Datasheet

1.3 x 10
-10
V
IN(MAX)
x 150 ns
R
ON
t
0 25 50 75 100 125
0.0
0.4
0.8
1.2
1.6
2.0
2.4
MAXIMUM I
OUT
(A)
AMBIENT TEMPERATURE (°C)
LMR24220
SNVS737E –OCTOBER 2011–REVISED APRIL 2013 www.ti.com
Thermal Derating
Temperature rise increases with frequency, load current, input voltage and smaller board dimensions. On a
typical board, the LMR24220 is capable of supplying 2A below an ambient temperature of 50°C under worst case
operation with input voltage of 42V. Figure 22 shows a thermal derating curve for the output current without
thermal shutdown against ambient temperature up to 125°C. Obtaining 2A output current is possible at higher
temperature by increasing the PCB ground plane area, adding air flow or reducing the input voltage or operating
frequency
θ
JA
=40°C/W, Vo = 3.3V, fs = 500kHz
(tested on the evaluation board)
Figure 22. Thermal Derating Curve
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
R
FB1
and R
FB2
: These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ,
satisfying the following ratio:
R
FB1
/R
FB2
= (V
OUT
/0.8V) - 1 (9)
For V
OUT
= 0.8V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than
20 µA. This is needed because the converter operation needs a minimum inductor current ripple to maintain
good regulation when no load is connected.
R
ON
: Equation 2 can be used to select R
ON
if a desired operating frequency is selected. But the minimum value
of R
ON
is determined by the minimum on-time. It can be calculated as follows:
(10)
If R
ON
calculated from Equation 2 is smaller than the minimum value determined in Equation 10, a lower
frequency should be selected to re-calculate R
ON
by Equation 2. Alternatively, V
IN(MAX)
can also be limited in
order to keep the frequency unchanged. The relationship of V
IN(MAX)
and R
ON
is shown in Figure 23.
On the other hand, the minimum off-time of 260 ns can limit the maximum duty ratio.
14 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated
Product Folder Links: LMR24220