MIC47300 Datasheet, PDF (7/11 Pages) Micrel Semiconductor – 3A, Low Voltage, Adjustable LDO Regulator with Dual Input Supply

Micrel, Inc.
Functional Description
The MIC47300 is an ultra-high performance, low-dropout
linear regulator designed for high current applications
requiring fast transient response. The MIC47300 utilizes
two input supplies, significantly reducing dropout
voltage, perfect for low-voltage, DC-to-DC conversion.
The MIC47300 requires a minimum of external
components and obtains a bandwidth of up to 10MHz.
As a µCap regulator, the output is tolerant of virtually
any type of capacitor including ceramic type and
tantalum type capacitors.
The MIC47300 regulator is fully protected from damage
due to fault conditions, offering linear current limiting and
thermal shutdown.
Bias Supply Voltage
V_BIAS, requiring relatively light current, provides power to
the control portion of the MIC47300. V_BIAS requires
approximately 50mA for a 3A load current. Dropout
conditions require higher currents. Most of the biasing
current is used to supply the base current to the pass
transistor. This allows the pass element to be driven into
saturation, reducing the dropout to 230mV at a 3A load
current. Bypassing on the bias pin is recommended to
improve performance of the regulator during line and
load transients. Small ceramic capacitors from V_BIAS to
ground help reduce high frequency noise from being
injected into the control circuitry from the bias rail and
are good design practice. Good bypass techniques
typically include one larger capacitor such as 1µF
ceramic and smaller valued capacitors such as 0.01µF
or 0.001µF in parallel with that larger capacitor to
decouple the bias supply. The V_BIAS input voltage must
be 2.1V above the output voltage with a minimum V_BIAS
inpt voltage of 3 volts.
Input Supply Voltage
V_IN provides the high current to the collector of the pass
transistor. The minimum input voltage is 1.4V, allowing
conversion from low voltage supplies.
Output Capacitor
The MIC47300 is designed to be stable with a minimal
capacitance value and without ESR constraints.
However, proper capacitor selection is important to
ensure desired transient response. A 1µF ceramic chip
capacitor should satisfy most applications and output
capacitance can be increased without bound. See
“Typical Characteristic” for examples of load transient
response.
X7R dielectric 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
October 2009
MIC47300
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 or a
tantalum capacitor to ensure the same capacitance
value over the operating temperature range. Tantalum
capacitors have a very stable dielectric (10% over their
operating temperature range) and can also be used with
this device.
Input Capacitor
Additional bypass capacitance is recommended when
the device is more than 2 to 3 inches away from the bulk
supply capacitance, or when the supply is a battery.
Small, surface-mount, ceramic chip capacitors can be
used for the bypassing. A 1μF or greater ceramic input
capacitor should be placed next to 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.
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 × I_IN + V_BIAS × I_BIAS – V_OUT × I_OUT
The input current will be less than the output current at
high output currents as the load increases. The bias
current is a sum of base drive and ground current.
Ground current is constant over load current. Then the
heat sink thermal resistance is determined with this
formula:
( ) θ_SA
=
⎜⎜⎝⎛
T_J(MAX) −
P_D
T_A
⎟⎟⎠⎞ ^−
θ _JC
+ θ_CS
The heat sink may be significantly reduced in
applications where the maximum input voltage is known
and 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 the MIC47300
allow significant reductions in regulator power dissipation
and the associated heat sink without compromising
performance. When this technique is employed, a
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M9999-102309-A