X RP 6 6 5 7
1.5A 1.3MHz Synchronous Step Down Converter
PCB L
AYOUT
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.
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
E
XAMPLE
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
1
∆
1
S
ELF
E
NABLE
A
PPLICATION
A self Enable function is easily implemented
through the following arrangement.
Substituting V
OUT
=1.8V, V
IN
=4.2V,
ΔI
L
=600mA
and f=1.3MHz gives
1.32
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
A resistor ratio R
3
/R
4
=1/1.5 is recommended.
O
UTPUT
V
OLTAGE
R
IPPLE IN
LDO M
ODE
The XRP6657 enters the LDO mode when
input voltage is close to the selected output
R
ECOMMENDED
C
OMPONENTS
Supplier
Inter-Technical
Supplier
Murata
Murata
Inductance
1.5µH
Capacitance
4.7µF
22µF
I
SAT
2.5A
DCR
MAX
47mΩ
Package
0805
0805
Dimensions
(mm)
4.5x5x2
Part #
SD52-1R5M
Part #
GRM219R61A475K
GRM219R60J226M
© 2009 Exar Corporation
9/11
Rev. 1.0.0
EXAR [ EXAR CORPORATION ]
