Datasheet

R
FB1
=
- 1
V
OUT
0.8
x R
FB2
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
)
LM20333
SNVS558D –MAY 2008–REVISED APRIL 2013 www.ti.com
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.
Table 1. Suggested Values for R
FB1
and R
FB2
R
FB1
(kΩ) R
FB2
(kΩ) V
OUT
short open 0.8
4.99 10 1.2
8.87 10.2 1.5
12.7 10.2 1.8
21.5 10.2 2.5
31.6 10.2 3.3
52.3 10 5.0
If different output voltages are required, R
FB2
should be selected to be between 4.99 kΩ to 49.9 kΩ and R
FB1
can
be calculated using the equation below.
(6)
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