Datasheet

Micrel MIC5239
December 2007 10
M9999-121007
Current Limit
Figure 4 displays a method for reducing the steady state
short-circuit current. The duration that the supply
delivers current is set by the time required for the error
flag output to discharge the 4.7µF capacitor tied to the
enable pin. The off time is set by the 200k resistor as it
recharges the 4.7µF capacitor, enabling the regulator.
This circuit reduces the short-circuit current from 800mA
to 40mA while allowing for regulator restart once the
short is removed.
C
OUT
V
OUT
V
IN
5V
V
ERR
IN
MIC5239
EN
200k
1N4148
200k
4.7µF
OUT
GND
SHUTDOWN
ENABLE
FLG
Figure 4. Remote Enable with Short-Circuit
Current Foldback
Thermal Characteristics
The MIC5239 is a high input voltage device, intended to
provide 500mA of continuous output current in two very
small profile packages. The power MSOP-8 allows the
device to dissipate about 50% more power than their
standard equivalents.
Power MSOP-8 Thermal Characteristics
One of the secrets of the MIC5239’s performance is its
power MSOP-8 package featuring half the thermal
resistance of a standard MSOP-8 package. Lower
thermal resistance means more output current or higher
input voltage for a given package size.
Lower thermal resistance is achieved by joining the four
ground leads with the die attach paddle to create a
single piece electrical and thermal conductor. This
concept has been used by MOSFET manufacturers for
years, proving very reliable and cost effective for the
user.
Thermal resistance consists of two main elements, 
JC
(junction-to-case thermal resistance) and 
CA
(case-to-
ambient
thermal resistance). See Figure 5. 
JC
is the
resistance
from the die to the leads of the package. 
CA
is the resistance from the leads to the ambient air and it
includes 
CS
(case-to-sink thermal resistance) and 
SA
(sink-to-ambient thermal resistance).
Figure 5. Thermal Resistance
Using the power MSOP-8 reduces the 
JC
dramatically
and allows the user to reduce 
CA
. The total thermal
resistance, 
JA
(junction-to-ambient thermal resistance)
is the limiting factor in calculating the maximum power
dissipation capability of the device. Typically, the power
MSOP-8 has a 
JC of 80°C/W, this is significantly lower
than the standard MSOP-8 which is typically 200°C/W.

CA
is reduced because pins 5 through 8 can now be
soldered directly to a ground plane which significantly
reduces the case-to-sink thermal resistance and sink to
ambient thermal resistance.
Low-dropout linear regulators from Micrel are rated to a
maximum junction temperature of 125°C. It is important
not to exceed this maximum junction temperature during
operation of the device. To prevent this maximum
junction temperature from being exceeded, the
appropriate ground plane heatsink must be used.
2
Figure 6. Copper Area vs. Power-MSOP
Power Dissipation (∆T
JA
)
Figure 6 shows copper area versus power dissipation
with each trace corresponding to a different temperature
rise above ambient.
From these curves, the minimum area of copper
necessary for the part to operate safely can be
determined. The maximum allowable temperature rise
must be calculated to determine operation along which
curve.