BD9137MUV Datasheet, PDF (8/17 Pages) Rohm – High Efficiency Step-down Switching Regulator

●Consideration on permissible dissipation and heat generation
As this IC functions with high efficiency without significant heat generation in most applications, no special consideration is needed
on permissible dissipation or heat generation. In case of extreme conditions, however, including lower input voltage, higher
output voltage, heavier load, and/or higher temperature, the permissible dissipation and/or heat generation must be carefully
considered.
For dissipation, only conduction losses due to DC resistance of inductor and ON resistance of FET are considered. Because the
conduction losses are considered to play the leading role among other dissipation mentioned above including gate
charge/discharge dissipation and switching dissipation.
4.5
4.0
①3.56W
3.0
②2.21W
2.0
① 4 layers (copper foil area : 5505mm²)
(copper foil in each layers)
θj-a=35.1℃/W
② 4 layers (1st,4thcopper foil area : 10.29mm²)
(2nd ,3rd copper foil area : 5505mm²)
θj-a=56.6℃/W
③ 1 layer (copper foil area :10.29mm²)
θj-a=178.6℃/W
④IC only
θj-a=367.6℃/W
P=IOUT²Ã—RON
RON=D×RONP+(1-D)RONN
D：ON duty (=VOUT/VCC)
RONH：ON resistance of Highside MOS FET
RONL：ON resistance of Lowside MOS FET
IOUT：Output current
1.0 ③0.70W
④0.34W
0
0
25
50
75
100105 125
150
Ambient temperature:Ta [℃]
Fig.26 Thermal derating curve
(VQFN020V4040)
If VCC=3.3V, VOUT=1.8V, RONH=82mΩ, RONL=70mΩ
IOUT=3A, for example,
D=VOUT/VCC=1.8/3.3=0.545
RON=0.545×0.082+(1-0.545)×0.07
=0.0447+0.0319
=0.0766[Ω]
P=3²Ã—0.0766＝0.6894[W]
As RONH is greater than RONL in this IC, the dissipation increases as the ON duty becomes greater. With the consideration on
the dissipation as above, thermal design must be carried out with sufficient margin allowed.
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