rt8012a Richtek Technology Corporation, rt8012a Datasheet - Page 11

• Manufacturer Part Number: rt8012a
• Description: Dual 1a/1.5a-1.2mhz Synchronous Step-down Converters
• Manufacturer: Richtek Technology Corporation

Applications Information
The basic RT8012A 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 ΔI
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 ΔI
highest V
below a specified maximum, the inductor value should be
chosen according to the following equation :
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.
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
DS8012A-01 March 2007
L
ΔI
=
L
L
= 0.4(I
⎡
⎢ ⎣
=
f
×
⎡
⎢ ⎣
V
IN
V
Δ
f
OUT
. To guarantee that the ripple current stays
OUT
I
×
MAX
L(MAX)
IN
L
and C
). The largest ripple current occurs at the
⎤
⎥ ⎦
L
×
increases with higher V
⎤
⎥ ⎦
⎡
⎢ ⎣
1
×
−
OUT
⎡
⎢
⎣
1
V
−
V
OUT
.
IN
V
IN(MAX)
V
OUT
⎤
⎥ ⎦
⎤
⎥
⎦
IN
and decreases
Preliminary
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!
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. However, they are
usually more expensive than the similar powered iron
inductors. The choice of which style inductor to use mainly
depends 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
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.
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, ΔV
RMS
IN
I
ΔV
RMS
and C
= I
OUT
=
OUT
I
OUT(MAX)
≤
OUT
ΔI
/2. This simple worst-case condition is
L
Selection
⎡
⎢ ⎣
ESR
OUT
V
V
OUT
IN
OUT
is determined by the effective series
+
8fC
, is determined by :
V
V
OUT
1
OUT
IN
IN
, is needed to filter the
−
⎤
⎥ ⎦
1
RT8012A
IN
= 2V
www.richtek.com
OUT
, where
11