MIC37252 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – 2.5A, Low Voltage UCap LDO Regulator

MIC37252
Applications 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 perfor-
mance 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 tem-
perature 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 Characteris-
tics.” 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^oC and θ_CS is between 0^oC and 2^oC/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 signifi-
cant reductions in regulator power dissipation and the asso-
ciated 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.
Micrel
Output Capacitor
The MIC37252 requires an output capacitor for stable opera-
tion. 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 the
MIC37252 is 10µF.
For 47µF or greater the ESR range recommended is less than
1Ω. Ultra-low ESR, ceramic capacitors are recommended for
output capacitance of 47µF or greater to help improve tran-
sient response and noise reduction at high frequency. X7R/
X5R dielectric-type ceramic capacitors are recommended
because of their temperature performance. X7R-type capaci-
tors change capacitance by 15% over their operating tem-
perature 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. 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 rejec-
tion by bypassing the input to the regulator, further improving
the integrity of the output voltage.
Transient Response and 5V to 3.3V or 3.3V to 1.8V
Conversions
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 10µF output
capacitor, preferably tantalum, is all that is required. Larger
values help to improve performance even further.
By virtue of its low-dropout voltage, this device does not
saturate into dropout as readily as similar NPN-based de-
signs. When converting from 3.3V to 1.8V, the NPN-based
regulators are already operating in dropout, with typical
dropout requirements of 2V or greater. To convert down to
1.8V without operating in dropout, NPN-based regulators
require an input voltage of 3.8V at the very least. The
MIC37252 regulator will provide excellent performance with
an input as low as 3.0V. This gives the PNP-based regulators
a distinct advantage over older NPN-based linear regulators.
MIC37252
6
January 2003