MP5121_MP5221_MP5421 –High Speed, single-supply Rail-to-Rail Input-Output Op Amps
OPERATION DESCRIPTION
The MP5121/MP5221/MP5421 are high-speed,
high slew rate, rail-to-rail input-output operational
amplifiers. These devices can operate up to
50mA output current and 20MHz bandwidth.
transients created on the power supply lines
when the MP5X21 load current changes
suddenly. This can damage the part.
Transients
In addition to the ripple and noise on the power
supplies, there are also transient voltage
changes. This can be caused by another device
on the same power supply suddenly drawing
current or suddenly stopping a current draw. The
design engineer should insure that there are no
damaging transients induced on the power
supply lines when the op amp suddenly changes
current delivery.
INPUT
The MP5X21 can operate with inputs from rail to
rail. It does this through the use of two differential
pairs. A traditional PNP differential pair is used
from 0.5V below the negative rail to 1V below the
positive rail. At that point the input is switched to
a NPN differential pair to operate up to 0.5V
above the positive rail. The transition from one
input differential pair to the other can cause
distortion. Inputs near the rails can also cause
distortion and degradation of other specifications.
LAYOUT
Ground Plane
Connect the opamp to a ground plane rather than
ground traces for very low impedance. If this is
not possible then make the ground traces as fat
and short as possible.
Decoupling
High performance devices such as the MP5X21,
with high slew rates and high currents, need
large decoupling capacitors. These should be
placed as close to the supply pins as possible.
Use ground and power planes to make these
decoupling capacitors as effective as possible. If
that is not realistic then make the ground and
power traces as thick and short as possible.
OUTPUT
Current Rating
The MP5X21 can sink or source 50mA. It can
provide high values of peak current, and much
reduced value of average current. When the
output voltages are near the rails the ability to
provide current will be reduced.
Output Power
Make sure that the rms power is such that the die
junction temperature will remain below 125ºC
Power Requirements
The MP5X21 operates from a voltage supply, of
±Vs and ground, or from a Vs split supply.
Single-ended voltage range is +3.2V to +20V.
PSRR and Noise
A common figure of merit is the PSRR (Power
Supply Rejection Ratio). The PSRR is a measure
of how much noise gets from the supply rails into
the output. Notice that the PSRR falls with
increasing frequency. In order to have good
PSRR the ripple voltages and frequencies of the
systems switching power supplies should be
measured. If the PSRR is not acceptable,
inductors can be inserted in series with the power
supply rails to provide improved PSRR. Also
make sure there are no
MP5121_MP5221_MP5421 Rev. 0.91
www.MonolithicPower.com
12/9/2009
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