BD9123MUV_14 Datasheet, PDF (15/28 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9123MUV
Datasheet
6. 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 allowable 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. The
reason is that 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
3.0
①2.66W
2.0
②1.77W
1.0
③0.62W
④0.27W
0
0
25
50
75 100 105 125 150
Ambient temperature:Ta [°C]
Figure 33. Thermal Derating Curve
(VQFN016V3030)
① 4 layers (Copper foil area : 5505mm²)
copper foil in each layers.
θj-a=47.0°C/W
② 4 layers (1^st and 4^th copper foil area : 6.28m²)
(2^nd and 3^rd copper foil area: 5505m²)
(copper foil in each layers)
θj-a=70.62°C/W
③ 1 layer (Copper foil area : 6.28m²)
θj-a=201.6°C/W
④ IC only.
θj-a=462.9°C/W
  ^{P  IOUT 2  RON}
R_ON  D  R_ONH  1  D R_ONL
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=1.2V, RONP=0.35mΩ, RONN=0.25mΩ
IOUT=1.2A, for example,
D=VOUT/VCC=1.2/5=0.24
RON=0.24x0.35+(1-0.24)x0.25
=0.084+0.19
=0.274[Ω]
P=1.2²x0.274=0.394[W]
As RONP is greater than RONN in this IC, the dissipation increases as the ON time becomes greater. With the consideration on
the dissipation as mentioned above, thermal design must be carried out with sufficient margin allowed.
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