MIC49300_05 Datasheet, PDF (6/8 Pages) Micrel Semiconductor – 3.0A Low Voltage LDO Regulator w/Dual Input Voltages

MIC49300
Applications Information
The MIC49300 is an ultra-high performance, low dropout linear
regulator designed for high current applications requiring fast
transient response. The MIC49300 utilizes two input supplies,
significantly reducing dropout voltage, perfect for low-voltage,
DC-to-DC conversion. The MIC49300 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 and tantalum.
The MIC49300 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 MIC49300. V_BIAS requires approximately
33mA for a 1.5A 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
300mV at a 1.5A load current. Bypassing on the bias pin is
recommended to improve performance of the regulator dur-
ing 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 in-
clude one larger capacitor such as a 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 1.6V above the output voltage with a
minimum V_BIAS input voltage of 3V.
Input Supply Voltage
V_IN provides the high current to the collector of the pass
transistor. The minimum input voltage is 1.4V, allowing con-
version from low voltage supplies.
Output Capacitor
The MIC49300 requires a minimum of output capacitance
to maintain stability. However, proper capacitor selection
is important to ensure desired transient response. The
MIC49300 is specifically designed to be stable with virtually
any capacitance value and ESR. A 1µF ceramic chip capaci-
tor should satisfy most applications. Output capacitance can
be increased without bound. See typical characteristics 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 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.
Micrel, Inc.
Input Capacitor
An input capacitor of 1µF 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, surfac-
emount, 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.
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:
( ) ^θSA
=



T_J(MAX)
P_D – θ_JC
±
+
T_A
θ_CS



The heat sink may be significantly reduced in applications
where the maximum input voltage is known and large com-
pared 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
MIC49300 allow significant reductions in regulator power dis-
sipation and the associated heat sink without compromising
performance. When this technique is employed, a capacitor of
at least 1µF is needed directly between the input and regula-
tor ground. Refer to Application Note 9 for further details and
examples on thermal design and heat sink specification.
Minimum Load Current
The MIC49300, unlike most other high current regulators,
does not require a minimum load to maintain output voltage
regulation.
Power Sequencing
There is no power sequencing requirement for V_IN and V_BIAS,
giving more flexibility to the user.
M9999-082605-B
6
August 2005