SP6669_12 Datasheet, PDF (9/11 Pages) Exar Corporation – 600mA 1.5MHz Synchronous Step Down Converter

SP6 6 6 9
6 0 0 m A 1 .5 M H z Sy n ch r o n o u s St e p Do w n Co n v e r t e r
4- The SW and VFB nodes should be kept as
separate as possible to minize 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.
OUPTUT V OLTAGE RIPPLE FOR V_IN CLOSE TO
V _OUT
When the input voltage V _IN is close to the
output voltage V _OUT, the SP6669 transitions
smoothly from the switching PWM converter
mode into a LDO mode. The following diagram
shows the output voltage ripple versus the
input voltage for a 3.3 V output setting and a
200mA current load.
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
L
f
1
˜'I _L
V_{OUT ¨¨©§}1^VVOIUNT
¸¸¹·
Substituting V _OUT=1.8V, V _IN 9_L=18Æ©0m,A
to 240mA (30% to 40%) and f=1.3MHz gives
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A 3.3 µH 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 I
LOAD(MAX) ^/2
and low ESR. In most cases, a ceramic
capacitor will sati sfy this requirement.
Fig. 24 : VOUT Ripple Voltage
for VIN decreasing close to V OUT
© 2012 Exar Corporation
9/ 11
Rev. 2. 2.0