MIC49300 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 requir-
ing 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 tanta-
lum.
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 approxi-
mately 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 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 tech-
niques typically include 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
conversion 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 capaci-
tance value and ESR. A 1µF ceramic chip capacitor should
satisfy most applications. Output capacitance can be in-
creased without bound. See typical characteristics for ex-
amples of load transient response.
X7R dielectric ceramic capacitors are recommended be-
cause of their temperature performance. X7R-type capaci-
tors change capacitance by 15% over their operating tem-
perature 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 oper-
ating 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
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,
surfacemount, ceramic chip capacitors can be used for the
bypassing. 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 regula-
tor, 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
dissipation and the associated heat sink without compromis-
ing performance. When this technique is employed, a capaci-
tor 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 speci-
fication.
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.
MIC49300
6
October 2003