zl2006 Intersil Corporation, zl2006 Datasheet - Page 22

• Manufacturer Part Number: zl2006
• Description: Adaptive Digital Dc-dc Controller With Drivers And Current Sharing
• Manufacturer: Intersil Corporation

I
where I
calculate the core loss of the selected inductor. Since
this calculation is specific to each inductor and
manufacturer, refer to the chosen inductor datasheet.
Add the core loss and the ESR loss and compare the
total loss to the maximum power dissipation
recommendation in the inductor datasheet.
5.8.3
Several trade-offs must also be considered when
selecting an output capacitor. Low ESR values are
needed to have a small output deviation during
transient load steps (V
ripple (V
as semi-stable (X5R and X7R) dielectric ceramic
capacitors, also have relatively low capacitance values.
Many designs can use a combination of high
capacitance 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 or down to the new steady state output
current value.
As a starting point, apportion one-half of the output
ripple voltage to the capacitor ESR and the other half
to capacitance, as shown in the following equations:
Lrms
is given by
Output Capacitor Selection
OUT
orip
). However, capacitors with low ESR, such
is the maximum output current. Next,
I
Lrms
C
P
OUT
LDCR
ESR
22
=
=
=
I
8
osag
=
DCR
OUT
×
) and low output voltage
2
f
V
2
×
sw
I
orip
+
opp
×
I
×
opp
I
( )
I
V
Lrms
opp
12
orip
2
2
2
ZL2006
Use these values to make an initial capacitor selection,
using a single capacitor or several capacitors in
parallel.
After a capacitor has been selected, the resulting output
voltage ripple can be calculated using the following
equation:
Because each part of this equation was made to be less
than or equal to half of the allowed output ripple
voltage, the V
maximum output ripple.
5.8.4
It is highly recommended that dedicated input
capacitors be used in any point-of-load design, even
when the supply is powered from a heavily filtered 5 or
12 V “bulk” supply from an off-line power supply.
This is because of the high RMS ripple current that is
drawn by the buck converter topology. This ripple
(I
equation:
Without capacitive filtering near the power supply
circuit, this current would flow through the supply bus
and return planes, coupling noise into other system
circuitry. The input capacitors should be rated at 1.2X
the
overheating of the capacitors due to the high ripple
current, which can cause premature failure. Ceramic
capacitors with X7R or X5R dielectric with low ESR
and 1.1X the maximum expected input voltage are
recommended.
5.8.5
The high-side driver boost circuit utilizes an external
Schottky diode (D
capacitor (C
high-side MOSFET driver. D
V Schottky diode or equivalent device and C
be a 1 μF ceramic type rated for at least 6.3V.
CINrms
ripple
) can be determined from the following
Input Capacitor
Bootstrap Capacitor Selection
V
orip
I
B
CINrms
) to supply sufficient gate drive for the
current
=
orip
I
opp
=
should be less than the desired
×
B
I
) and an external bootstrap
OUT
ESR
calculated
×
+
B
8
D
should be a 20 mA, 30
×
×
f
1 ( D
December 15, 2010
sw
I
above
opp
−
×
C
)
OUT
FN6850.1
to
B
should
avoid