SC2441AEVB datasheet(27/37 Pages) SEMTECH

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SC2441A

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.

Output Capacitor (C

o) 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

esr) and

ESL (L

esl) (Figure 16).

Figure 16. C

o equivalent circuit

If the current through the branch is i

b(t), the voltage

across the terminals will then be

(

)

(

)

(

)

(

esr

esl

This basic equation illustrates the effects of ESR, ESL

and C

o on the output voltage.

The first term is the DC voltage across C

o 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.

Applications Information

Since the inductor current is a triangular waveform with

peak-to-peak value

δ * I

o, the ripple-voltage caused by

inductor current ripples is

The ripple-voltage due to ESL is

esl

ESL

and the ESR ripple-voltage is

esr

ESR

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

∆V

o, the ESR should satisfy

esr

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

esr

The required ESR value of the output capacitors should

esr = min{Resr1,Resr2 }.

In the aluminum capacitor selection, the working voltage

rating is normally suggested to be greater than 1.5V

The allowable current ripple (RMS) should be greater than

Lesl

Resr

b(t)

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