Micrel, Inc. MIC913
October 2007
12
M9999-101707
Application Information
The MIC913 is a high-speed, voltage-feedback opera-
tional amplifier featuring very low supply current. The
MIC913 is not unity-gain stable, it requires a minimum
gain of +2 or –1 to ensure stability. The device is
however stable even when driving high capacitance
loads.
Driving High Capacitance
The MIC913 is stable when driving any capacitance (see
“Typical Characteristics: Gain Bandwidth and Phase
Margin vs. Load Capacitance”) making it ideal for driving
long coaxial cables or other high-capacitance loads.
Phase margin remains constant as load capacitance is
increased. Most high-speed op amps are only able to
drive limited capacitance.
Note: increasing load capacitance does reduce
the speed of the device (see “Typical Character-
istics: Gain Bandwidth and Phase Margin vs.
Load”). In applications where the load capaci-
tance reduces the speed of the op amp to an
unacceptable level, the effect of the load capaci-
tance can be reduced by adding a small resistor
(<100Ω) in series with the output.
Feedback Resistor Selection
Conventional op amp gain configurations and resistor
selection apply, the MIC913 is NOT a current feedback
device. Resistor values in the range of 1k to 10k are
recommended.
Layout Considerations
All high speed devices require careful PCB layout. The
high stability and high PSRR of the MIC913 make this op
amp easier to use than most, but the following guidelines
should be observed: Capacitance, particularly on the two
inputs pins will degrade performance; avoid large copper
traces to the inputs. Keep the output signal away from
the inputs and use a ground plane.
It is important to ensure adequate supply bypassing
capacitors are located close to the device.
Power Supply Bypassing
Regular supply bypassing techniques are recom-
mended. A 10µF capacitor in parallel with a 0.1µF
capacitor on both the positive and negative supplies are
ideal. For best performance all bypassing capacitors
should be located as close to the op amp as possible
and all capacitors should be low ESL (equivalent series
inductance), ESR (equivalent series resistance).
Surface-mount ceramic capacitors are ideal.
Thermal Considerations
The SOT-23-5 package, like all small packages, has a
high thermal resistance. It is important to ensure the IC
does not exceed the maximum operating junction (die)
temperature of 85°C. The part can be operated up to the
absolute maximum temperature rating of 125°C, but
between 85°C and 125°C performance will degrade, in
particular CMRR will reduce.
A MIC913 with no load, dissipates power equal to the
quiescent supply current * supply voltage.
P
D(no load)
= (V
V+
– V
V–
)I
S
When a load is added, the additional power is dissipated
in the output stage of the op amp. The power dissipated
in the device is a function of supply voltage, output
voltage and output current.
P
D(output stage)
= (V
V+
– V
V–
)I
OUT
Total Power Dissipation = P
D(no load)
+ P
D(output stage)
Ensure the total power dissipated in the device is no
greater than the thermal capacity of the package. The
SOT23-5 package has a thermal resistance of 260°C/W.
260W
TT
nDissipatio Power AllowableMax.
A(max)J(max)
−
=
