November 2006 7 MIC29310/29312
MIC29310/29312 Micrel
Applications Information
The MIC29310 and MIC29312 are high performance low-drop-
out 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 effi ciency noise fi lters 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
drive requirement. But Micrel’s Super ßeta PNP™ process
reduces 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
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 volt-
ages in excess of the desired output voltage to be applied
without reverse current fl ow. 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.
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-specifi c
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.
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 signifi cantly reduced in applications
where the minimum input voltage is known and is large com-
pared 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 signifi cant 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 ex-
amples on thermal design and heat sink specifi cation.
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 capaci-
tors 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 sup-
plies 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 specifi ed between fi nite loads.
If the output current is too small, leakage currents dominate
