FAN53540 Fairchild Semiconductor, FAN53540 Datasheet - Page 13

• Manufacturer Part Number: FAN53540
• Description: The FAN53540 is a step-down switching voltage regulator that delivers an adjustable output from an input voltage supply of 2
• Manufacturer: Fairchild Semiconductor

© 2011 Fairchild Semiconductor Corporation
FAN53540 • Rev. 1.0.2
To calculate maximum operating temperature (<125°C) for a
specific application:
1.
2.
3.
4.
5.
Device temperature (T
Use efficiency graphs to determine efficiency for the
desired V
Calculate IC power dissipation using:
where η is efficiency from Figure 4 through Figure 9.
Compute inductor copper losses using:
Combine IC (step 2) and inductor losses (step 3) to
determine total dissipation:
Determine device operating temperature:
P
P
P

IC
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
L
D
T




0
I
P
LOAD
P
V
IC
D
OUT


IN
R
2
, V
P



L
Figure 29. Power Derating
JA
I
LOAD
DCR
25
OUT
2.88W, max.
and
, and load condition
IC
Ambient Temperature (C)

L
) should not exceed 125°C.

 

T

1
IC
50


1
T

 

AMB

75

T
100
(10)
(11)
(12)
(13)
125
13
A different approach, shown here as an example, uses the
same equations to determine maximum inductor DCR for a
specific application:
Suppose a design requires a 5.0V
Due to the +0.4%/°C temperature coefficient of copper,
inductor DCR must be further reduced to accommodate the
~50°C temperature rise.
To meet the design requirements, an inductor with a room
temperature DCR of <13.6mΩ is necessary.
Figure 30 shows the maximum ambient temperature where
FAN53540UC can be used for a continuous load, at 5.0V
Note:
6.
A.
B.
C. From Eq. 13, maximum P
D. From Eq. 12, P
E.
The graph was empirically determined using an ultra-low
DCR (2.6m) inductor. For physically smaller devices
with higher DCR, further derating may be necessary.
6
5
4
3
2
1
0
25
From Figure 4, η is ~82%.
From Eq. 10, P
From Eq. 11, DCR<16.4m
Figure 30. Load Current Derating
50
Ambient Temperature (C)
IC
L
=262mW.
=1,054mW.
75
IN
, 1.2V
D
=1,316mW for 50
OUT
www.fairchildsemi.com
, 4A
100
RMS
(6)
1.2 VOUT
1.8 VOUT
3.3 VOUT
, at 75
o
C rise.
125
o
IN
C:
: