MIC37300/37301/37302
Micrel
Output Capacitor
The MIC37300/01/02 requires an output capacitor for stable
operation. As a
µCap
LDO, the MIC37300/01/02 can operate
with ceramic output capacitors as long as the amount of
capacitance is 47µF or greater. For values of output capaci-
tance lower than 47µF, the recommended ESR range is
200mΩ to 2Ω. The minimum value of output capacitance
recommended for the MIC37300 is 10µF.
For 47µF or greater, the ESR range recommended is less
than 1Ω. Ultra-low ESR, ceramic capacitors are recom-
mended 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 capaci-
tor with Y5V dielectric, the value must be much higher than an
X7R ceramic capacitor to ensure the same minimum capaci-
tance over the equivalent operating temperature range.
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 3.3V to 2.5V, 2.5V to 1.8V or
1.65V, or 2.5V to 1.5V Conversions
The MIC37300/01/02 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, 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
designs. When converting from 3.3V to 2.5V, 2.5V to 1.8V or
1.65V, or 2.5V to 1.5V, the NPN-based regulators are already
operating in dropout, with typical dropout requirements of
1.2V or greater. To convert down to 2.5V without operating in
dropout, NPN-based regulators require an input voltage of
3.7V at the very least. The MIC37300/01/02 regulator will
provide excellent performance with an input as low as 3.0V or
2.25V, respectively. This gives the PNP-based regulators a
distinct advantage over older, NPN-based linear regulators.
Applications Information
The MIC37300/01/02 is a high-performance low-dropout
voltage regulator suitable for moderate to high-current regu-
lator applications. Its 500mV dropout voltage at full load and
overtemperature makes it especially valuable in battery-
powered systems and as high-efficiency noise filters in post-
regulator applications. Unlike older NPN-pass transistor de-
signs, there the minimum dropout voltage is limited by the
based-to-emitter voltage drop and collector-to-emitter satu-
ration 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 ßeta PNP
process reduces this
drive requirement to only 2% to 5% of the load current.
The MIC37300/01/02 regulator is fully protected from dam-
age 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 tem-
perature. 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°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’s Super ßeta 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.
M0307-112003
8
November 2003
MICREL [ MICREL SEMICONDUCTOR ]
