June 2000

9

MIC912

MIC912

Micrel

Applications Information

The MIC912 is a high-speed, voltage-feedback operational

amplifier featuring very low supply current and excellent

stability. This device is unity gain stable with R

capable of driving high capacitance loads.

Stability Considerations

The MIC912 is unity gain stable and it is capable of driving

unlimited capacitance loads, but some design considerations

are required to ensure stability.

The output needs to be

loaded with 200

cient load capacitance to achieve stability (refer to the

“Load Capacitance vs. Phase Margin” graph).

For applications requiring a little less speed, Micrel offers the

MIC910, a more heavily compensated version of the MIC912

which provides extremely stable operation for all load resis-

tance and capacitance.

Driving High Capacitance

The MIC912 is stable when driving high 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

(<100

Ω) in series with the output.

Feedback Resistor Selection

Conventional op amp gain configurations and resistor selec-

tion apply, the MIC912 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

following guidelines should be observed: Capacitance, par-

ticularly 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 Bypassing

Regular supply bypassing techniques are recommended. A

10

µF 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.

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.

An MIC912 with no load, dissipates power equal to the

quiescent supply current * supply voltage

PV

V

I

DV

V

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.

PV

V

I

DV

OUT

OUT

()

+

Total Power Dissipation

P

P

DD

t

=+

(no load)

(outpu 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.

Max Allowable Power Dissipation

TT

JA

.

=

−

(max)

(max)

260W