BD9123MUV_11 Datasheet, PDF (11/18 Pages) Rohm – Output 1.5A or Less High-efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9123MUV
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
3.0
①2.66W
2.0
②1.77W
① 4 layers (Copper foil area : 5505mm²)
copper foil in each layers.
θj-a=47.0℃/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℃/W
③ 1 layer (Copper foil area : 6.28m²)
θj-a=201.6℃/W
④ IC only.
θj-a=462.9℃/W
P=IOUT²Ã—RON
RON=D×RONP+(1-D)RONN
D：ON duty (=VOUT/VCC)
RONP：ON resistance of Highside MOS FET
RONN：ON resistance of Lowside MOS FET
IOUT：Output curren
1.0
③0.62W
④0.27W
0
0
25
50
75 100 105 125 150
Ambient temperature:Ta [℃]
Fig.33 Thermal derating curve
(VQFN016V3030)
If VCC=3.3V, VOUT=1.2V, RONP=0.35mΩ, RONN=0.25mΩ
IOUT=1.2A, for example,
D=VOUT/VCC=1.2/5=0.24
RON=0.24×0.35+(1-0.24)×0.25
=0.084+0.19
=0.274[Ω]
P=1.2²Ã—0.247=0.394[W]
As RONP is greater than RONN 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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11/17
2011.01 - Rev.A