MP5121_MP5221_MP5421 –High Speed, single-supply Rail-to-Rail Input-Output Op Amps

MP5121_MP5221_MP5421 Rev. 0.91

www.MonolithicPower.com

9

12/9/2009

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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.

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.

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

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.

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.