Datasheet

LM26400Y
www.ti.com SNVS457C –FEBRUARY 2007–REVISED APRIL 2013
Example:
V
IN
= 5V, V
OUT1
= 3.3V, I
OUT1
= 2A, V
OUT2
= 1.2V, I
OUT2
= 1.5A, R
DS
= 170mΩ, R
DC
= 30mΩ. (I
OUT1
is the same as
I
1
in the input ripple RMS current equation, I
OUT2
is the same as I
2
).
First, find out the duty cycles. Plug the numbers into the duty cycle equation and we get D1 = 0.75, and D2 =
0.33. Next, follow the decision tree in Figure 37 to find out the values of d1, d2 and d3. In this case, d1 = 0.5, d2
= D2 + 0.5 - D1 = 0.08, and d3 = D1 - 0.5 = 0.25. I
av
= I
OUT1
·D1 + I
OUT2
·D2 = 1.995A. Plug all the numbers into
the input ripple RMS current equation and the result is I
irrm
= 0.77A.
Figure 37. Determining d1, d2 and d3
CATCH DIODE SELECTION
The catch diode should be at least 2A rated. The most stressful operation for the diode is usually when the
output is shorted under high line. Always pick a Schottky diode for its lower forward drop and higher efficiency.
The reverse voltage rating of the diode should be at least 25% higher than the highest input voltage. The diode
junction temperature is a main concern here. Always validate the diode's junction temperature in the intended
thermal environment to make sure its thermally derated maximum current is not exceeded. There are a few 2A,
30V surface mount Schottky diodes available in the market. Notice that diodes have a negative temperature
coefficient, so do not put two diodes in parallel to achieve a lower temperature rise. Current will be hogged by
one of the diodes instead of shared by the two. Use a larger package for that purpose.
THERMAL CONSIDERATIONS
Due to the low thermal impedance from junction to the die-attach pad (or DAP, exposed metal at the bottom of
the package), thermal performance heavily depends on PCB copper arrangement. The minimum requirement is
to have a top-layer thermal pad that is exactly the same size as the DAP. There should be at least nine 8-mil
thermal vias in the pad. The thermal vias should be connected to internal ground plane(s) (if available) and to a
ground plane on the bottom layer that is as large as allowed.
In boards that have internal ground planes, extending the top-layer thermal pad outside the body of the package
to form a "dogbone" shape offers little performance improvement. However, for two-layer boards, the dogbone
shape on the top layer will provide significant help.
Predicting on paper with reasonable accuracy the junction temperature of the LM26400Y in a real-world
application is still an art. Major factors that contribute to the junction temperature but not directly associated with
the thermal performance of the LM26400Y itself include air speed, air temperature, nearby heating elements and
arrangement of PCB copper connected to the DAP of the LM26400Y. The θ
JA
value published in the datasheet is
based on a standard board design in a single heating element mode and measured in a standard environment.
The real application is usually completely different from those conditions. So the actual θ
JA
will be significantly
different from the datasheet number. The best approach is still to assign as much copper area as allowed to the
DAP and prototype the design.
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