ZL2006 Datasheet, PDF (22/45 Pages) Intersil Corporation – Adaptive Digital DC-DC Controller with Drivers and Current Sharing

ZL2006
P_LDCR = DCR × I _Lrms²
I_Lrms is given by
I _Lrms =
( ) I 2
OUT
+
I2
opp
12
where I_OUT is the maximum output current. Next,
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 Output Capacitor Selection
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_osag) and low output voltage
ripple (V_orip). However, capacitors with low ESR, such
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:
C_OUT
=
8×
I _opp
f
sw
×
V_orip
2
ESR = ^Vorip
2 × I _opp
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:
V_orip
=
I _opp
× ESR +
8×
I _opp
f _sw × C_OUT
Because each part of this equation was made to be less
than or equal to half of the allowed output ripple
voltage, the V_orip should be less than the desired
maximum output ripple.
5.8.4 Input Capacitor
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_CINrms) can be determined from the following
equation:
I_CINrms = I _OUT × D × (1 − D)
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 ripple current calculated above to avoid
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 Bootstrap Capacitor Selection
The high-side driver boost circuit utilizes an external
Schottky diode (D_B) and an external bootstrap
capacitor (C_B) to supply sufficient gate drive for the
high-side MOSFET driver. D_B should be a 20 mA, 30
V Schottky diode or equivalent device and C_B should
be a 1 μF ceramic type rated for at least 6.3V.
22
Data Sheet Revision 2/18/2009
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