SC2447TSTRT datasheet(16/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

Setting the Switching Frequency

The switching frequency is set with an external resistor

connected from Pin 3 to the ground. The set frequency is

inversely proportional to the resistor value (Figure 8).

100

200

300

400

500

600

700

800

100

150

200

250

Rosc (k Ohm)

Figure 8. Free Running Frequency vs. R

OSC

Inductor (L) and Ripple Current

Both step-down controllers in the SC2447 operate in

synchronous continuous-conduction mode (CCM)

regardless of the output load. The output inductor

selection/design is based on the output DC and transient

requirements. Both output current and voltage ripples

are reduced with larger inductors but it takes longer to

change the inductor current during load transients.

Conversely, smaller inductors 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*I

o, the inductance value will then be

)

(

−

The peak current in the inductor becomes (1+

δ/2)*Io

and the RMS current is

rms

Consider the following when choosing inductors:

a) Inductor core material: For high efficiency applications

above 350kHz, ferrite, Kool-Mu and polypermalloy

materials should be used. Low-cost powdered iron cores

can be used for cost sensitive-applications below 350kHz

but with attendant higher core losses.

b) Select inductance value: Sometimes the calculated

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

can choose the next larger standard inductance value.

The inductance varies with temperature and DC current.

It is a good engineering practice to re-evaluate the

resulting 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

The output capacitor filters the inductor current in the

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 (Lesl) (Figure

9).

Lesl

Resr

Figure 9. An Equivalent Circuit of C

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 effect 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 voltage variation caused by the

Application Information (Cont.)

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