Micrel, Inc. MIC49500
July 2007
8
M9999-071307
Applications Information
The MIC49500 is an ultra-high performance, low
dropout linear regulator designed for high current
applications requiring fast transient response. The
MIC49500 utilizes two input supplies, significantly
reducing dropout voltage, perfect for low-voltage, DC-
to-DC conversion. The MIC49500 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 MIC49500 regulator is fully protected from
damage due to fault conditions, offering constant
current limiting and thermal shutdown.
Bias Supply Voltage
V
BIAS
, requiring relatively light current, provides power
to the control portion of the MIC49500. V
BIAS
requires
approximately 70mA for 5A load current. 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 290mV
at a 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 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 3.0V.
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 MIC49500 requires a minimum of output
capacitance to maintain stability. However, proper
capacitor selection is important to ensure desired
transient response. The MIC49500 is specifically
designed to be stable with a wide range of
capacitance values and ESR. A 10µF ceramic chip
capacitor 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.
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, 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
The input current will be less than the output current
as the output 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:
SA
=
T
J(MAX)
± T
A
P
D JC 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
