MIC37139
Micrel
Refer to
“Application Note 9”
for further details and examples
on thermal design and heat sink applications.
Output Capacitor
The MIC37139 requires an output capacitor for stable opera-
tion. As a
µCap
LDO, the MIC37139 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
MIC37139 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
transient response and noise reduction at high frequency.
X7R/X5R dielectric-type ceramic capacitors are recom-
mended because of their temperature performance. X7R-
type capacitors change capacitance by 15% over their op-
erating 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.
V
IN
MIC37139
IN
C
IN
OUT
GND
C
OUT
V
OUT
Applications Information
The MIC37139 is a high-performance low-dropout voltage
regulator suitable for moderate to high-current regulator
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 MIC37139 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°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 ßeta 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.
Figure 1. Capacitor Requirements
Input Capacitor
An input capacitor of 1.0µF or greater is recommended when
the device is more than 4 inches away from the bulk and
supply capacitance, or when the supply is a battery. Small,
surface-mount chip capacitors can be used for the bypass-
ing. 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.
Transient Response and 3.3V to 2.5V, 2.5V to 1.8V or
1.65V, or 2.5V to 1.5V Conversions
The MIC37139 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.
M9999-112003
8
November 2003
MICREL [ MICREL SEMICONDUCTOR ]
