MIC915 Micrel
MIC915 10 September 2000
Applications Information
The MIC915 is a high-speed, voltage-feedback operational
amplifier featuring very low supply current and excellent
stability. This device is unity gain stable and capable of
driving high capacitance loads.
Driving High Capacitance
The MIC915 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 Characteris-
tics: 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 selec-
tion apply, the MIC915 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 MIC915 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 capaci-
tors are located close to the device.
Power Supply Consideration
Regular supply bypassing techniques are recommended. A
10µF capacitor in parallel with a 0.1µF capacitor on both the
positive and negative supplies are ideal. For best perfor-
mance 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 resis-
tance). Surface-mount ceramic capacitors are ideal. Both V–
pins must be externally shorted together.
Thermal Considerations
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 de-
grade, in particular CMRR will reduce.
A MIC915 with no load, dissipates power equal to the quies-
cent supply current * supply voltage
PVVI
DVV
S
(noload)
=−
()
+−
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.
PVVI
DV
OUT OUT
(output stage)
=−
()
+
Total Power Dissipation P P
DDt
=+
(noload) (outpu stage)
Ensure the total power dissipated in the device is no greater
than the thermal capacity of the package. The MSOP-10
package has a thermal resistance of TBD°C/W.
Max AllowablePower Dissipation
TT
W
JA
. =
−
(max) (max)
TBD
