XRP6657_12 Datasheet, PDF (9/11 Pages) Exar Corporation – 1.5A 1.3MHz Synchronous Step Down Converter

XRP6 6 5 7
1 .5 A 1 .3 M H z Sy n ch r o n o u s St e p D o w n Co n v e r t e r
PCB LAYOUT
The following PCB layout guidelines should be
taken into account to ensure proper operation
and performance of the XRP6657:
1- The GND, SW and VIN traces should be
kept short, direct and wide.
2- VFB pin must be connected directly to the
feedback resistors. The resistor divider
network must be connected in parallel to the
C_OUT capacitor.
3- The input capacitor C _IN must be kept as
close as possible to the VIN pin.
4- The SW and VFB nodes should be kept as
separate as possible to minimize possible
effects from the high frequency and voltage
swings of the SW node.
5- The ground plates of C _IN and C _OUT should be
kept as close as possible.
6- Connect all analog grounds to a common
node and connect the common node to the
power ground via an independent path.
SELF ENABLE A PPLICATION
A self Enable function is easily implemented
through the following arrange ment.
A resistor ratio R ₃/R ₄=1/1.5 is recommended.
OUTPUT V OLTAGE RIPPLE IN LDO M ODE
The XRP6657 enters the LDO mode when
input voltage is close to the selected output
voltage. The transition from PWM mode to
LDO mode is smooth. Figure 24 illustrates the
amount of output voltage ripple for an output
voltage of 3.3V providing 200mA.
Fig. 24 : Output Voltage Ripple in LDO mode
D ESIGN EXAMPLE
In a single Lithium - Ion battery powered
application, the V _IN range is about 2.7V to
4.2V. The desired output voltage is 1.8V.
The inductor value needed can be calculated
using the following equation
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Substituting V _OUT=1.8V, V _IN 9_L=45Æ©0m,A
to 600mA (30% to 40%) and f=1.3MHz gives
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A 1.5µH inductor can be chosen with this
application. An inductor of greater value with
less equivalent series resistance would provide
better efficiency. The C _IN capacitor requires an
RMS current rating of at least
ILOAD(MAX) ^{/2 and}
low ESR. In most cases, a ceramic capacitor
will satisfy this requirement.
See
recommended components section below
© 2012 Exar Corporation
9/ 11
Rev. 1.4.0