Micrel, Inc.
MIC2954
Thermal Calculations
Layout Considerations
The MIC2954-07BM/-08BM (8-pin surface-mount pack-
age) has the following thermal characteristics when
mounted on a single layer copper-clad printed circuit
board.
PC Board Dielectric Material
θ
JA
FR4
160°C/W
Ceramic
120°C/W
Multilayer boards having a ground plane, wide traces
near the pads, and large supply bus lines provide better
thermal conductivity.
Our calculations will use the “worst case” value of
160°C/W, which assumes no ground plane, minimum
trace widths, and a FR4 material board.
Pad Layout
(minimum recommended geometry)
* See Application Information
Figure 1. /Error Output Timing
or about 0.01µF. When doing this, the output capacitor
must be increased to 3.3µF to maintain stability. These
changes reduce the output noise from 430µV to
100µV
rms
for a100kHz bandwidth at 5V output. With the
bypass capacitor added, noise no longer scales with
output voltage so that improvements are more dramatic
at higher output voltages.
Automotive Applications
The MIC2954 is ideally suited for automotive applica-
tions for a variety of reasons. It will operate over a wide
range of input voltages with very low dropout voltages
(40mV at light loads), and very low quiescent currents
(75µA typical). These features are necessary for use in
battery powered systems, such as automobiles. It is a
“bulletproof” device with the ability to survive both
reverse battery (negative transients up to 20V below
ground), and load dump (positive transients up to 60V)
conditions. A wide operating temperature range with low
temperature coefficients is yet another reason to use
these versatile regulators in automotive designs.
Nominal Power Dissipation and Die Temperature
The MIC2954-07BM/-08BM at a 55°C ambient temp-
erature will operate reliably at up to 440mW power
dissipation when mounted in the “worst case” manner
described above. This power level is equivalent to a die
temperature of 125°C, the recommended maximum
temperature for nonmilitary grade silicon integrated
circuits.
September 2007
11
M9999-090607
MICREL [ MICREL SEMICONDUCTOR ]
