RT8020EPQW データシートの表示（PDF） - Richtek Technology
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RT8020EPQW Datasheet PDF : 14 Pages
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Applications Information
The basic RT8020E 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 CIN and COUT.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current ΔIL increases with higher VIN and decreases
with higher inductance.
ΔIL
=
⎡ VOUT
⎢⎣ f × L
⎤
⎥⎦
×
⎢⎣⎡1
−
VOUT
VIN
⎤
⎥⎦
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 ΔIL = 0.4(IMAX). The largest ripple current occurs at the
highest VIN. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation :
L
=
⎡
⎢
⎣
f
×
VOUT
ΔIL(MAX)
⎤
⎥
⎦
×
⎢⎡1 −
⎣
VOUT
VIN(MAX)
⎤
⎥
⎦
Inductor Core Selection
Once the value for L is known, the type of inductor can be
selected. High efficiency converters generally cannot afford
the core loss found in low cost powdered iron cores, thus,
limiting the use to 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 higher copper losses.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, thus allowing design goals
to concentrate on copper loss and saturation prevention.
Ferrite core material saturates “hard”, which means that
inductance collapses abruptly when the peak design
current is exceeded.
RT8020E
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 type of inductor to
use mainly depends on the price vs. size requirements
and any radiated field/EMI requirements.
CIN and COUT Selection
The input capacitance, CIN, is needed to filter the
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 :
IRMS
= IOUT(MAX)
VOUT
VIN
VIN − 1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT/2. This simple worst-case condition is
commonly used for design because even significant
deviations do not result in much difference. Note that ripple
current ratings from capacitor manufacturers are often
based on a life time of only 2000 hours, 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 the
size or height requirements in the design.
The selection of COUT is determined by the effective series
resistance (ESR) that is required to minimize voltage ripple
and load step transients, as well as by 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, ΔVOUT, is determined by :
ΔVOUT
≤
ΔIL
⎢⎣⎡ESR +
1
8fCOUT
⎤
⎥⎦
DS8020E-02 March 2011
www.richtek.com
11
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