OPERATING
CONSIDERATIONS
MSA260
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CALCULATING INTERNAL POWER DISSIPATION
Detailed calculation of internal power dissipation is complex
but can be approximated with simple equations. Conduction
loss is given by:
W = I * 2.5 + I
2
* 0.095
where I = output current
Switching loss is given by:
W = 0.00046 * I * Vsupply * Fswitching
Combine these two losses to obtain total loss. Calculate
heatsink ratings and case temperatures as would be done for
a linear amplifier. For calculation of junction temperatures,
assume half the loss is dissipated in each of two switches:
Tj = Ta + Wtotal * RØhs + 1/2Wtotal * RØjc, where:
RØhs = heatsink rating
RØjc = junction-to-case thermal resistance of the
MSA260.
The SOA typical performance graphs below show perfor-
mance with the MSA260 mounted with thermal grease on the
Apex HS26. The Free Air graph assumes vertical orientation
of the heatsink and no obstruction to air flow in an ambient
temperature of 30°C. The other two graphs show performance
with two levels of forced air. Note that air velocity is given
in linear feet per minute. As fans are rated in cubic delivery
capability, divide the cubic rating by the square area this air
flows through to find velocity. As fan delivery varies with static
pressure, these calculations are approximations, and heatsink
ratings vary with amount of power dissipated, there is no sub-
stitute for temperature measurements on the heatsink in the
center of the amplifier footprint as a final check.
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This
MICROTECHNOLOGY CORPORATION • TELEPHONE (520) 690-8600 • FAX
is assumed for possible
ORDERS
or omissions. All specifications are subject to change without notice.
APEX
data sheet has been carefully checked and is believed to be reliable, however, no responsibility
(520) 888-3329 •
inaccuracies
(520) 690-8601 • EMAIL prodlit@apexmicrotech.com
MSA260U REV B JULY 2004 © 2004 Apex Microtechnology Corp.
5
APEX [ CIRRUS LOGIC ]
