CS5305 datasheet(27/33 Pages) ONSEMI

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CS5305

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Inductors

There are many factors to consider when choosing the

output inductors. Maximum load current, core and winding

losses, ripple current, short circuit current, saturation

characteristics, component height and cost are all variables

that the designer should consider. However, the most

important consideration in designing for the VRM 9.x

specifications may be the effect inductor value has on

transient response.

The amount of overshoot or undershoot exhibited during

a current transient is defined as the product of the current

step and the output filter capacitor ESR. To some degree,

adaptive voltage positioning is used to “pre−position” the

output voltage so the voltage step during a current transient

will not cause the output voltage to exceed the Power Good

window. However, adaptive positioning will not completely

eliminate the overshoot or undershoot conditions, and

choosing the inductor value appropriately can minimize the

amount of energy that must be transferred from the inductor

to the capacitor or vice−versa. In the subsequent paragraphs,

we will determine the minimum value of inductance

required for our system and consider the trade−off of ripple

current vs. transient response.

In order to choose the minimum value of inductance, input

voltage, output voltage and output current must be known.

Most computer applications use reasonably well regulated

bulk power supplies so that, while the equations below

specify VIN(MAX) or VIN(MIN), it is possible to use the

nominal value of VIN in these calculations with little error.

Current in the inductor while operating in the continuous

current mode is defined as the load current plus ripple

current.

IL + ILOAD ) IRIPPLE

The ripple current waveform is triangular, and the current

is a function of voltage across the inductor, switch FET

on−time and the inductor value. FET on−time can be defined

as the product of duty cycle and switch frequency, and duty

cycle can be defined as a ratio of VOUT to VIN. Thus,

IRIPPLE +

(VIN * VOUT)VOUT

(fOSC)(L)(VIN)

Peak inductor current is defined as the load current plus

half of the peak current. Peak current must be less than the

maximum rated FET switch current, and must also be less

than the inductor saturation current. Thus, the maximum

output current for a single phase can be defined as:

IOUT(MAX) + ISWITCH(MAX) *

VIN(MAX) * VOUT VOUT

2 fOSC L VIN(MAX)

Since the maximum output current must be less than the

maximum switch current, the minimum inductance required

for a single phase can be determined.

L(MIN) +

(VIN(MIN) * VOUT)VOUT

(fOSC)(ISWITCH(MAX))(VIN(MIN))

This equation identifies the value of inductor that will

provide the full rated switch current as inductor ripple

current, and will usually result in inefficient system

operation. The system will sink current away from the load

during some portion of the duty cycle unless load current is

greater than half of the rated switch current. Some value

larger than the minimum inductance must be used to ensure

the converter does not sink current. Choosing larger values

of inductor will reduce the ripple current, and inductor value

can be designed to accommodate a particular value of ripple

current by replacing ISWITCH(MAX) with a desired value of

IRIPPLE:

L(RIPPLE) +

(VIN(MIN) * VOUT)VOUT

(fOSC)(IRIPPLE)(VIN(MIN))

However, reducing the ripple current will cause transient

response times to increase. The response times for both

increasing and decreasing current steps are shown below.

TRESPONSE(INCREASING) +

(L)(DIOUT)

(VIN * VOUT)

TRESPONSE(DECREASING) +

(L)(DIOUT)

(VOUT)

Inductor value selection also depends on how much output

ripple voltage the system can tolerate. Output ripple voltage

is defined as the product of the output ripple current and the

output filter capacitor ESR. However, since the CS5305 has

three paralleled phases, the net effect is that the switching

frequency as seen by the output capacitance is tripled relative

to the CS5305 operating frequency. This is because each

phase switches in sequence and the ripple currents in each

phase are superimposed on the output capacitance. This is

illustrated graphically in Figures 45 and 46.

Thus, output ripple voltage can be calculated as:

VRIPPLE + ESRC IRIPPLE +

ESRC VIN * VOUT VOUT

3 fOSC L VIN

It is also important to note that the maximum value of

inductor ESR is limited by the single−phase pulse−by−pulse

current limit. The specified minimum value for this

parameter is 80 mV, so the maximum inductor ESR is:

ESRL(MAX)(in ohms) +

(0.08 V)

single phase current limit value in Amps

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