XRP7662 Datasheet, PDF (13/19 Pages) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

XRP7 66 2
Pow er Blox ^TM
1 2 A 3 0 0 KHz Sy n ch r on ou s St ep Dow n Reg u lat or
The switching frequency and the inductor
operating point determine the inductor value
as follows:
ܮൌܸ_ூ^ܸ_{ேሺைெ௎஺்௑ሻ}^{ൈൈሺܸ݂}_{௦ூൈ}^{ேሺெ}_ܭ^{஺ோ௑ሻ}_{ൈെܫை}^{ܸை௎௎்ሺெ்ሻ}_{஺௑ሻ}
where:
f_s = switching frequency
K_R = ratio of the AC inductor ripple current to
the maximum output current
The peak -to - peak inductor ripple current is:
ܫ_௉௉ൌܸ^ை௎_ܸ^்ൈ_{ூேሺ}^ሺ_ெ^{ܸூ஺ே௑ሺሻெ}_ൈ^஺_݂^{௑ሻ௦െൈܸ}_ܮ^{ை௎்ሻ}
Once the required inductor value is selected,
the proper selection of core material is based
on peak inductor current and efficiency
requirements. The core must be large en
ough
not to saturate at the peak inductor current
ܫ_{௉ா஺௄}ൌܫ_{ை௎்ሺெ஺௑ሻ}൅^ܫ௉_ʹ^௉
and provide low core loss at the high switching
frequency. Low cost powdered -iron cores have
a gradual saturation characteristic but can
introduce considerabl e AC core loss, especially
when the inductor value is relatively low and
the ripple current is high. Ferrite materials,
although more expensive, have an abrupt
saturation characteristic with the inductance
dropping sharply when the peak design
current is e xceeded. Nevertheless, they are
preferred at high switching frequencies
because they present very low core loss while
the designer is only required to prevent
saturation. In general, ferrite or
molypermalloy materials are a better choice
for all but the mo st cost sensitive applications.
OPTIMIZING EFFICIENCY
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 currents where the
copper losses dominate. Core loss information
is usually available from the magnetics
vendor. Proper inductor selection can affect
the resulting power supply efficiency by more
than 15%!
The copper loss in the inductor can be
calculated using the following equation:
ܲ_{௅ሺ஼௨ሻ}ൌܫ_{௅ଶሺோெௌሻ}ൈܴ_{ௐூே஽ேூீ}
where I _L(RMS) is the RMS inductor current that
can be calculated as follows:
I L( RMS)
I OUT( MAX )
x
1
1

¨ ^§
I PP
¸ ^·2
3 _©I OUT( MAX ) _¹
OUTPUT CAPACITOR SELECTION
The required ESR (Equivalent Series
Resistance) and capacitance drive the
selection of the type and quantity of the
output capacitors. The ESR must be small
enough that both the resistive voltage
deviation due to a step change in the load
current and the output ripple voltage do not
exceed the tolerance limits expected on the
output voltage. During an output load
transient, t he output capacitor must supply all
the additional current demanded by the load
until the XRP7662 adjusts the inductor current
to the new value.
In order to maintain V _OUT, the capacitance
must be large enough so that the output
voltage is held up while the inductor current
ramps to the value corresponding to the new
load current. Additionally, the ESR in the
output capacitor causes a step in the output
voltage equal to the current. Because of the
fast transient response and inherent 100% to
0% duty cycle capability provided by the
XRP7662 when exposed to output load
transients, the output capacitor is typically
chosen for ESR, not for capacitance value.
The ESR of the outpu t capacitor, combined
with the inductor ripple current, is typically the
main contributor to output voltage ripple. The
maximum allowable ESR required to maintain
a specified output voltage ripple can be
calculated by:
RESR d ^'VOUT
I _{PK PK}
© 2012 Exar Corporation
13 / 19
Rev. 2. 2.0