SP6136ER1 datasheet(11/18 Pages) EXAR

Part #	SP6136ER1

Description	Synchronous Buck Controller

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Manufacturer	EXAR [Exar Corporation]

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Oct 3-06 Rev L

SP636 Synchronous Buck Controller

than internal resistors, then the Vin start

threshold is given by:

Vin(start) = 2.5 • (R4+R5)/R5................ (8)

For example, if it is required to have a Vin

start threshold of 7V, then let R5 = 5KW and

using equation (9) we get R4 = 9.09KW.

Inductor Selection

There are many factors to consider in select-

ing the inductor including cost, efficiency,

size and EMI. In a typical SP636 circuit,

the inductor is chosen primarily for value,

saturation current and DC resistance. In-

creasing the inductor value will decrease

output voltage ripple, but degrade transient

response. Low inductor values provide the

smallest size, but cause large ripple cur-

rents, poor efficiency and need more output

capacitance to smooth out the larger ripple

current. The inductor must also be able to

handle the peak current at the switching

frequencywithoutsaturating,andthecopper

resistance in the winding should be kept as

low as possible to minimize resistive power

loss. A good compromise between size, loss

and cost is to set the inductor ripple current

to be within 20% to 40% of the maximum

output current.

The switching frequency and the inductor

operating point determine the inductor value

as follows:

L =

Vout • (Vin(max) - Vout)

Vin(max)

• Fs • Kr • Iout(max)

where:

Fs = switching frequency

Kr = ratio of the ac inductor ripple current

to the maximum output current

The peak to peak inductor ripple current is:

Ipp =

Vout • (Vin(max) - Vout)

Vin(max) • Fs • L

Once the required inductor value is selected,

the proper selection of core material is based

on peak inductor current and efficiency re-

quirements. The core must be large enough

not to saturate at the peak inductor current

IpEak = Iout(max) + Ipp

/2

and provide low core loss at the high switch-

ing frequency. Low cost powdered iron cores

have a gradual saturation characteristic

but can introduce considerable AC core

loss, especially when the inductor value is

relatively low and the ripple current is high.

Ferrite materials, on the other hand, are more

expensive and have an abrupt saturation

characteristic with the inductance dropping

sharply when the peak design current is

exceeded. Nevertheless, they are preferred

at high switching frequencies because they

present very low core loss and the design

only needs to prevent saturation. In general,

ferrite or molypermalloy materials are the

better choice for all but the most cost sensi-

tive applications.

The power dissipated in the inductor is equal

to the sum of the core and copper losses.

To minimize copper losses, the winding

resistance needs to be minimized, but this

usually comes at the expense of a larger

inductor.Corelosseshaveamoresignificant

contribution at low output current where the

copper losses are at a minimum, and can

typically be neglected at higher output cur-

rents where the copper losses dominate.

Core loss information is usually available

from the magnetic vendor.

The copper loss in the inductor can be cal-

culated using the following equation:

PL(cu) = I

2

L

(rms) • Rwinding

where IL(rms) is the RMS inductor current that

can be calculated as follows:

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