SC2542 datasheet(15/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

Applications Information (Cont.)

Input voltage range: Vin,min and Vin,max

Input voltage ripple (peak-to-peak): DVin

Output voltage: Vo

Output voltage accuracy: e

Output voltage ripple (peak-to-peak): DVo

Nominal output (load) current: Io

Maximum output current limit: Io,max

Output (load) current transient slew rate: dIo/dt (A/s)

Circuit efficiency:

Inductor (L) and Ripple Current

Both step-down controllers in the SC2542 operate in

synchronous continuous-conduction mode (CCM)

regardless except in light-load mode. The output

inductor selection/design is based on the output DC

and transient requirements. Both output current and

voltage ripples are reduced with larger inductance but

it takes longer to change the inductor current during

load transients. Conversely smaller inductance results

in lower DC copper losses but the AC core losses (flux

swing) and the winding AC resistance losses are higher.

A compromise is to choose the inductance such that

peak-to-peak inductor ripple-current is 20% to 30% of

the rated output load current.

Assuming that the inductor current ripple (peak-to-

peak) value is

δ Io, the inductance value will then be:

)

−

The peak current in the inductor becomes:

)

(

and RMS current is:

rms

The followings are to be considered when choosing in-

ductors.

a) Inductor core material: For higher efficiency applica-

tions above 300 KHz, ferrite, Kool-Mu and polypermalloy

materials should be used. Low-cost powdered iron cores

can be used for cost sensitive-applications below 300

KHz but with attendant higher core losses.

b) Select inductance value: Sometimes the calculated

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

signer can choose the adjacent (larger) standard induc-

tance value. The inductance varies with temperature and

DC current. It is a good engineering practice to re-evalu-

ate 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 cur-

rent under all conditions.

Output Capacitor (Co) and Vout Ripple

The output capacitor provides output current filtering in

steady state and serves as a reservoir during load tran-

sient. The output capacitor can be modeled as an ideal

capacitor in series with its parasitic ESR and ESL as shown

in Figure 13.

Figure 13. An equivalent circuit of output.

If the current through the branch is ib(t), the voltages

across the terminals will then 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 voltage variation caused by the

charge balance between the load and the converter out-

put. The third term is 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. Since the inductor current is a triangular

waveform with peak-to-peak value ä*Io, the ripple-volt-

age caused by inductor current ripples is:

≈

∆

the ripple-voltage due to ESL is:

esl

ESL

∆

and the ESR ripple-voltage is:

esr

ESR

∆

1( )

()

().

di t

V t

i t dt Lesl

sri t

∫

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