripple at

the rated DC output current.

c) Current rating: The saturation current of the inductor

should be at least 1.5 times of the peak inductor current

under all conditions.

Since the inductor current is a triangular waveform with

peak-to-peak value δ * Io, the ripple-voltage caused by

inductor current ripples is

Dv C

»

dIo

8Cofs

.

The ripple-voltage due to ESL is

DvESL

= Leslfs

dIo

D

Output Capacitor (Co) and Vout Ripple in Step-down

Sections

The output capacitor provides output current filtering in

steady state and serves as a reservoir during load

transient. The output capacitor can be modeled as an

ideal capacitor in series with its parasitic ESR (R ) and

esr

ESL (L ) (Figure 16).

esl

Co

ib(t)

Lesl

Resr

and the ESR ripple-voltage is

DvESR = Resr dIo.

Aluminum capacitors (e.g. electrolytic, solid OS-CON,

POSCAP, tantalum) have high capacitances and low

ESL’s. The ESR has the dominant effect on the output

ripple voltage. It is therefore very important to minimize

the ESR.

When determining the ESR value, both the steady state

ripple-voltage and the dynamic load transient need to be

considered. To keep the steady state output ripple-voltage

< ∆Vo, the ESR should satisfy

R esr1

<

DVo

dIo

.

Figure 16. Co equivalent circuit

If the current through the branch is i (t), the voltage

b

across the terminals will then be

ò vo (t) =

Vo

+

1

Co

t

ib (t)dt + Lesl

0

dib (t)

dt

+ Resrib (t).

To limit the dynamic output voltage overshoot/

undershoot within α (say 3%) of the steady state output

voltage) under 0 to full load current swing, the ESR value

should be

R esr 2

<

aVo

Io

.

The required ESR value of the output capacitors should

be

This basic equation illustrates the effects of ESR, ESL

and Co on the output voltage.

The first term is the DC voltage across Co at time t=0.

The second term is the ripple-voltage caused by the

inductor ripple-current. The third term is the voltage ripple

due to ESL and the fourth term is the voltage ripple due

to ESR. The total output voltage ripple is then a vector

sum of the last three terms.

Resr = min{Resr1,Resr2 }.

In the aluminum capacitor selection, the working voltage

rating is normally suggested to be greater than 1.5Vo.

The allowable current ripple (RMS) should be greater than

 2006 Semtech Corp.

27

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