XRP7662 Datasheet, PDF (12/19 Pages) Exar Corporation – 12A 300KHz Synchronous Step Down Regulator

XRP7 66 2
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1 2 A 3 0 0 KHz Sy n ch r on ou s St ep Dow n Reg u lat or
sinking. Maximum output current could be
limited by thermal limitations of a particular
application by taking advantage of the
integrated - over - temperature
protective
scheme employed in the
XRP7662 . The
XRP7662 incor porates a built -in over
temperature protection to prevent internal
overheating.
For additional information on thermal
performance, read ANP -25 ³3RZHU^T%^M OR[
7KHUPDO$Q. DO\VLV´
The second feature is a 100% duty cycle
timeout that ensures synchronized refreshing
of the BST capacitor at very high duty ratios.
In the event that the high side NFET is on for
20 continuous clock cycles, a r eset is given to
the PWM flip flop half way through the 21
st
cycle. This forces GL to rise for the cycle, in
turn refreshing the BST capacitor. The boost
capacitor is used to generate a high voltage
drive supply for the high side switch, which is
V_CC above V_IN .
I NTEGRATED POWER MOSFET S
The XRP7662 contains a pair of integrated low
resistance N -channel switches designed to
drive up to 12A of output current. Care should
be taken to de -rate the output current based
on the thermal conditions in the system such
as ambient temperature, airflow and heat
SETTING OUTPUT V OLTAGES
The XRP7662 can be set to different output
voltages. The relationship in the following
formula is based on a voltage divider from the
output to the feedback pin VFB, which is set to
an internal reference voltage of 0.80V.
Standard 1% metal film resistors of surface
mount size 0603 are recommended.
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So
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Where R ₁  NÆ» DQG_O_UT I=R0U.80V9
setting, simply remove R ₂ from the board.
Furthermore, one could select the value of the
R₁ and R ₂ combination to meet the exact
output voltage setting by
restricting the R ₁
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for overall system loop stability.
APPLICATIONS INFORMA TION
I NDUCTOR SELECTION
There are many factors to consider in selecting
the inductor including core material,
inductance vs. frequency, current handling
capability, efficiency, size and EMI. In a typical
XRP7662 circuit, the inductor is chosen
primarily for value, saturation current and DC
resistance. Increasing the inductor value will
decrease output voltage ripple, but degrade
transient response. Low inductor values
provide the smallest size, but cause large
ripple currents, poor efficiency and require
more output capacitance to smooth out the
larger ripple current. The inductor must be
able to handle the peak current at the
switching fr equency without saturating, and
the copper resistance in the winding should be
kept as low as possible to minimize resistive
power loss. A good compromise between size,
loss and cost is to set the inductor ripple
current to be within 20% to 40% of the
maxi mum output current.
© 2012 Exar Corporation
12 / 19
Rev. 2. 2.0