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3.5
A different approach, shown here as an example, uses the
same equations to determine maximum inductor DCR for a
3.0
2.5
2.0
1.5
2.88W, max.
specific application:
If a design requires a 5.0V IN , 1.2 V OUT , 4 A RMS , at 75°C:
A. From Figure 4, η is ~82%.
B. From Eq. 10, P IC =1,054 mW.
C. From Eq. 13, maximum P D =1,316 mW for 50°C rise.
D. From Eq. 12, P L =262 mW.
1.0
E.
From Eq. 11, DCR<16.4 m ??
0.5
Due to the +0.4%/°C temperature coefficient of copper,
inductor DCR must be further reduced to accommodate the
0.0
0
25
50
75
100
125
~50°C temperature rise.
Ambient Temperature (C)
Figure 29. Power Derating
To meet the design requirements, an inductor with a room
temperature DCR of <13.6 m ? is necessary.
Figure 30 shows the maximum ambient temperature where
FAN53540UC can be used for a continuous load, at 5.0 V IN :
To calculate maximum operating temperature (<125°C) for a
specific application:
6
1.2 VOUT
1.
2.
Use efficiency graphs to determine efficiency for the
desired V IN , V OUT , and load condition
Calculate IC power dissipation using:
5
4
1.8 VOUT
3.3 VOUT
P IC ? V OUT ? I LOAD ? ? ?
? 1 ? ?
? 1
? ?
?
?
(10)
3
2
where η is efficiency from Figure 4 through Figure 9.
3.
Compute inductor copper losses using:
1
P L ? I LOAD ? DCR L
2
(11)
0
25
50
75
100
125
4.
Combine IC (step 2) and inductor losses (step 3) to
Ambient Temperature (C)
5.
determine total dissipation:
P D ? P IC ? P L
Determine device operating temperature:
? T ? P D ? R ? JA and T IC ? T AMB ? ? T
(12)
(13)
Figure 30. Load Current Derating (6)
Note:
6. The graph was empirically determined using an ultra-low
DCR (2.6 m ? ) inductor. For physically smaller devices
with higher DCR, further derating may be necessary.
Device temperature (T IC ) should not exceed 125°C.
? 2011 Fairchild Semiconductor Corporation
FAN53540 ? Rev. 1.0.3
13
www.fairchildsemi.com
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