Micrel, Inc.
MIC29302A
First, we calculate the power dissipation of the regulator
from these numbers and the device parameters from this
datasheet:
P
D
=
I
OUT
(
1.02V
IN
−
V
OUT
)
Application Information
The MIC29302A is a high-performance, low-dropout
voltage regulator suitable for all moderate to high-current
voltage regulation applications. Its 450mV typical
dropout voltage at full load makes it especially valuable
in battery-powered systems and as high efficiency noise
filters in “post-regulator” applications. Unlike older NPN-
pass transistor designs, where the minimum dropout
voltage is limited by the base-emitter voltage drop and
collector-emitter saturation voltage, dropout performance
of the PNP output is limited merely by the low V
CE
saturation voltage.
A trade-off for the low-dropout voltage is a varying base
driver requirement. But Micrel’s Super ßeta PNP
®
process reduces this drive requirement to merely 1% of
the load current.
The MIC29302A regulator is fully protected from damage
due to fault conditions. Current limiting is linear; output
current under overload conditions is constant. Thermal
shutdown disables the device when the die temperature
exceeds the 125°C maximum safe operating
temperature. The output structure of the regulators
allows voltages in excess of the desired output voltage
to be applied without reverse current flow. The
MIC29302A offer a logic level ON/OFF control: when
disabled, the devices draw nearly zero current.
Where the ground current is approximated by 2% of I
OUT
.
Then the heat sink thermal resistance is determined with
this formula:
−
T
A
T
θ
SA
=
JMAX
−
(
θ
JC
+
θ
CS
)
P
D
where:
T
JMAX
≤
125°C and
θ
CS
is between 0 and 2°C/W.
The heat sink may be significantly reduced in
applications where the minimum input voltage is known
and is large compared to the dropout voltage. A series
input resistor can be used to drop excessive voltage and
distribute the heat between this resistor and the
regulator. The low-dropout properties of Micrel Super
βeta
PNP
®
regulators allow very significant reductions in
regulator power dissipation and the associated heat sink
without compromising performance. When this technique
is employed, a capacitor of at least 0.1µF is needed
directly between the input and regulator ground.
Please refer to
Application Note 9
and
Application Hint
17
on Micrel’s website (www.micrel.com) for further
details and examples on thermal design and heat sink
specification.
With no heat sink in the application, calculate the
junction temperature to determine the maximum power
dissipation that will be allowed before exceeding the
maximum junction temperature of the MIC29302A. The
maximum power allowed can be calculated using the
thermal resistance (θ
JA
) of the D-Pak (TO252) adhering
to the following criteria for the PCB design: 2 oz. copper
and 100mm
2
copper area for the MIC29302A.
For example, given an expected maximum ambient
temperature (T
A
) of 75°C with V
IN
= 3.3V, V
OUT
= 2.5V,
and I
OUT
= 1.5A, first calculate the expected P
D
using:
P
D
=
(
3.3V
−
2.5V
)
×
3A
−
(
3.3V
)
×
(
0.016A
)
=
2.3472W
Figure 3. Linear Regulators Require Only Two Capacitors
for Operation
Thermal Design
Linear regulators are simple to use. The most
complicated set of design parameters to consider are
thermal characteristics. Thermal design requires the
following application-specific parameters:
•
•
•
•
Maximum ambient temperature, T
A
Output Current, I
OUT
Output Voltage, V
OUT
Input Voltage, V
IN
August 2011
10
M9999-080411-A
MICREL [ MICREL SEMICONDUCTOR ]
