SC475AEVB datasheet(18/27 Pages) SEMTECH

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SC475A

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POWER MANAGEMENT

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 create 46mV ripple:

ESRMAX = VRIPPLE/IRIPPLEMAX = 46mV / 4.91A

ESRMAX = 9.4 mΩ

The output capacitance is typically chosen based on

transient requirements. A worst-case load release, from

maximum 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 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 • (I

OUT

+ 1/2 • IRIPPLEMAX)2 / (VPEAK2 - VOUT2)

With a peak voltage VPEAK of 1.230 (80mV rise above

1.15 upon load release), the required capacitance is:

COUT

MIN = 0.7μH•(10 + 1/2•4.91)

/(1.242 - 1.152)

COUT

MIN = 570μF

The above requirements (570μF, 9.4mΩ) will be met using

two capacitors, 330μF 6mΩ.

If the load release is relatively slow, the output capacitance

can be reduced. At heavy loads during normal switching,

when the FB pin is above the 0.75V reference, 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 downslope or falling 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

change in load current, and there will be relatively less

overshoot from a load release. The following can used to

calculate the needed capacitance for a given dILOAD/dt.

Peak inductor current,

ILPEAK = ILOADMAX + 1/2 • IRIPPLEMAX

ILPEAK = 10 + 1/2 • 4.91 = 12.45A

Rate of change of Load current = dILOAD/dt

IMAX = maximum DC load current = 10A

COUT = ILPEAK • (L •ILPEAK / VOUT - IMAX/dILOAD /dt)

2 • (VPEAK - VOUT)

Example: Load dI/dt = 2.5A/μsec

This would cause the output current to move from 10A to

zero in 4μsec.

COUT = 12.45•(0.7μH•12.45/1.15 - 10/(2.5/1μsec)

2 •(1.23 - 1.15)

COUT = 278 μF

Stability Considerations

Unstable operation shows up in two related but distinctly

different ways: double-pulsing and fast-feedback loop

instability. double-pulsing occurs due to switching noise

seen at the FB input or because the ESR is too low, causing

insufﬁcient voltage ramp in the FB signal. This causes

the error ampliﬁer to trigger prematurely after the 350ns

minimum off-time has expired. Double-pulsing will result

in higher ripple voltage at the output but in most cases is

harmless. In some cases, however, double-pulsing can

indicate the presence of loop instability, which is caused

by insufﬁcient ESR.

One simple way to solve this problem is to add some

trace resistance between the VOUT/FB sense point and

the output capacitor in the high current output path. A

side effect of doing this is output voltage droop with load.

Another way to eliminate doubling-pulsing is to add a small

(e.g. 10pF) capacitor across the upper feedback resistor

Applications Information (continued)

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