SC416 datasheet(20/34 Pages) SEMTECH

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SC416

Applications Information (continued)

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 ½ of the peak-to-peak ripple.

Changing the ripple voltage will lead to a change in DC

output voltage.

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

750mV reference tolerance is 1%, and assuming 1% toler-

ance for the FB resistor divider, this allows 2% tolerance

due to VOUT ripple. Since this 2% error comes from ½ of

the ripple voltage, the allowable ripple is 4%, or 72mV for

a 1.8V output. Although this is acceptable from a regula-

tion standpoint, 72mV ripple is high for a 1.8V output and

therefore more realistic ripple value of 36mV will be used

(2% of VOUT).

The maximum ripple current of 4.2A creates a ripple volt-

age across the ESR. The maximum ESR value allowed

would create 36mV ripple:

ESR

MAX

= V

RIPPLE

RIPPLEMAX

= 36mV / 4.2A

ESR

MAX

= 8.6 mΩ

While the ESR is chosen to meet ripple requirements,

the output capacitance (μF) is typically chosen based on

transient requirements. A worst-case load release, from

maximum load to no load at the exact moment when in-

ductor current is at the peak, defines the required capaci-

tance. If the load release is instantaneous (load changes

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

capacitor 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 × I

RIPPLEMAX

___________________

PEAK

2 - VOUT2)

With a peak voltage VPEAK of 1.98V (180mV or 10% rise

above 1.8V upon load release), the required capacitance

is:

COUT

MIN

1.5μH × (10 + 1/2 × 4.2)2

___________________

(1.982 - 1.82)

COUT

MIN

= 323μF

The previous requirements (323μF, 6.4mΩ) will be met

using a single 330μF 6mΩ device.

Note that output voltage ripple is often higher than ex-

pected due to the ESL (inductance) of the capacitor. See

the FB/VOUT Ripple section.

If the load release is relatively slow, the output capaci-

tance can be reduced. At heavy loads during normal

switching, when the FB pin is above the 750mV refer-

ence, the DL output is high and the low-side MOSFET is

on. During this time, the voltage across the inductor is

approximately -VOUT. This causes a down-slope or fall-

ing di/dt in the inductor. If the load di/dt is not much

faster than the di/dt in the inductor, then the inductor

current can track the changing load current, and there

will be relatively less overshoot from a load release. The

following can be used to calculate the needed capaci-

tance for a given dILOAD/dt.

Peak inductor current:

PEAK

= ILOAD

MAX

+ 1/2 × I

RIPPLEMAX

PEAK

= 10 + 1/2 × 4.2 = 12.1A

Rate of change of Load current = dILOAD/dt

IMAX = maximum DC load current = 10A

COUT = IL

PEAK

× (L ×IL

PEAK

/ V

OUT

- IMAX/(dILOAD /dt))

___________________________________

2 × (V

PEAK

- VOUT)

Example: Load dI/dt = 2.5A/μsec

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