FAN53541 Datasheet, PDF (13/15 Pages) Fairchild Semiconductor – 2.4 MHz, 5 A TinyBuck Synchronous Buck Regulator

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
2.88W, max.
25
50
75
100
125
Ambient Temperature (C)
Figure 29. Power Derating
To calculate maximum operating temperature (<125°C) for a
specific application:
1. Use efficiency graphs to determine efficiency for the
desired V_IN, V_OUT, and load condition
2. Calculate IC power dissipation using:
P_IC

V_OUT
 I_LOAD


1

 1_{}
(10)
where η is efficiency from Figure 4 through Figure 9.
3. Compute inductor copper losses using:
P_L  I_LOAD2  DCR_L
(11)
4. Combine IC (step 2) and inductor losses (step 3) to
determine total dissipation:
P_D  P_IC  P_L
(12)
5. Determine device operating temperature:
T  P_D  R_JA and T_IC  T_AMB  T
(13)
Device temperature (T_IC) should not exceed 125°C.
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 5.0 V_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.
E. From Eq. (11), DCR<16.4 m
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 <13.6 mΩ is necessary.
Figure 30 shows the maximum ambient temperature where
FAN53541 can be used for a continuous load, at 5.0 V_IN:
6
1.2 VOUT
1.8 VOUT
5
3.3 VOUT
4
3
2
1
0
25
50
75
100
125
Ambient Temperature (C)
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.
© 2013 Fairchild Semiconductor Corporation
FAN53541 • Rev. 1.0.2
13
www.fairchildsemi.com