MIC37501 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – 5A, Low Voltage μCap LDO Regulator

Micrel, Inc.
Application Information
The MIC37501/02 is a high-performance, low dropout
voltage regulator suitable for moderate to high-current
regulator applications. Its 500mV dropout voltage at
full load makes it especially valuable in battery-
powered systems and as a high-efficiency noise filter
in post-regulator applications. Unlike older NPN-pass
transistor designs, where the minimum dropout
voltage is limited by the based-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 MIC37501/02 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.
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 data sheet.
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” sections. The heat sink
thermal resistance is then 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
February 2007
MIC37501/37502
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 MIC37501/02 requires an output capacitor for
stable operation. As a µCap LDO, the MIC37501/02
can operate with ceramic output capacitors as long as
the amount of capacitance is 100µF or greater. For
values of output capacitance lower than 100µF, the
recommended ESR range is 200mΩ to 2Ω. The
minimum value of output capacitance recommended
for the MIC37501/02 is 47µF.
For 100µF or greater, the ESR range recommended is
less than 1Ω. Ultra-low ESR ceramic capacitors are
recommended for output capacitance of 100µF 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. X7Rtype 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 with Y5V dielectric,
the value must be much higher than an X7R ceramic
capacitor to ensure the same minimum capacitance
over the equivalent operating temperature range.
Input Capacitor
An input capacitor of 1.0µF or greater is recom-
mended when the device is more than 4 inches away
from the bulk supply capacitance, or when the supply
is a battery. Small, surfacemount chip capacitors can
be used for the bypassing. The capacitor should be
placed 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.
Transient Response and 3.3V to 2.5V, 2.5V to 1.8V
or 1.65V, or 2.5V to 1.5V Conversions
The MIC37501/02 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
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