RT8008
Applications Information
current is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage ripple.
Do not allow the core to saturate!
The basic RT8008 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
Different core materials and shapes will change the size/
current and price/current relationship of an inductor.
followed by CIN and COUT
.
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 depends on the price vs size requirements and
any radiated field/EMI requirements.
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.
V
f ×L
V
OUT
V
IN
⎡
⎤⎡
⎦⎣
⎤
OUT
ΔI =
L
1−
⎥⎢
⎢
⎣
⎥
⎦
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 :
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.
Areasonable 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 :
VOUT
V
IN
IRMS = IOUT(MAX)
−1
V
VOUT
IN
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 based on only 2000
hours of life which makes it advisable to further derate 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.
⎡
⎤⎡
1−
⎤
V
f × ΔI
V
OUT
V
IN(MAX)
OUT
L =
⎢
⎣
⎥⎢
⎥
⎦
L(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,
forcing the use of more expensive ferrite or mollypermalloy
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. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
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 as described in a later section.
The output ripple, ΔVOUT, is determined by :
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
⎡
⎣
⎤
1
ΔV
≤ ΔI ESR +
OUT
L
⎢
⎥
⎦
8fC
OUT
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
DS8008-07 March 2011
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9
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