XRP6657
1.5A 1.3MHz Synchronous Step Down Converter
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.
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
COUT capacitor.
3- The input capacitor CIN 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.
Fig. 24: Output Voltage Ripple in LDO mode
5- The ground plates of CIN and COUT should be
kept as close as possible.
D
ESIGN EXAMPLE
6- Connect all analog grounds to a common
node and connect the common node to the
power ground via an independent path.
In a single Lithium-Ion battery powered
application, the VIN 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
S
ELF ENABLE APPLICATION
A self Enable function is easily implemented
through the following arrangement.
1
ꢆ
ꢥ ꢂ
ꢆꢇꢈꢉ ꢧ1 ꢋ ꢇꢈꢉꢪ
ꢦ ꢅ ∆ꢀꢁ
ꢆ
ꢨꢩ
Substituting VOUT=1.8V, VIN=4.2V, ΔIL=600mA
and f=1.3MHz gives
ꢥ ꢂ 1.32ꢫꢬ
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 CIN capacitor requires an
RMS current rating of at least ILOAD(MAX)/2 and
low ESR. In most cases, a ceramic capacitor
A resistor ratio R3/R4=1/1.5 is recommended.
O
UTPUT VOLTAGE RIPPLE IN LDO MODE
will
satisfy
this
requirement.
See
The XRP6657 enters the LDO mode when
input voltage is close to the selected output
recommended components section below
R
ECOMMENDED
Supplier
C
OMPONENTS
Dimensions
Inductance
ISAT
DCRMAX
Part #
(mm)
SD52-1R5M
1.5µH
2.5A
47mΩ
4.5x5x2
Inter-Technical
Supplier
Capacitance
Package
Part #
0805
0805
4.7µF
22µF
GRM219R61A475K
GRM219R60J226M
Murata
Murata
© 2009 Exar Corporation
9/11
Rev. 1.0.0
EXAR [ EXAR CORPORATION ]
