Datasheet
LMR61428
SNVS815A –JUNE 2012–REVISED APRIL 2013
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BOOTSTRAPPING
When the output voltage (V
OUT
) is between 2.5V and 5.0V a bootstrapped operation is suggested. This is
achieved by connecting the V
DD
pin (Pin 6) to V
OUT
. However if the V
OUT
is outside this range, the V
DD
pin should
be connected to a voltage source whose range is between 2.5V and 5V. This can be the input voltage (V
IN
), V
OUT
stepped down using a linear regulator, or a different voltage source available in the system. This is referred to as
non-bootstrapped operation. The maximum acceptable voltage at the BOOT pin (Pin 7) is 10V.
SETTING THE SWITCHING FREQUENCY
The switching frequency of the oscillator is selected by choosing an external resistor (R
FQ
) connected between
FREQ and V
DD
pins. See the following graph, Figure 15, for choosing the R
FQ
value to achieve the desired
switching frequency. A high switching frequency allows the use of very small surface mount inductors and
capacitors and results in a very small solution size. A switching frequency between 300kHz and 2MHz is
recommended.
Figure 15. Switching Frequency vs R
FQ
INDUCTOR SELECTION
The LMR61428's high switching frequency enables the use of a small surface mount inductor. A 6.8µH shielded
inductor is suggested for a typical application. The inductor should have a saturation current rating higher than
the peak current it will experience during circuit operation (see the following graph, Peak Inductor vs Load
Current). Less than 100mΩ ESR is suggested for high efficiency.
Figure 16. Peak Inductor Current vs Load Current
Open-core inductors cause flux linkage with circuit components and interfere with the normal operation of the
circuit. They should be avoided. For high efficiency, choose an inductor with a high frequency core material, such
as ferrite, to reduce the core losses. To minimize radiated noise, use a toroid, pot core or shielded core inductor.
The inductor should be connected to the SW pin as close to the IC as possible. See Connection Diagram.
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