BD33C0AWFP Datasheet, PDF (8/13 Pages) Rohm – Standard Fixed Output LDO Regulators with Shutdown Switch

BD33C0AWFP, BD50C0AWFP
●Thermal Design
Technical Note
5
4
3
2
1.30
1
Mounted on a Rohm standard board
Board size : 70 ㎜×70 ㎜×1.6 ㎜
Copper foil area :7 ㎜×7 ㎜
TO252-5θja=96.2(℃/W)
5
③4.80
4
②3.50
3
①1.85
2
1
Mounted on a Rohm standard board
Board size : 70 ㎜×70 ㎜×1.6 ㎜
Copper foil area :7 ㎜×7 ㎜
①2-layer board
(back surface copper foil area :15 ㎜×15 ㎜)
②2-layer board
(back surface copper foil area :70 ㎜×70 ㎜)
③4-layer board
(back surface copper foil area :70 ㎜×70 ㎜)
①:θja=67.6℃/W
②:θja=35.7℃/W
③:θja=26.0℃/W
0
0
25
50
75
100
125
150
AMBIENT T EMPERAT URE:Ta[℃]
Fig.26
0
0
25
50
75
100
125
150
AMBIENT T EMPERAT URE:Ta[℃]
Fig.27(Reference data)
When using at temperatures over Ta=25℃, please refer to the heat reducing characteristics shown in Fig.26 and Fig.27.
The IC characteristics are closely related to the temperature at which the IC is used, so it is necessary to operate the IC
at temperatures less than the maximum junction temperature Tjmax.
Fig.26 and Fig.27 shows the acceptable loss and heat reducing characteristics of the TO252-5 package. Even when the
ambient temperature Ta is a normal temperature (25℃), the chip (junction) temperature Tj may be quite high so please
operate the IC at temperatures less than the acceptable loss Pd.
The calculation method for power consumption Pc(W) is as follows :(Fig.27③)
Pc=(Vcc－Vo)×Io+Vcc×Ib
Acceptable loss Pd≧Pc
Solving this for load current Io in order to operate within the acceptable loss,
Io≦ ^{Pd－Vcc×Ib}
Vcc－Vo
(Please refer to Fig.9,21 for Ib.)
Vcc: Input voltage
Vo: Output voltage
Io: Load current
Ib: Circuit current
Ishort: Short current
It is then possible to find the maximum load current IoMax with respect to the applied voltage Vcc at the time of thermal
design.
Calculation Example) When Ta=85℃
BD33C0AWFP:Vcc=8.3V,Vo=3.3V BD50C0AWFP:Vcc=10V,Vo=5.0V
Io≦ ^{2.496－10×Ib}
5
Io≦498.2mA (Ib:0.5mA)
Fig.27③:θja=26.0℃/W → -38.4mW/℃
25℃=4.80W → 85℃=2.496W
Please refer to the above information and keep thermal designs within the scope of acceptable loss for all operating
temperature ranges. The power consumption Pc of the IC when there is a short circuit (short between Vo and GND) is :
Pc=Vcc×(Ib+Ishort)
(Please refer to Fig.5,17 for Ishort.)
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8/12
2011.07 - Rev.A