PRELIMINARY
CM3112
Application Information (cont’d)
Power Dissipation/Handling
Input Capacitor
The overall junction to ambient thermal resistance
If the VIN pin is within a few inches of the main input fil-
ter, a capacitor may not be necessary. Otherwise an
(θJA) for device power dissipation (P ) consists prima-
D
input filter capacitor (C ) of 0.1uF to 1uF will ensure
IN
rily of two paths in series. The first path is the junction
adequate filtering.
to the case (θJC) which is defined by the package style,
and the second path is case to ambient (θCA) thermal
Enable/Disable
resistance which is dependent on board layout. The
final operating junction temperature for any set of con-
ditions can be estimated by the following thermal equa-
tion:
Whenever this input is taken low, the regulator pass
transistor is forced into a high impedance mode and an
internal discharge resistance (500Ω) will be applied
from the output to ground.
T
= T
+ P (θJC ) + P (θCA
)
JUNC
AMB
D
D
= T
+ P (θJA)
AMB
D
Power Good
The CM3112-12 uses a SOT23-5 package. When this
package is mounted on a double sided printed circuit
board with two square inches of copper allocated for
"heat spreading", the resulting θJA is 175°C/W.
This is an open drain output signal. It works as a sup-
ply voltage supervisor for the output voltage.
It is asserted when the output falls below 84% (when
2.5V<Vin<5.5V) of its nominal value. The signal
becomes inactive when the three following conditions
are met (valid) for more than 1-10ms:
Based on a maximum power dissipation of 315mW
(2.1Vx150mA), with an ambient of 70°C the resulting
junction temperature will be:
a) EN > 1.5V
T
= T
+ P (θJA
)
JUNC
AMB
D
b) 2.5V < V < 5.5V
IN
= 70°C + 315mW (175°C/W)
= 70°C + 55°C = 125°C
c) V
> 97% of V
OUTNOM
OUT
Thermal characteristics were measured using a double
sided board with two square inches of copper area
connected to the GND pins for "heat spreading".
Measurements showing performance up to junction
temperature of 125°C were performed under light load
conditions (1mA). This allows the ambient temperature
to be representative of the internal junction tempera-
ture.
Note: The use of multi-layer board construction with
separate ground and power planes will further enhance
the overall thermal performance. In the event of no
copper area being dedicated for heat spreading, a
multi-layer board construction, using only the minimum
size pad layout, will provide the CM3112-12 with an
overall θJA of 175°C/W which allows up to 450mW to
be safely dissipated.
© 2004 California Micro Devices Corp. All rights reserved.
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L
Tel: 408.263.3214 L Fax: 408.263.7846
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13
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