SP6136 Datasheet, PDF (11/18 Pages) Sipex Corporation – 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 = 5KΩ 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 SP6136 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
frequency without saturating, and the copper
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
APPLICATION INFORMATION
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. Core losses have a more significant
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(c_u)
=
i²
l(rms)
•
rwinding
where IL(RMS) is the RMS inductor current that
can be calculated as follows:
Oct 31-06 Rev L
SP6136 Synchronous Buck Controller
11
© 2006 Sipex Corporation