BD9134MUV_14 Datasheet, PDF (12/25 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9134MUV
5. Consideration on Permissible Dissipation and Heat Generation
Since 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. This is
because conduction losses are the most significant among other dissipation mentioned above such as 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 [°C]
① 4 layers (Copper foil area : 5505mm²)
copper foil in each layers.
θj-a=35.1°C/W
② 4 layers (Copper foil area : 10.29m²)
copper foil in each layers.
θj-a=103.3°C/W
③ 1 layers (Copper foil area : 10.29m²)
θj-a=178.6°C/W
④IC only.
θj-a=367.6°C/W
Figure 27. Thermal Derating Curve
(VQFN020V4040)
P  I_OUT ²  R_ON
R_ON  D  R_{O N H} 1  D R_{O N L}
Where:
D is the ON duty (=VOUT/VCC).
RONH is the ON-Resistance of Highside MOSFET.
RONL is the ON-Resistance of Lowside MOSFET.
IOUT is the Output current.
If VCC=5V, VOUT=3.3V, RONH=82mΩ, RONL=70mΩ
IOUT=3A, for example,
D V_OUT / V_CC  3.3 / 5.0  0.66
R_ON  0.66 0.082 1 0.66  0.07
 0.05412 0.0238
 0.07792  
P  3²  0.07792 0.70128 W 
Since RONH is greater than RONL in this IC, the dissipation increases as the ON duty increases. Taking into consideration the
dissipation stated above, thermal design must be carried out with sufficient margin.
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TSZ22111・15・001
12/21
TSZ02201-0J3J0AJ00150-1-2
03.Oct.2014 Rev.002