RT6246B
the gate driver supply voltage for the high sideN-channel
MOSFET switch.
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,
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.
The internal power MOSFET switch gate driver is
optimized to turn the switch on fast enough for low power
loss and good efficiency, but also slow enough to reduce
EMI. Switch turn-on is when most EMI occurs since VLX
rises rapidly. In some cases, it is desirable to reduce EMI
further, by the expense of some additional power
dissipation. The switch turn-on can be slowed by placing
a small (10Ω ≤ RBOOT ≤ 30Ω) resistance between BOOT
and the external bootstrap capacitor. This will slow the
high-side switch turn-on and VLX's rise. In order to improve
EMI performance and enhancement of the internal
MOSFET switch.
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
Once an inductor value is chosen, the ripple current (ΔIL)
is calculated to determine the required peak inductor
current.
R1
R2
VOUT,VALLEY = 1+
0.6V
V
OUT
VOUT (VIN VOUT
)
IL
and
R1
VIN fSW L
FB
IL
2
IL(PEAK) IOUT(MAX)
RT6246B
R2
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
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12
DS6246B-01 March 2018
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