MIC921

Micrel

MIC921

10

December 2001

Applications Information

The MIC921 is a high-speed, voltage-feedback operational

amplifier featuring very low supply current and excellent

stability. This device is unity gain stable, capable of driving

high capacitance loads.

Driving High Capacitance

The MIC921 is stable when driving high capacitance, making

it ideal for driving long coaxial cables or other high-capaci-

tance loads. Most high-speed op amps are only able to drive

limited capacitance.

Note: increasing load capacitance does reduce

the speed of the device. In applications where

the load capacitance reduces the speed of the

op amp to an unacceptable level, the effect of

the load capacitance 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 MIC921 is NOT a current feedback device.

Also, for minimum peaking, the feedback resistor should

have low parasitic capacitance, usually 470

Ω is ideal. To use

the part as a follower, the output should be connected to input

via a short wire.

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 SC70-5 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 MIC921 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 SC70-5

package has a thermal resistance of 450

°C/W.

Max AllowablePowerDissipation

TT

JA

.

=

−

°

(max)

(max)

450 C / W