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CS51313

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where ESRCAP = maximum ESR per capacitor (specified in

manufacturer’s data sheet).

The designer must also verify that the inductor value

yields reasonable inductor peak and valley currents (the

inductor current is a triangular waveform):

IL(PEAK) + IOUT ) D

2.0

where:

IL(PEAK) = inductor peak current;

IOUT = load current;

ΔIL = inductor ripple current.

IL(VALLEY) + IOUT * D

2.0

where IL(VALLEY) = inductor valley current.

Given the requirements of an application such as a buck

converter, it is found that a toroid powdered iron core is quite

suitable due to its low cost, low core losses at the switching

frequency, and low EMI.

Step 5: Selection of the Input Capacitors

These components must be selected and placed carefully

to yield optimal results. Capacitors should be chosen to

provide acceptable ripple on the input supply lines. A key

specification for input capacitors is their ripple current

rating. The input capacitor should also be able to handle the

input RMS current IIN(RMS).

The combination of the input capacitors CIN discharges

during the on−time.

The input capacitor discharge current is given by:

ICINDIS(RMS) +

IL(PEAK)2

) (IL(PEAK)

IL(VALLEY))

) IL(VALLEY)2

3.0

where:

ICINDIS(RMS) = input capacitor discharge current;

IL(PEAK) = inductor peak current;

IL(VALLEY) = inductor valley current.

CIN charges during the off−time, the average current

through the capacitor over one switching cycle is zero:

ICIN(CH) + ICIN(DIS)

1.0 * D

where:

ICIN(CH) = input capacitor charge current;

ICIN(DIS) = input capacitor discharge current;

D = Duty Cycle.

The total Input RMS current is:

ICIN(RMS) +

(ICIN(DIS)2 D)

) (ICIN(CH)2

(1.0 * D))

The number of input capacitors required is then

determined by:

NCIN +

ICIN(RMS)

IRIPPLE

where:

NCIN = number of input capacitors;

ICIN(RMS) = total input RMS current;

IRIPPLE = input capacitor ripple current rating (specified

in manufacturer’s data sheets).

The total input capacitor ESR needs to be determined in

order to calculate the power dissipation of the input

capacitors:

ESRCIN +

ESRCAP

NCIN

where:

ESRCIN = total input capacitor ESR;

ESRCAP = maximum ESR per capacitor (specified in

manufacturer’s data sheets);

NCIN = number of input capacitors.

Once the total ESR of the input capacitors is known, the

input capacitor ripple voltage can be determined using the

formula:

VCIN(RMS) + ICIN(RMS)

ESRCIN

where:

VCIN(RMS) = input capacitor RMS voltage;

ICIN(RMS) = total input RMS current;

ESRCIN = total input capacitor ESR.

The designer must determine the input capacitor power

loss in order to ensure there isn’t excessive power

dissipation through these components. The following

formula is used:

PCIN(RMS) + ICIN(RMS)2

ESRCIN

where:

PCIN(RMS) = input capacitor RMS power dissipation;

ICIN(RMS) = total input RMS current;

ESRCIN = total input capacitor ESR.

Step 6: Selection of the Input Inductor

A CPU switching regulator, such as the one in a buck

topology, must not disturb the primary +5.0 V supply. One

method of achieving this is by using an input inductor and

a bypass capacitor. The input inductor isolates the +5.0 V

supply from the noise generated in the switching portion of

the microprocessor buck regulator and also limits the inrush

current into the input capacitors upon power up. The

inductor’s limiting effect on the input current slew rate

becomes increasingly beneficial during load transients. The

worst case is when the CPU load changes from no load to full

load (load step), a condition under which the highest voltage

change across the input capacitors is also seen by the input

inductor. The inductor successfully blocks the ripple current

while placing the transient current requirements on the input

bypass capacitor bank, which has to initially support the

sudden load change.

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