herunterladen
Application Note 110
AN110-1
an110fa
May 2007
air fl ow with and without a heatsink. 24V input designs
are analyzed for a worse case temperature rise due to the
relatively lower effi ciency exhibited.
THERMAL MODEL
The thermal model is shown in Figure 1. A μModule
regulator is attached to a 4-layer PCB with a size of
95mm ¥ 76mm. To analyze this physical system, a sim-
plifi ed 1- D thermal model presented in Figure 1(b), is
employed to show the heat paths in the system. The heat
is generated from the μModule regulator and fl ows to the
top and bottom sides. For the topside heat path, R
JT
is used
to represent the thermal resistance from the junction to the
top package surface, while R
TA
represents the resistance
from the top package surface to ambient. Similarly, for
the bottom side, R
JB
is the thermal resistance from the
junction to the bottom surface, and R
BA
is the resistance
from the bottom surface to ambient. The double-sided
cooling scheme can be realized easily, especially if a heat
sink is used for the top side.
Figure 1. Design Thermal Model
INTRODUCTION
The LTM4601 DC/DC μModule regulator is a complete
high power density stepdown regulator for up to 12A
continuous (14A peak) loads. The device is housed in
a small 15mm ¥ 15mm ¥ 2.8mm LGA surface mount
package, thus the large power dissipation is a challenge
in some applications. This thermal application note will
provide guidelines for using the μModule regulator in
ambient environments with or without air fl ow. Load
current derating curves are provided for several input
voltages and output voltages versus ambient temperature
and air fl ow. These derating curves provide guidelines
for using the LTM4601 in ambient environments with
regard to safe-operating-area (SOA). Also included are
effi ciency curves that are used to extrapolate the power
loss curves used in this thermal application note. The ap-
proach is to measure the temperature of a design, derive
thermal models for different cases and fi nally determine
the junction-to-ambient thermal resistance (q
JA
) in units of
°C/W in the heat path. The data includes power loss curves,
safe operating curves (SOA), thermal camera images and
current derating curves versus ambient temperature and
LTM4601 DC/DC µModule
®
Regulator Thermal
Performance
Eddie Beville, Jian Yin
R
TA
T
A
T
A
R
JT
R
JB
R
BA
PCB
μModule REGULATOR
R
JT
R
TA
R
JB
R
BA
T
A
T
J
(a) (b)
AN110 F01
L, LT, LTC, LTM, Linear Technology, μModule and the Linear logo are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
