CS51313 datasheet(13/20 Pages) CHERRY

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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 +

where

IL(PEAK) = inductor peak current;

IOUT = load current;

∆IL = inductor ripple current.

IL(VALLEY) = IOUT −

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 pro-

vide acceptable ripple on the input supply lines. A key

specification for input capacitors is their ripple current rat-

ing. 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) =

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) ×

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 − D))

The number of input capacitors required is then deter-

mined by:

NCIN =

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 capac-

itors:

ESRCIN =

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 dissipa-

tion 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 +5V supply. One

method of achieving this is by using an input inductor and

a bypass capacitor. The input inductor isolates the +5V

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 require-

ments on the input bypass capacitor bank, which has to

initially support the sudden load change.

The minimum inductance value for the input inductor is

therefore:

ESRCAP

NCIN

ICIN(RMS)

IRIPPLE

1 − D

(IL(PEAK)2 + (IL(PEAK) × IL(VALLEY)) + IL(VALLEY)2 × D

∆IL

Application Information: continued

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