Datasheet

R
T
=
78000
f
SW
- 55
R
FB1
=
- 1
V
OUT
0.8
x R
FB2
LM20146
SNVS563C –FEBRUARY 2008–REVISED APRIL 2013 www.ti.com
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, shown below, 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
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.
(7)
Adjusting the Operating Frequency (R
T
)
The operating frequency of the LM20146 can be adjusted by connecting a resistor from the RT pin to ground.
The equation shown below can be used to calculate the value of R
T
for a given operating frequency.
where
• f
SW
is the switching frequency in kHz
• R
T
is the frequency adjust resistor in kΩ (8)
Please refer to the curve Oscillator Frequency verses R
T
in the Typical Performance Characteristics section If the
R
T
resistor is omitted the device will not operate.
Loop Compensation (R
C1
, C
C1
)
The purpose of loop compensation is to meet static and dynamic performance requirements while maintaining
adequate stability. Optimal loop compensation depends on the output capacitor, inductor, load, and the device
itself. Table 2 below gives values for the compensation network that will result in a stable system when using a
100 µF, 6.3V ceramic X5R output capacitor and 1 µH inductor.
Table 2. Recommended Compensation for
C
OUT
= 100 µF, L = 1.5 µH & f
SW
= 500 kHz
V
IN
V
OUT
C
C1
(nF) R
C1
(kΩ)
5.00 3.30 2.2 15.4
5.00 2.50 2.2 13.3
5.00 1.80 2.2 10.7
5.00 1.50 2.2 9.31
5.00 1.20 2.2 7.87
5.00 0.80 2.7 4.42
3.30 2.50 2.7 8.45
3.30 1.80 2.7 7.5
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