Micrel, Inc.
MIC29510/29512
Thermal Design
Linear regulators are simple to use. The most
complicated 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
Applications Information
The MIC29510 and MIC29512 are high performance
low-dropout voltage regulators suitable for all moderate
to high-current voltage regulator applications. Their
600mV of dropout voltage at full load make them
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 of these devices is limited merely by the low VCE
saturation voltage.
A trade-off for the low dropout voltage is a varying base
drive requirement. But Micrel’s Super ßeta PNP™
process reduces this drive requirement to merely 2 to
5% of the load current.
MIC29510/512 regulators are fully protected from
damage due to fault conditions. Current limiting is
provided. This limiting is linear; output current under
overload conditions is constant. Thermal shutdown
disables the device when the die temperature exceeds
the maximum safe operating temperature.
Transient protection allows device (and load) survival
even when the input voltage spike above and below
nominal. The output structure of these regulators allows
voltages in excess of the desired output voltage to be
applied without reverse current flow. The MIC29512
version offers a logic level ON/OFF control: when
disabled, the devices draw nearly zero current.
An additional feature of this regulator family is a common
pinout: a design’s current requirement may change up or
down yet use the same board layout, as all of Micrel’s
high-current Super ßeta PNP™ regulators have identical
pinouts.
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
)
Where the ground current is approximated by 3% of I
OUT
,
then the heat sink thermal resistance is determined with
this formula:
θ
SA
=
T
J(MAX)
−
T
A
P
D
−
(θ
JC
+
θ
CS
)
Where T
J MAX
≤
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 with the dropout voltage. Use a
series input resistor to drop excessive voltage and
distribute the heat between this resistor and the
regulator. The low dropout properties of Micrel Super
ßeta 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 for further details and
examples on thermal design and heat sink specification.
Capacitor Requirements
For stability and minimum output noise, a capacitor on
the regulator output is necessary. The value of this
capacitor is dependent upon the output current; lower
currents allow smaller capacitors. MIC29510/2
regulators are stable with a minimum capacitor value of
47µF at full load.
Figure 3. The MIC29510 requires only
two capacitors for operation
This capacitor need not be an expensive low ESR type:
aluminum electrolytics are adequate. In fact, extremely
low ESR capacitors may contribute to instability.
Tantalum capacitors are recommended for systems
where fast load transient response is important.
May 2006
7
M9999-051706
(408) 955-1690
MICREL [ MICREL SEMICONDUCTOR ]
