bd6709fs ROHM Co. Ltd., bd6709fs Datasheet - Page 38

• Manufacturer Part Number: bd6709fs
• Description: Dc Brushless Motor Drivers For Cooling Fans Speed Controllable Single-phase Full-wave Dc Brushless Fan Motor Drivers
• Manufacturer: ROHM Co. Ltd.

● Thermal derating curve
Power dissipation (total loss) indicates the power that can be consumed by IC at Ta = 25 º C (normal temperature). IC is
heated when it consumes power, and the temperature of IC chip becomes higher than ambient temperature. The
temperature that can be accepted by IC chip depends on circuit configuration, manufacturing process, etc, and
consumable power is limited. Power dissipation is determined by the temperature allowed in IC chip (maximum junction
temperature) and thermal resistance of package (heat dissipation capability). The maximum junction temperature is in
general equal to the maximum value in the storage temperature range.
Heat generated by consumed power of IC is radiated from the mold resin or lead frame of package. The parameter
which indicates this heat dissipation capability (hardness of heat release) is called heat resistance, represented by the
symbol θja [ ℃ /W]. The temperature of IC inside the package can be estimated by this heat resistance. Fig.37 shows the
model of heat resistance of the package.
Heat resistance θja, ambient temperature Ta, junction temperature Tj, and power consumption P can be calculated by
the equation below:
Thermal derating curve indicates power that can be consumed by IC with reference to ambient temperature. Power that
can be consumed by IC begins to attenuate at certain ambient temperature. This gradient is determined by thermal
resistance θja.
Thermal resistance θja depends on chip size, power consumption, package ambient temperature, packaging condition,
wind velocity, etc., even when the same package is used. Thermal derating curve indicates a reference value measured
at a specified condition. Fig.38 shows a thermal derating curve (Value when mounting FR4 glass epoxy board 70 [mm] x
70 [mm] x 1.6 [mm] (copper foil area below 3 [%]))
Pd(mW
812
1000
400
200
800
600
＊ Reduce by 6.5mW/ ℃ over 25 ℃ .
.
5
0
)
(On 70.0mm × 70.0mm × 1.6mm glass epoxy board)
25
Fig.104 Thermal derating curve
θ ja ＝ (Tj － Ta) / P
θ ja = (Tj-Ta) / P
Fig.103 Thermal resistance
50
Chip surface temperature Tj[℃]
Power consumption P[W]
38/40
Ambient temperature Ta[℃]
75
95
100
[ ℃ /W]
[ ℃ /W]
BD6718FV,BD6709FS
BD6721FS,BD6722FS
125
150
Ta(℃)