BD9134MUV_0909 Datasheet, PDF (10/18 Pages) Rohm – Output 2A or More High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9134MUV
Technical Note
●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.
4.0
①3.56W
3.0
2.0
②1.21W
1.0 _③0.70W
④0.34W
0
0 25 50 75 100105 125 150
Ambient temperature:Ta [℃]
① 4 layers (Copper foil area : 5505mm²)
copper foil in each layers.
θj-a=35.1℃/W
② 4 layers (Copper foil area : 10.29m²)
copper foil in each layers.
θj-a=103.3℃/W
③ 4 layers (Copper foil area : 10.29m²)
θj-a=178.6℃/W
④IC only.
Fig.26 Thermal derating curve
(VQFN020V4040)
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
If VCC=5V, VOUT=3.3V, RONH=82mΩ, RONL=70mΩ
IOUT=3A, for example,
D=VOUT/VCC=3.3/5.0=0.66
RON=0.66×0.082+(1-0.66)×0.07
=0.05412+0.0238
=0.07792[Ω]
P=3²Ã—0.07792＝0.70128[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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10/17
2009.09 - Rev.B