rt8020 Richtek Technology Corporation, rt8020 Datasheet - Page 10

• Manufacturer Part Number: rt8020
• Description: Dual High-efficiency Step-down Dc-dc Converter
• Manufacturer: Richtek Technology Corporation

RT8020
Applications Information
The basic RT8020 application circuit is shown in Typical
Application Circuit. External component selection is
determined by the maximum load current and begins with
the selection of the inductor value and operating frequency
followed by C
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current I
with higher inductance.
Having a lower ripple current reduces the ESR losses in
the output capacitors and the output voltage ripple. Highest
efficiency operation is achieved at low frequency with small
ripple current. This, however, requires a large inductor.
A reasonable starting point for selecting the ripple current
is I
highest V
below a specified maximum, the inductor value should be
chosen according to the following equation :
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10
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite or permalloy
cores. Actual core loss is independent of core size for a
fixed inductor value but it is very dependent on the
inductance selected. As the inductance increases, core
losses decrease. However, increased inductance requires
more turns of wire and therefore copper losses will
increase.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design
current is exceeded.
L
ΔI
L
L
f
= 0.4(I
V
V
f
OUT
OUT
I
IN
L(MAX)
L
. To guarantee that the ripple current stays
MAX
IN
and C
). The largest ripple current occurs at the
L
increases with higher V
1
1
V
OUT
V
OUT
IN
V
.
IN(MAX)
V
OUT
IN
and decreases
This results in an abrupt increase in inductor ripple current
and consequent output voltage ripple.
Do not allow the core to saturate!
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don't radiate energy but generally cost more
than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depend on the price vs. size requirements and
any radiated field/EMI requirements.
C
The input capacitance, C
trapezoidal current at the source of the top MOSFET. To
prevent large ripple voltage, a low ESR input capacitor
sized for the maximum RMS current should be used. RMS
current is given by :
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. Note that ripple current
ratings from capacitor manufacturers are often based on
only 2000 hours of life which makes it advisable to further
de-rate the capacitor, or choose a capacitor rated at a
higher temperature than required. Several capacitors may
also be paralleled to meet size or height requirements in
the design.
The selection of C
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as the amount of bulk
capacitance that is necessary to ensure that the control
loop is stable. Loop stability can be checked by viewing
the load transient response as described in a later section.
The output ripple, V
I
ΔV
RMS
RMS
IN
OUT
and C
= I
I
OUT(MAX)
OUT
ΔI
OUT
L
/2. This simple worst-case condition is
Selection
ESR
V
V
OUT
OUT
IN
OUT
is determined by the effective series
8fC
V
, is determined by :
V
OUT
1
OUT
IN
IN
, is needed to filter the
1
DS8020-03 August 2007
IN
= 2V
OUT
, where