Micrel, Inc. MIC59150
December 2008 8
M9999-121808-A
Application Information
The MIC59150 is an ultra-high performance, low-dropout
linear regulator designed for high current applications
requiring a fast transient response. The MIC59150
utilizes two input supplies, significantly reducing dropout
voltage, making it perfect for low-voltage, DC-to-DC
conversion. The MIC59150 requires a minimum number
of external components, and as a μCap regulator, the
output is tolerant of virtually any type of capacitor,
including ceramic type and tantalum type capacitors.
The MIC59150 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 MIC59150. V
BIAS
requires
approximately 12mA 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 100mV 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 VBIAS
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 VBIAS
input voltage must
be 2.1V above the output voltage with a minimum VBIAS
input voltage of 3V.
Input Supply Voltage
VIN provides the high current to the collector of the pass
transistor. The minimum input voltage is 1.0V, allowing
conversion from low voltage supplies.
Output Capacitor
The MIC59150 requires a minimum of output
capacitance to maintain stability. However, proper
capacitor selection is important to ensure desired
transient response. The MIC59150 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 the “Functional
Characteristics” subsection 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 (P
D
) of the
regulator from these numbers and the device
parameters from this datasheet.
OUTOUTBIASBIASININD
IVIVIVP
−×
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:
()
CSJC
D
AMAXJ
SA
P
TT
θθθ
+−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
)(
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 MIC59150
allow significant reductions in regulator power dissipation
and the associated heat sink without compromising
