RT8048
Application Information
The basic RT8048 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
IN
and C
OUT
. Although frequency as high as
3MHz are possible, the minimum on-time of the RT8048
imposes a minimum limit on the operating duty cycle.
The minimum duty is equal to 70ns’ f
OSC
(Hz)’ 100%.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current
ΔI
L
increases with higher V
IN
and decreases
with higher inductance :
⎡
V
⎤⎡
V
⎤
Δ
I
L
=
⎢
OUT
⎥ ⎢
1
−
OUT
⎥
V
IN
⎦
⎣
f
OSC
×
L
⎦ ⎣
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
ΔI
L
= 0.4 (I
MAX
). The largest ripple current occurs at the
highest V
IN
. To guarantee that the ripple current stays
below a specified maximum, the inductor value should be
chosen according to the following equation :
⎡
⎤⎡
⎤
V
OUT
V
L=
⎢
⎥ ⎢
1
−
OUT
⎥
⎢
f
OSC
× Δ
I
L(MAX)
⎥ ⎢
V
IN(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, forcing
the use of more expensive ferrite or molypermalloy 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, more copper losses.
Ferrite designs have very low core losses and are preferred
at high switching frequencies. Hence, design goals should
concentrate on copper loss and saturation prevention.
Ferrite core material saturates “ hard” , which means that
the inductance collapses abruptly when the peak design
DS8048-01 June 2011
current is exceeded. This result 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
inductor type to use mainly depends on the price vs. size
requirements and any radiated field/EMI requirements.
C
IN
and C
OUT
Selection
The input capacitance, C
IN
, 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 :
I
RMS
=
I
OUT(MAX)
V
OUT
V
IN
V
IN
−
1
V
OUT
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
=
I
OUT
/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 either further derate the
capacitor or choose a capacitor rated at a higher
temperature than required. Several capacitors may also
be placed in parallel to meet size or height requirements
in the design. The selection of C
OUT
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 examined by viewing the load transient response as
described in a later section. The output ripple,
ΔV
OUT
, is
determined by :
⎡
⎤
1
Δ
V
OUT
≤ Δ
I
L
⎢
ESR
+
⎥
8f
OSC
C
OUT
⎦
⎣
The output ripple is highest at maximum input voltage
since
ΔI
L
increases with input voltage. Multiple capacitors
placed in parallel may be needed to meet the ESR and
RMS current handling requirements. Dry tantalum, special
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