MIC29310/29312
Micrel
P
D
= I
OUT
×
(1.02V
IN
– V
OUT
)
Where the ground current is approximated by 2% of I
OUT
.
Then the heat sink thermal resistance is determined with this
formula:
T
J MAX
– T
A
θ
SA
= —————— – (θ
JC
+
θ
CS
)
P
D
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. MIC29310/2 regulators are stable with a
minimum capacitor value of 10µF at full load.
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 ca-
pacitors are recommended for systems where fast load
transient response is important.
Where the regulator is powered from a source with a high AC
impedance, a 0.1µF capacitor connected between Input and
GND is recommended. This capacitor should have good
characteristics to above 250kHz.
Transient Response and 5V to 3.3V Conversion
The MIC29310/2 have excellent response to variations in
input voltage and load current. By virtue of their low dropout
voltage, these devices do not saturate into dropout as readily
as similar NPN-based designs. A 3.3V output Micrel LDO will
maintain full speed and performance with an input supply as
low as 4.2V, and will still provide some regulation with
supplies down to 3.8V, unlike NPN devices that require 5.1V
or more for good performance and become nothing more
than a resistor under 4.6V of input. Micrel’s PNP regulators
provide superior performance in “5V to 3.3V” conversion
applications than NPN regulators, especially when all toler-
ances are considered.
Minimum Load Current
The MIC29310/2 regulators are specified between finite
Applications Information
The MIC29310 and MIC29312 are high performance low-
dropout voltage regulators suitable for all moderate to high-
current voltage regulator applications. Their 600mV of drop-
out 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 tran-
sistor designs, where the minimum dropout voltage is limited
by the base-emitter voltage drop and collector-emitter satu-
ration 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 drive
requirement. But Micrel’s Super ßeta PNP™ process re-
duces this drive requirement to merely 2% to 5% of the load
current.
MIC29310/312 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 tem-
perature. Transient protection allows device (and load) sur-
vival 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 MIC29312 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.
V
IN
IN
OUT
GND
V
OUT
Figure 3. The MIC29310 regulator requires 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
• 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.
July 1999
7
MIC29310/29312
MICREL [ MICREL SEMICONDUCTOR ]
