XRP6124 Datasheet, PDF (9/12 Pages) Exar Corporation – Non-Synchronous PFET Step-Down Controller

XRP6124
Non-Synchronous PFET Step-Down Controller
OUTPUT INDUCTOR
Select the output inductor L1 for inductance L,
DC current rating I_DC and saturation current
rating I_SAT. I_DC should be larger than regulator
output current. I_SAT, as a rule of thumb, should
be 50% higher than the regulator output
current. Calculate the inductance from:
L
=
(V_IN
− V_OUT
)_{⎜⎜⎝⎛}
ΔI _L
V_OUT
× fs ×V_IN
⎟⎟⎠⎞
Where:
ΔI_L is peak-to-peak inductor current ripple
nominally set to 30% of I_OUT
f_S is nominal switching frequency from table 2
OUTPUT CAPACITOR C_OUT
Select the output capacitor for voltage rating,
capacitance C_OUT and Equivalent Series
Resistance ESR. The voltage rating, as a rule
of thumb, should be twice the output voltage.
When calculating the required capacitance,
usually the overriding requirement is current
load-step transient. If the unloading transient
requirement (i.e., when I_OUT transitions from a
high to a low current) is met, then usually the
loading transient requirement (when I_OUT
transitions from a low to a high current) is met
as well. Therefore calculate the C_OUT
capacitance based on the unloading transient
requirement from:
( ) ^COUT
=
L
×
⎜⎛
⎜⎝
V_OUT
I2
High
−
I _LOW
2
+ V_transient ² − V_OUT ²
⎟⎞
⎟⎠
Where:
L is the inductance calculated in the preceding
step
I_High is the value of I_OUT prior to unloading. This
is nominally set equal to regulator current
rating.
I_Low is the value of I_OUT after unloading. This is
nominally set equal to 50% of regulator
current rating.
V_transient is the maximum permissible voltage
transient corresponding to the load step
mentioned above. V_transient is typically specified
from 3% to 5% of V_OUT.
ESR of the capacitor has to be selected such
that the output voltage ripple requirement
V_OUT(ripple), nominally 1% of V_OUT, is met.
Voltage ripple V_OUT(ripple) is composed mainly of
two components: the resistive ripple due to
ESR and capacitive ripple due to C_OUT charge
transfer. For applications requiring low voltage
ripple, ceramic capacitors are recommended
because of their low ESR which is typically in
the range of 5mΩ. Therefore V_OUT(ripple) is
mainly capacitive. For ceramic capacitors
calculate the V_OUT(ripple) from:
V_OUT(ripple)
=
ΔI _L
8 × C_OUT
×
fs
Where:
C_OUT is the value calculated above
If tantalum or electrolytic capacitors are used
then V_OUT(ripple) is essentially a function of ESR:
V_OUT(ripple) = ΔI _L × ESR
INPUT CAPACITOR C_IN
Select the input capacitor for voltage rating,
RMS current rating and capacitance. The
voltage rating, as a rule of thumb, should be
50% higher than the regulator’s maximum
input voltage. Calculate the capacitor’s current
rating from:
( ) I_CIN,RMS = I_OUT × D × 1 − D
Where:
I_OUT is regulator’s maximum current
D is duty cycle (D=V_OUT/V_IN)
Calculate the C_IN capacitance from:
( ) _CIN
=
I _OUT
×V_OUT ×
fs ×V_IN ²
V_IN − V_OUT
× ΔV_IN
Where:
ΔV_IN is the permissible input voltage ripple,
nominally set to 1% of V_IN.
© 2011 Exar Corporation
9/12
Rev. 1.1.0