Datasheet

Micrel, Inc. MIC20xx Family
August 2011 28
M9999-080211-D
In Figure 12, die temperature is plotted against I
OUT
assuming a constant case temperature of 85°C. The
plots also assume a worst case R
ON
of 140m at a die
temperature of 135°C. Under these conditions it is clear
that an SOT-23 packaged device will be on the verge of
thermal shutdown, typically 140°C die temperature,
when operating at a load current of 1.25A. For this
reason we recommend using MLF
®
packaged switches
for any design intending to supply continuous currents
of 1A or more.
Power Dissipation
Power dissipation depends on several factors such as
the load, PCB layout, ambient temperature, and supply
voltage. Calculation of power dissipation can be
accomplished by the following equation:
2
)I(RP
OUT)ON(DSD
×
=
To relate this to junction temperature, the following
equation can be used:
0
20
40
60
80
100
120
140
160
0.2
OUTPUT CURRENT (A)
Die Temperature vs.
Output Current (T
CASE
=85°C)
SOT-23
0.4
0.6 0.8 1.0 1.2 1.4 1.6 1.82.0
MLF
A
)AJ(
DJ
TRPT +×=
−
θ
where:
T
J
= junction temperature
T
A
= ambient temperature
R
θ(J-A)
is the thermal resistance of the package
In normal operation the switch’s R
ON
is low enough that
no significant I
2
R heating occurs. Device heating is
most often caused by a short circuit, or very-heavy
load, when a significant portion of the input supply
voltage appears across the switch’s power MOSFET.
Under these conditions the heat generated will exceed
the package and PCB’s ability to cool the device and
thermal limiting will be invoked.
Figure 12. Die Temperature vs. I
OUT