MIC49200 Datasheet, PDF (10/12 Pages) Micrel Semiconductor – 2A LOW VOLTAGE LDO WITH DUAL INPUT VOLTAGES

Micrel, Inc.
Applications Information
The MIC49200 is an ultra-high performance, low-
dropout linear regulator designed for high current
applications requiring fast transient response. The
MIC49200 utilizes two input supplies, significantly
reducing dropout voltage, perfect for low-voltage, DC-
to-DC conversion. The MIC49200 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 MIC49200 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 MIC49200. V_BIAS requires
approximately 40mA 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 thereby reducing the dropout to 400mV
at a 2A 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 represent 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 input
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 MIC49200 requires a minimum of output
capacitance to maintain stability. However, proper
capacitor selection is important to ensure desired
transient response. The MIC49200 is specifically
designed to be stable with virtually any capacitance
value and ESR. A 1µF ceramic chip capacitor should
satisfy most applications. Output capacitance can be
increased without bound. See “Typical Characteristic”
subsection for examples of load transient response.
MIC49200
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.
Input Capacitor
An input capacitor of 1µF or greater is recommended
when the device is more than 4" 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. 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
As the load increases, the input current will be less
than the output current at high output currents. The
bias current is a sum of base drive and ground
current. Ground current is constant over load current.
The heat sink thermal resistance is determined with
this formula:
θ _SA
=
T_J(MAX) − T_A
P_D
January 2006
10
M9999-011306
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