zl2005 Intersil Corporation, zl2005 Datasheet - Page 21

• Manufacturer Part Number: zl2005
• Description: Digital-dc? Integrated Power Management And Conversion Ic
• Manufacturer: Intersil Corporation

struck between output ripple and optimal load tran-
sient performance. A good starting point is to select
the output inductor ripple current (I
expected load transient step magnitude (I
Now the output inductance can be calculated using the
following equation:
where V
The average inductor current is equal to the maximum
output current. The peak inductor current (IL
culated using the following equation where I
maximum output current:
Select an inductor rated for the average DC current
with a peak current rating above the peak current com-
puted above.
In over-current or short-circuit conditions, the inductor
may have currents greater than 2X the normal maxi-
mum rated output current. It is desirable to use an
inductor that is not saturated at these conditions to pro-
tect the load and the power supply MOSFETs from
damaging currents.
Once an inductor is selected, the DCR and core losses
in the inductor are calculated. Use the DCR specified
in the inductor manufacturer’s datasheet.
I
where I
culate the core loss of the selected inductor. Since this
calculation is specific to each inductor and manufac-
turer, refer to the chosen inductor’s datasheet. Add the
Lrms
is given by:
OUT
P
INM
L
I
LDCR
I
OUT
Lrms
IL
opp
is the maximum output current. Next, cal-
is the maximum input voltage.
pk
=
=
=
V
DCR
=
=
OUT
I
OUT
I
f
sw
OUT
I
×
×
2
ostep
×
(
1
+
I
I
−
Lrms
21
+
opp
I
opp
12
V
V
OUT
INM
I
2
opp
2
2
)
opp
) equal to the
(3)
(6)
(7)
ostep
(5)
(4)
OUT
pk
):
) is cal-
is the
ZL2005
core loss and the DCR loss and compare the total loss
to the maximum power dissipation recommendation in
the inductor datasheet.
Output Capacitor Selection
Several trade-offs also must be considered when
selecting an output capacitor. Low ESR values are
needed to have a small output deviation during tran-
sient load steps (V
(V
semi-stable (X5R and X7R) dielectric ceramic capaci-
tors, also have relatively low capacitance values.
Many designs can use a combination of high capaci-
tance devices and low ESR devices in parallel.
For high ripple currents, a low capacitance value can
cause a significant amount of output voltage ripple.
Likewise, in high transient load steps, a relatively
large amount of capacitance is needed to minimize the
output voltage deviation while the inductor current
ramps up to the new steady state output current value.
As a starting point, allocate one-half of the output volt-
age ripple to the capacitor ESR and the other half to its
capacitance, as shown in the following equations:
Use these values to make an initial capacitor selection,
using a single capacitor or several capacitors in paral-
lel.
After a capacitor has been selected, the resulting out-
put voltage ripple can be calculated using the follow-
ing equation:
Because each part of this equation was made to be less
than or equal to half of the allowed output ripple volt-
age, the V
output ripple.
orip
V
). However, capacitors with low ESR, such as
ESR
orip
C
OUT
orip
=
=
I
=
opp
should be less than the desired maximum
2
V
×
8
×
orip
×
I
opp
ESR
osag
f
I
sw
opp
) and low output voltage ripple
×
+
V
orip
8
2
×
f
sw
I
opp
×
C
OUT
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February 18, 2009
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FN6848.0