SC2620 datasheet(11/26 Pages) SEMTECH

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SC2620

POWER MANAGEMENT

)

)(

(

)

)(

(

OUT

−

(4)

where L is in

µH and f is in MHz.

Equation (3) shows that for a given

OUT

∆ increases as

D decreases. If

V varies over a wide range, then choose

L based on the nominal input voltage. Always verify

converter operation at the input voltage extremes.

The peak current limits of both SC2620 power transistors

are internally set at 3.2A. The peak current limits are duty-

cycle invariant and are guaranteed higher than 2.3A. The

maximum load current is therefore conservatively:

)

MAX

(

OUT

∆

−

∆

−

(5)

⋅

∆

, then

)

MAX

(

OUT

⋅

−

∆

−

The saturation current of the inductor should be 20-30%

higher than the peak current limit (2.3A). Low-cost powder

iron cores are not suitable for high-frequency switching

power supplies due to their high core losses. Inductors

with ferrite cores should be used.

Power Line Input Capacitor

A buck converter draws pulse current with peak-to-peak

amplitude equal to its output current I

OUT from its input

supply. An input capacitor placed between the supply and

the buck converter filters the AC current and keeps the

current drawn from the supply to a DC constant. The input

capacitance C

IN should be high enough to filter the pulse

input current. Its equivalent series resistance (ESR) should

be low so that power dissipated in the capacitor does not

result in significant temperature rise and degrade reliability.

For a single channel buck converter, the RMS ripple current

in the input capacitor is

)

(

OUT

RMS

)

CIN

(

−

(6)

Applications Information

Power dissipated in the input capacitor is

)

ESR

(

RMS

)

CIN

(

⋅

Equation (6) has a maximum value of

OUT ( at

= ),

corresponding to the worst-case power dissipation

ESR

OUT ⋅

in C

IN.

A dual-channel step-down converter with interleaved

switching reduces the RMS ripple current in the input

capacitor to a fraction of that of a single-phase buck

converter. If both power transistors in the SC2620 were

to switch on in phase, the current drawn by the SC2620

would consist of current pulses with amplitude equal to

the sum of the channel output currents. If each channel

were delivering I

OUT and operating at 50% duty cycle, then

the input current would switch from zero to 2I

OUT. The RMS

ripple current in the input capacitor would then be I

OUT.

Power dissipated in C

IN would be

ESR

OUT ⋅

, 4 times that

of a single-channel converter. The SC2620 produces the

highest RMS ripple current in C

IN when only one channel is

running and delivering the maximum output current (2A).

The input capacitor therefore should have a RMS ripple

current rating of at least 1A.

Multi-layer ceramic capacitors, which have very low ESR

(a few m

Ω) and can easily handle high RMS ripple current,

are the ideal choice for input filtering. A single 4.7

µF or

µF X5R ceramic capacitor is adequate. For high voltage

applications, a small ceramic (1

µF or 2.2µF) can be placed

in parallel with a low ESR electrolytic capacitor to satisfy

both the ESR and bulk capacitance requirements.

Output Capacitor

The output ripple voltage

∆V

OUT of a buck converter can be

expressed as









∆

OUT

ESR

(7)

where C

OUT is the output capacitance.

Inductor ripple current

∆I

L increases as D decreases

(Equation (3)). The output ripple voltage is therefore the

highest when V

IN is at its maximum. The first term in (7)

results from the ESR of the output capacitor while the

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