RT6248B
EMI performance and enhancement of the internal
MOSFET switch.
and transient response is to use a ripple current (ΔIL) about
20-50% of the desired full output load current. Calculate
the approximate inductor value by selecting the input and
output voltages, the switching frequency (fSW), the
maximum output current (IOUT(MAX)) and estimating a ΔIL
as some percentage of that current.
Output Voltage Setting
Set the desired output voltage using a resistive divider
from the output to ground with the midpoint connected to
FB. The output voltage is set according to the following
equation :
V
(V V
)
OUT
IN
OUT
L
V f
IN SW
I
L
R1
R2
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
VOUT,VALLEY = 1+
0.6V
V
OUT
VOUT (VIN VOUT
)
R1
IL
and
VIN fSW L
FB
IL
2
RT6248B
R2
IL(PEAK) IOUT(MAX)
GND
To guarantee the required output current, the inductor
needs a saturation current rating and a thermal rating that
exceeds IL(PEAK). These are minimum requirements. To
maintain control of inductor current in overload and short-
circuit conditions, some applications may desire current
ratings up to the current limit value. However, the IC's
output under-voltage shutdown feature make this
unnecessary for most applications.
Figure 1. Output Voltage Setting
Place the FB resistors within 5mm of the FB pin. Choose
R2 between 10kΩ and 100kΩ to minimize power
consumption without excessive noise pick-up and
calculate R1 as follows :
R2 x (VOUT 0.6V)
R1 =
0.6V
For best efficiency, choose an inductor with a low DC
resistance that meets the cost and size requirements.
For low inductor core losses some type of ferrite core is
usually best and a shielded core type, although possibly
larger or more expensive, will probably give fewer EMI
and other noise problems.
For output voltage accuracy, use divider resistors with 1%
or better tolerance.
Inductor Selection
Selecting an inductor involves specifying its inductance
and also its required peak current. The exact inductor value
is generally flexible and is ultimately chosen to obtain the
best mix of cost, physical size, and circuit efficiency.
Lower inductor values benefit from reduced size and cost
and they can improve the circuit's transient response, but
they increase the inductor ripple current and output voltage
ripple and reduce the efficiency due to the resulting higher
peak currents. Conversely, higher inductor values increase
efficiency, but the inductor will either be physically larger
or have higher resistance since more turns of wire are
required and transient response will be slower since more
time is required to change current (up or down) in the
inductor. A good compromise between size, efficiency,
Input Capacitor Selection
High quality ceramic input decoupling capacitor, such as
X5R or X7R, with values greater than 20μF are
recommended for the input capacitor. The X5R and X7R
ceramic capacitors are usually selected for power regulator
capacitors because the dielectric material has less
capacitance variation and more temperature stability.
Voltage rating and current rating are the key parameters
when selecting an input capacitor. Generally, selecting an
input capacitor with voltage rating 1.5 times greater than
the maximum input voltage is a conservatively safe design.
The input capacitor is used to supply the input RMS
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12
DS6248B-01 May 2018
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