BD9151MUV Datasheet, PDF (9/17 Pages) Rohm – Output 1.5A or Less High Efficiency Step-down Switching Regulator with Built-in Power MOSFET

BD9151MUV
Technical Note
●Consideration on permissible dissipation and heat generation
As BU9151MUV 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
②2.21W
2.0
①①4 l4ay層er基s (板co(表pp層er放fo熱il a銅re箔a :55550055mmmm²²))
(co(各pp層erにfo銅il i箔n e積a層ch)layers)
θθj-aj=-a3=53.15℃.1℃/W/W
②②4 l4ay層er基s (板co(表pp層er放fo熱il a銅re箔a :1100..2299mmmm²²))
(co(2p,p3e層r foにil銅in箔2-積3 層lay5e5rs0:55m5m05²m) m²)
θjθ-aj=-a1=0130.33℃.3℃/W/W
③③1 l1ay層er基(c板op(表pe層r f放oil熱ar銅ea箔: 1100.2.299mmmm²²))
θjθ-aj=-a1=7187.68℃.6℃/W/W
④④ICICo単nly体時
θjθ-aj=-a3=6376.67℃.6℃/W/W
P=IOUT²Ã—RON
RON=D×RONH+(1-D)RONL
D：ON Duty(=VOUT/VCC)
RONH：ON resistance of High side MOS FET
RONL：ON resistance of Low side MOS FET
IOUT：Output current
1.0 _③0.70W
④0.34W
0
0
25
50
75 100 105 125
150
Ambient Temperature :Ta [℃]
Fig.32 Heat radiation characteristics (VQFN020V4040)
If VCC=3.3V, VOUT1=1.8V, VOUT2=1.2V, RONH1=0.27Ω, RONL1=0.18Ω, RONH2=0.27Ω, RONL2=0.18Ω
IOUT1=0.4A, IOUT2=0.8A, for example,
D1=VOUT1/VCC=1.8/3.3=0.55
D2=VOUT2/VCC=1.2/3.3=0.36
RON1=0.55×0.27+(1-0.55)×0.18
=0.1485+0.081
=0.2295[Ω]
RON2=0.36×0.27+(1-0.36)×0.18
=0.0972+0.1152
=0.2124[Ω]
P=0.4²Ã—0.2295+0.8²Ã—0.2124=0.173[W]
As RONH is greater than RONL in BU9151MUV, 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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9/16
2009.08 - Rev.A