Datasheet

I
IN-RMS
= I
OUT
D(1 - D)
V
DROOP
= 'I
OUTSTEP
x R
ESR
+
L x 'I
OUTSTEP
2
C
OUT
x (V
IN
- V
OUT
)
'V
OUT
= 'i
L
x
1
8 x f
SW
x C
OUT
R
ESR
+
LM20343
www.ti.com SNVS559B –MAY 2008–REVISED APRIL 2013
where
• ΔV
OUT
(V) is the amount of peak to peak voltage ripple at the power supply output
• R
ESR
(Ω) is the series resistance of the output capacitor
• f
SW
(Hz) is the switching frequency
• C
OUT
(F) is the output capacitance used in the design (3)
The amount of output ripple that can be tolerated is application specific; however a general recommendation is to
keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic capacitors are sometimes
preferred because they have very low ESR; however, depending on package and voltage rating of the capacitor
the value of the capacitance can drop significantly with applied voltage. The output capacitor selection will also
affect the output voltage droop during a load transient. The peak droop on the output voltage during a load
transient is dependent on many factors; however, an approximation of the transient droop ignoring loop
bandwidth can be obtained using the following equation:
where
• C
OUT
(F) is the minimum required output capacitance
• L (H) is the value of the inductor
• V
DROOP
(V) is the output voltage drop ignoring loop bandwidth considerations
• ΔI
OUTSTEP
(A) is the load step change
• R
ESR
(Ω) is the output capacitor ESR
• V
IN
(V) is the input voltage
• V
OUT
(V) is the set regulator output voltage (4)
Both the tolerance and voltage coefficient of the capacitor should be examined when designing for a specific
output ripple or transient droop target.
INPUT CAPACITOR SELECTION
Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the
switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they
provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over temperature performance and also minimize the DC
voltage derating that occurs on Y5V capacitors. The input capacitors C
IN1
and C
IN2
should be placed as close as
possible to the VIN and GND pins on both sides of the device.
Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good
approximation for the required ripple current rating is given by the relationship:
(5)
As indicated by the RMS ripple current equation, highest requirement for RMS current rating occurs at 50% duty
cycle. For this case, the RMS ripple current rating of the input capacitor should be greater than half the output
current. For best performance, low ESR ceramic capacitors should be placed in parallel with higher capacitance
capacitors to provide the best input filtering for the device.
SETTING THE OUTPUT VOLTAGE (R
FB1
, R
FB2
)
The resistors R
FB1
and R
FB2
are selected to set the output voltage for the device. Table 1 provides suggestions
for R
FB1
and R
FB2
for common output voltages.
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