Micrel, Inc. MIC37252
February 2006
6
M9999-021606
(408) 955-1690
Application Information
The MIC37252 is a high-performance low-dropout
voltage regulator suitable for moderate to high-current
regulator applications. Its 550mV 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-to-emitter voltage drop and collector-to-
emitter saturation voltage, dropout performance of the
PNP output of these devices is limited only by the low
V
CE
saturation voltage.
A trade off for the low dropout voltage is a varying base
drive requirement. Micrel’s Super βeta PNP
®
process
reduces this drive requirement to only 2% to 5% of the
load current.
The MIC37252 regulator is fully protected from damage
due to fault conditions. Current limiting is provided. This
limiting is linear; output current during 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
spikes 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.
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
)
• Ground current (I
GND
)
First, calculate the power dissipation of the regulator
from these numbers and the device parameters from this
datasheet.
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
I
GND
Where the ground current is approximated by using
numbers from the “Electrical Characteristics” or “Typical
Characteristics.” 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°C 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
βeta PNP
®
regulators allow significant reductions in
regulator power dissipation and the associated heat sink
without compromising performance. When this technique
is employed, a capacitor of at least 1.0µF is needed
directly between the input and regulator ground. Refer to
Application Note 9 for further details and examples on
thermal design and heat sink applications.
Output Capacitor
The MIC37252 requires an output capacitor for stable
operation. As a µCap LDO, the MIC37252 can operate
with ceramic output capacitors as long as the amount of
capacitance is 47µF or greater. For values of output
capacitance lower than 47µF, the recommended ESR
range is 200mΩ to 2Ω. The minimum value of output
capacitance recommended for theMIC37252 is 10µF.
For 47µF or greater the ESR range recommended is
less than1Ω. Ultra-low ESR, ceramic capacitors are
recommended for output capacitance of 47µF or greater
to help improve transient response and noise reduction
at high frequency. X7R/X5R dielectric-type ceramic
capacitors are recommended because of their
temperature performance. X7R-type capacitors change
capacitance by 15% over their operating temperature
range and are the most stable type of ceramic
capacitors. Z5U and Y5V dielectric capacitors change
value by as much as 50% and 60% respectively over
their operating temperature ranges. To use a ceramic
chip capacitor withY5V dielectric, the value must be
much higher than an X7Rceramic capacitor to ensure
the same minimum capacitance over the equivalent
operating temperature range. The MIC37252 has
excellent transient response to variations in input voltage
and load current. The device has been designed to
respond quickly to load current variations and input
voltage variations. Large output capacitors are not
required to obtain this performance. A standard 47µF
output capacitor is all that is required. Larger values help
to improve performance even further.
Input Capacitor
An input capacitor of 1.0µF or greater is recommended
when the device is more than 4 inches away from the
bulk supply capacitance, or when the supply is a battery.
Small, surface-mount chip capacitors can be used for
the bypassing. The capacitor should be place within 1” of
the device for optimal performance. Larger values will
help to improve ripple rejection by bypassing the input to
the regulator, further improving the integrity of the output
voltage.
