SC412AEVB datasheet(12/22 Pages) SEMTECH

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SC412A

POWER MANAGEMENT

Applications Information (continued)

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

switching 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 / I

RIPPLE

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

associated with maximum VBAT.

TON = 182 nsec at 20VBAT, 1.1VOUT

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

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

decrease the maximum I

RIPPLE to 4.91A.

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

rent plus 1/2 of the ripple current.

The ripple current under minimum VBAT conditions is also

checked.

TON

VBATMIN = 2560 • (1.15/10) + 35 = 329 nsec

RIPPLE = (VBAT - VOUT) • TON / L

RIPPLE_VBATMIN = (10 - 1.15) • 329 nsec / 0.7μH = 4.16A

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.

The design goal is +/-4% output regulation. The internal

0.75V reference tolerance is 1%, assuming 1% tolerance

for the FB resistor divider, this allows 2% tolerance due to

VOUT ripple. Since this 2% error comes from 1/2 of the

ripple voltage, the allowable ripple is 4%, or 46mV for a

1.15V output.

The maximum ripple current of 4.05A creates a ripple

voltage across the ESR. The maximum ESR value allowed

would be 44mV:

ESR

MAX = VRIPPLE/IRIPPLEMAX = 46mV / 4.91A

ESR

MAX = 9.4 mΩ

The output capacitance is typically chosen based on tran-

sient requirements. A worst-case load release, from maxi-

mum load to no load at the exact moment when inductor

current is at the peak, deﬁnes the required capacitance.

If the load release is instantaneous (load changes from

maximum to zero in a very small time), the output capaci-

tor must absorb all the inductor’s stored energy. This will

cause a peak voltage on the capacitor according to the

equation:

COUT

MIN = L • (IOUT + 1/2 • IRIPPLEMAX)

/ (V

PEAK

- VOUT2)

Assuming a peak voltage VPEAK of 1.230 (80mV rise upon

load release), and a 10 amp load release, the required

capacitance is:

COUT

MIN = 0.7μH•(10 + 1/2 • 4.91)

/ (1.232 - 1.152)

COUT

MIN = 570μF

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