RT8015
Application Information
The basic RT8015 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
The transition from low current operation begins when the
peak inductor current falls below the minimum peak
current. Lower inductor values result in higher ripple current
which causes this to occur at lower load currents. This
causes a dip in efficiency in the upper range of low current
operation.
followed by CIN and COUT
.
Operating Frequency
4.5
4
Selection of the operating frequency is a tradeoff between
efficiency and component size. High frequency operation
allows the use of smaller inductor and capacitor values.
Operation at lower frequency improves efficiency by
reducing internal gate charge and switching losses but
requires larger inductance and/or capacitance to maintain
low output ripple voltage.
3.5
3
RT = 154k for 2MHz
2.5
2
1.5
1
RT = 332k for 1MHz
The operating frequency of the RT8015 is determined by
an external resistor that is connected between the RT pin
and ground. The value of the resistor sets the ramp current
that is used to charge and discharge an internal timing
capacitor within the oscillator. The RT resistor value can
be determined by examining the frequency vs. RT curve.
Although frequencies as high as 4MHz are possible, the
minimum on-time of the RT8015 imposes a minimum limit
on the operating duty cycle. The minimum on-time is
typically 110ns. Therefore, the minimum duty cycle is
equal to 100 x 110ns x f(Hz).
0.5
0
0
100 200 300 400 500 600 700 800 900 100
1000
0
RRRRTT ((kkW))
Figure 1
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.
Inductor Selection
For a given input and output voltage, the inductor value
and operating frequency determine the ripple current. The
ripple current DIL increases with higher VIN and decreases
with higher inductance.
V
f ´ L
V
OUT
V
IN
é
ùé
ù
OUT
DI =
L
1-
úê
ê
ú
ë
ûë
û
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
current is exceeded.
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 DI = 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 :
This result in an abrupt increase in inductor ripple current
and consequent output voltage ripple.
Do not allow the core to saturate!
é
ùé
ù
ú
V
OUT
V
OUT
L =
1-
ê
úê
f ´ DI
V
IN(MAX)
L(MAX)
ë
ûë
û
www.richtek.com
10
DS8015-03 March 2011
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
