SC471AEVB datasheet(17/27 Pages) SEMTECH

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SC471/SC471A

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POWER MANAGEMENT

In general, four parameters are needed to deﬁne the

design:

1) Nominal output voltages (VOUT)

2) Static or DC output tolerance

3) Transient response

4) Maximum load current (IOUT)

Therearetwovaluesofloadcurrenttoconsider:continuous

load current and peak load current. Continuous load

current is concerned with thermal stresses which drive

the selection of input capacitors, MOSFETs and diodes.

Peak load current determines instantaneous component

stresses and ﬁltering requirements such as inductor

saturation, output capacitors and design of the current

limit circuit.

Design example:

VBAT = 10V min, 20V max

VOUT1 = 0.9V +/- 4%

VOUT2 = 1.05V +/-4%

VOUT3 = 1.1V +/-4%

VOUT4 = 1.15V+/-4%

Load = 20A maximum

Inductor Selection

Low inductor values result in smaller size but create

higher ripple current. Higher inductor values will reduce

the ripple current but are larger and more costly. Because

wire resistance varies widely for different inductors and

because magnetic core losses vary widely with operating

conditions, it is often difﬁcult to choose which inductor

will optimize efﬁciency. The general rule is that higher

inductor values have better efﬁciency at light loads due

to lower core losses and lower peak currents, but at high

load the smaller inductors are better because of lower

resistance. The inductor selection is generally based on

the ripple current which is typically set between 20% to

50% of the maximum load current. Cost, size, output ripple

and efﬁciency all play a part in the selection process.

The switching frequency is optimized for 325kHz. The

equation for on-time is:

TON (nsec) = 2560 • (VOUT/VBAT) + 35

During the DH on-time, voltage across the inductor is (VBAT

- VOUT). To determine the inductance, the ripple current

must be deﬁned. Smaller ripple current will give smaller

output ripple and but will lead to larger inductors. The ripple

current will also set the boundary for PSAVE operation.

The switcher will typically enter PSAVE operation when the

load current decreases to 1/2 of the ripple current; (i.e.

if ripple current is 4A then PSAVE operation will typically

start for loads less than 2A. If ripple current is set at 40%

of maximum load current, then PSAVE will commence for

loads less than 20% of maximum current).

The equation for determining inductance is:

L = (VBAT - VOUT) • TON / IRIPPLE

Use the maximum value for VBAT, and for TON use the

value associated with maximum VBAT. For selecting the

inductor, we start with the highest VOUT setting and a

maximum ripple current of 5A.

TON = 182 nsec at 20VBAT, 1.15VOUT

L = (20 - 1.15) • 182 nsec / 5A = 0.69μH

We will select a slightly larger value of 0.7μH, which will

decrease the maximum IRIPPLE to 4.91A.

Note: the inductor must be rated for the maximum DC load

current plus 1/2 of the ripple current.

The minimum ripple current under is also checked .This

occurs when VBAT and VOUT are set to their minimum

values of 10V and 0.9V.

TONVBATMIN = 2560 • (0.9/10) + 35 = 265 nsec

IRIPPLE = (VBAT - VOUT) • TON / L

IRIPPLE_VBATMIN = (10 - 0.9) • 265 nsec / 0.7μH = 3.45A

Capacitor Selection

The output capacitors are chosen based on required ESR

and capacitance. The ESR requirement is driven by the

output ripple requirement and the DC tolerance. The

output voltage has a DC value that is equal to the valley

of the output ripple, plus 1/2 of the peak-to-peak ripple.

Change in the ripple voltage will lead to a change in DC

voltage at the output.

Applications Information (continued)

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