Micrel, Inc.
MIC29150/29300/29500/29750
•
Output Current, I
OUT
•
Output Voltage, V
OUT
•
Input Voltage, V
IN
First, we calculate the power dissipation of the regulator
from these numbers and the device parameters from this
datasheet.
Application Information
The
MIC29150/29300/29500/29750
are
high
performance low-dropout voltage regulators suitable for
all moderate to high-current voltage regulator
applications. Their 350mV to 425mV typical 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 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 MIC29150/29300/29500/29750 family of 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 125°C maximum safe operating
temperature. Line transient protection allows device (and
load) survival even when the input voltage spikes
between –20V and +60V. When the input voltage
exceeds approximately 32V, the over voltage sensor
disables the regulator. The output structure of these
regulators allows voltages in excess of the desired
output voltage to be applied without reverse current flow.
MIC29xx1 and MIC29xx2 versions offer 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 these
regulators have identical pinouts.
MIC29XXX
V
IN
IN
OUT
V
OUT
P
D
=
I
OUT
(
1.01 V
IN
−
V
OUT
)
Where the ground current is approximated by 1% of I
OUT
.
Then the heat sink thermal resistance is determined with
this formula:
T
−
T
A
θ
SA
=
JMAX
−
(
θ
JC
+ θ
CS
)
P
D
GND
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 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 and Application Hint
17 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 MIC29152. The
maximum power allowed can be calculated using the
thermal resistance (θ
JA
) of the D-Pak adhering to the
following criteria for the PCB design: 2 oz. copper and
100mm
2
copper area for the MIC29152.
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
Equation (1);
P
D
=(3.3V–2.5V)1.5A–(3.3V)(0.016A)=1.1472W
Next, calcualte the junction temperature for the expected
power dissipation.
T
J
=(θ
JA
×P
D
)+T
A
=(56°C/W×1.1472W)+75°C=139.24°C
Now determine the maximum power dissipation allowed
that would not exceed the IC’s maximum junction
temperature (125°C) without the useof a heat sink by
P
D(MAX)
=(T
J(MAX)
–T
A
)/θ
JA
=(125°C–75°C)/(56°C/W) =0.893W
Figure 3. Linear regulators require only two capacitors for
operation.
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
January 2012
17
M9999-013112-B
MICREL [ MICREL SEMICONDUCTOR ]
