SP6669 Datasheet, PDF (8/10 Pages) Exar Corporation – 1.5MHZ, 600mA SYNCHRONOUS STEP DOWN CONVERTER

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 Voltage 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 3.3V output setting and a 200mA
current load.
THEORY OF OPERATION
Design 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
⋅ ΔI
L
V_OUT
⎜⎜⎝⎛¹
−
V_OUT
V_IN
⎟⎟⎠⎞
Subsituting
V_OUT=1.8V,
V_IN=4.2V,
ΔI_L=240mA and f=1.5MHz gives
L = 2.86μH
A 2.2µH inductor can be choosen 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
satisfy this requirement.
Fig.20: V_OUT Ripple Voltage
for V_IN decreasing close to V_OUT
Rev G – 12/02/2008
SP6669 1.5MHz 0.6A Sync. Step Down Converter © 2008 Exar Corporation
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