MIC39150 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – 1.5A Low-Voltage Low-Dropout Regulator

MIC39150/39151
Applications Information
The MIC39150/1 is a high-performance low-dropout voltage
regulator suitable for moderate to high-current voltage 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, where the minimum dropout voltage is limited by the
base-to-emitter voltage drop and collector-to-emitter satura-
tion 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 MIC39150/1 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 tem-
perature. Transient protection allows device (and load) sur-
vival 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.
MIC39150-x.x
V_IN
V_OUT
IN
OUT
GND
C_IN
C_OUT
Figure 1. Capacitor Requirements
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° and 2°C/W.
The heat sink may be significantly reduced in applications
where the minimum input voltage is known and is large
Micrel
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 1µF 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 specification.
Output Capacitor
The MIC39150/1 requires an output capacitor to maintain
stability and improve transient response. Proper capacitor
selection is important to ensure proper operation. The
MIC39150/1 output capacitor selection is dependent upon
the ESR (equivalent series resistance) of the output capacitor
to maintain stability. When the output capacitor is 10µF or
greater, the output capacitor should have an ESR less than
2Ω. This will improve transient response as well as promote
stability. Ultralow ESR capacitors (<100mΩ), such as ce-
ramic chip capacitors may promote instability. These very low
ESR levels may cause an oscillation and/or underdamped
transient response. A low-ESR solid tantalum capacitor works
extremely well and provides good transient response and
stability over temperature. Aluminum electrolytics can also
be used, as long as the ESR of the capacitor is < 2Ω.
The value of the output capacitor can be increased without
limit. Higher capacitance values help to improve transient
response and ripple rejection and reduce output noise.
Input Capacitor
An input capacitor of 1µF or greater is recommended when
the device is more than 4 inches away from the bulk ac supply
capacitance, or when the supply is a battery. Small, surface-
mount, ceramic chip capacitors can be used for the bypass-
ing. 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
or 2.5V to 1.8V Conversion
The MIC39150/1 has excellent transient response to varia-
tions 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 2.5V, or 2.5V to 1.8V, 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 MIC39150/1 regulator will provide excellent performance
with an input as low as 3.0V or 2.5V, respectively. This gives
MIC39150/39151
8
May 2000