MIC2009YM6-TR Datasheet, PDF (28/30 Pages) Micrel Semiconductor – Fixed and Adjustable Current Limiting Power Distribution Switches

Micrel, Inc.
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
P_D = R_DS(ON) × (I_OUT )
To relate this to junction temperature, the following
equation can be used:
T_J = P_D × R_{θ(J−A)} + T_A
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 I²R 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.
MIC20xx Family
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 140mΩ 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.
Die Temperature vs.
Output Current (T _CASE=85°C)
160
140
120 ^SOT-23
100
MLF
80
60
40
20
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
OUTPUT CURRENT (A)
Figure 12. Die Temperature vs. I_OUT
August 2011
28
M9999-080211-D