Datasheet

R
T
=
78000
f
SW
- 55
R
FB1
=
- 1
V
OUT
0.8
x R
FB2
LM20343
SNVS559B –MAY 2008–REVISED APRIL 2013 www.ti.com
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)
ADJUSTING THE OPERATING FREQUENCY (R
RT
)
The operating frequency of the LM20343 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
RT
for a given operating frequency.
where
• f
SW
is the switching frequency in kHz
• R
RT
is the frequency adjust resistor in kΩ (7)
Please refer to the curve Oscillator Frequency versus R
RT
in the Typical Performance Characteristics section. If
the R
RT
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
150 µF, 6.3V POSCAP output capacitor (6TPB150MAZB).
Table 2. Recommended Compensation for
C
OUT
= 150 µF, I
OUT
= 3A, f
SW
= 500kHz
V
IN
V
OUT
L (µH) R
C
(kΩ) C
C1
(nF)
12 5 6.8 43.2 4.7
12 3.3 5.6 43.2 3.3
12 2.5 4.7 48.7 2.2
12 1.5 3.3 30.1 2.2
12 1.2 2.2 23.2 2.2
12 0.8 1.5 34 1
5 3.3 2.2 43.2 3.3
5 2.5 3.3 30.1 3.3
5 1.5 2.2 30.1 2.2
5 1.2 2 34 3.3
5 0.8 1.5 30.1 2.2
14 Submit Documentation Feedback Copyright © 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM20343