RT8056 查看數據表（PDF） - Richtek Technology
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RT8056 Datasheet PDF : 11 Pages
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RT8056
Applications Information
The basic RT8056 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 must
be selected. High efficiency converters generally cannot
afford the core loss found in low-cost powdered iron cores,
thus 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, results in higher copper
losses.
Ferrite designs have very low core losses and are preferred
at high switching frequencies, so design goals can
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.
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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 kind 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 offer much relief. Note that ripple current
ratings from capacitor manufacturers are often only based
on 2000 hours of life-time 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 COUT is determined by the Effective Series
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. The output ripple, ΔVOUT, is
determined by :
ΔVOUT
≤
ΔIL
⎢⎣⎡ESR +
1
8fCOUT
⎤
⎥⎦
The output ripple is highest at maximum input voltage since
ΔIL increases with input voltage. Multiple capacitors placed
in parallel may be needed to meet the ESR and RMS
current handling requirements. Dry tantalum, special
DS8056-02 April 2011
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