Technical Note
11/18
www.rohm.com
2009.08 - Rev.A
?2009 ROHM Co., Ltd. All rights reserved.
BD9153MUV
?SPAN class="pst BD9153MUV-E2_2471427_7">Consideration on permissible dissipation and heat generation
As BU9153MUV 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.
(Example) VCC=5V, VOUT1=3.3V, VOUT2=1.2V, RONH=170m&, RONL=130m&
IOUT=1.5A, for example,
D1=VOUT1/VCC=3.3/5=0.66
D2=VOUT2/VCC=1.2/5=0.24
RON1=0.66?.170+(1-0.66)?.130
=0.1122+0.0442
=0.1564[&]
RON2=0.24?.170+(1-0.24)?.130
=0.0408+0.0988
=0.1397[&]
P=1.5
2
?.1564+1.5
2
?.1397=0.666[W]
As RONH
is greater than RONL in BU9153MUV, 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.
Fig.34 Thermal derating curve
(VQFN024V4040)
P=IOUT
2
識ON
RON=D識ONP+(1-D)RONN
DON duty (=VOUT/VCC)
RONHON resistance of Highside MOS FET
RONLON resistance of Lowside MOS FET
IOUTOutput current
Ambient temperature :Ta []
0 25
SW1
75 100 125 150
0
2.0
3.0
4.0
a2.2W
`3.56W
1.0
b0.69W
c0.34W
105
` 4 layers (copper foil area : 5505mm
2
)
(Copper foil in each layers)
竕-a=35.1/W
a 4 layers (copper foil area : 10.29mm
2
)
(Copper foil in 2nd and 3rd layers)
竕-a=56.8/W
b 1 layer (Copper foil area : 0mm
2
)
竕-a=181.2/W
cIC only
竕-a=367.6/W
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