SC2440AEVB datasheet(15/28 Pages) SEMTECH

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Applications Information (Continued)

SC2440A

Figure 5. Normalized C

IN RMS Ripple Current as a

Function of the Duty Cycle D for the Following

Regulators:

(a) Two step-down converters switching

in phase and at the same duty cycle. Each

regulator delivers I

OUT to its corresponding

output for a total output current of 2I

OUT.

(b) A 180o out of phase switching dual step-

down regulator. The output currents and

the duty cycles of the individual regulators

are identical. Each regulator delivers I

OUT to

its corresponding output for a total output

current of 2I

OUT.

Equation (6) has a maximum of

when

, corre-

spondingtotheworst-casepowerdissipationof

in C

IN. For example, if one power transistor in the SC2440A

is switching from zero to 2A and operating at 50% duty

cycle while the other channel is disabled, then the in-

put capacitor will carry A of RMS ripple current. If both

power transistors in the SC2440A were to switch on in

phase, the current drawn by the SC2440A would consist

of current pulses with amplitude equal to the sum of the

channel switch currents. If both channels were delivering

full load to their outputs and operating at 50% duty cycle,

then the input current would switch from zero to 4A. The

RMS ripple current in the input capacitor would then be

2A. Power dissipated in C

IN would be (2A)

(ESR), four times

the maximum due to one channel alone. The SC2440A

produces the highest RMS ripple current in C

IN when only

one channel is switching at current limit (< 3.4A). The in-

put capacitor therefore should have a RMS ripple current

rating of at least .7A.

Figure 5 compares the RMS ripple currents produced in

the input capacitor by (a) two identical step-down con-

verters switching in phase and (b) a dual step-down con-

verter with 80o out of phase switching (as implemented

in the SC2440A) as a function of the switching duty cycle

D. For simplicity, each individual converter in both cases is

assumed to operate at the same duty cycle and deliver the

same output current I

OUT for a total output current of 2IOUT.

Case (a) produces a maximum C

IN RMS ripple current of IOUT

when D = 0.5. Whereas the corresponding ripple current

is reduced to

in Case (b). At 50% duty cycle, 80o

out of phase switching nulls C

IN ripple current. Figure 5(b)

also shows that slight deviation from 80o phase shift has

no major impact on input ripple reduction. Interleaved

switching therefore generates lower input voltage

noise and requires a smaller input ceramic capacitor

for filtering. This saves cost for V

IN > 25V as high voltage

ceramic capacitors are not cheap. Predicting the input ca-

pacitor RMS ripple current of a dual step-down converter

operating at different duty cycles and delivering different

output currents is not easy. However, the aforementioned

advantages of interleaved switching are still valid.

Figure 6 compares the input voltage ripple generated

by the DC-DC converter in Figure with either channel

or both channels switching. The low-noise advantage of

interleaved switching is clearly evident.

Multi-layer ceramic capacitors, which have very low ESR

(a few mW) and can easily handle high RMS ripple current

are the ideal choice for input filtering. A single 4.7mF or

0mF X5R ceramic capacitor is adequate. For high voltage

applications, a small ceramic (mF or 2.2mF) can be placed

in parallel with a low ESR electrolytic capacitor to satisfy

both the ESR and bulk capacitance requirements.

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