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MP111

POWER OPERATIONAL AMPLIFIER

Apex Microtechnology

MP111 Datasheet PDF : 4 Pages

1 2 3 4

OPERATING

CONSIDERATIONS

MP111

GENERAL

Please read Application Note 1 "General Operating Consid-

erations" which covers stability, power supplies, heat sinking,

mounting, current limit, SOA interpretation, and speciﬁcation

interpretation. Visit www.apexmicrotech.com for design tools

that help automate tasks such as calculations for stability,

internal power dissipation, current limit, heat sink selection,

Apex's complete Application Notes library, Technical Seminar

Workbook and Evaluation Kits.

GROUND PINS

The MP111 has two ground pins (pins 3, 32). These pins

provide a return for the internal capacitive bypassing of the

small signal portions of the MP111. The two ground pins are

not connected together on the substrate. Both of these pins

are required to be connected to the system signal ground.

SAFE OPERATING AREA

The MOSFET output stage of the MP111 is not limited by

second breakdown considerations as in bipolar output stages.

Only thermal considerations and current handling capabilities

limit the SOA (see Safe Operating Area graph on previous page).

The output stage is protected against transient ﬂyback by the

parasitic body diodes of the output stage MOSFET structure.

However, for protection against sustained high energy ﬂyback

external fast-recovery diodes must be used.

COMPENSATION

The external compensation capacitor CC is connected

between pins 5 and 6. Unity gain stability can be achieved with

any capacitor value larger than 100pF for a minimum phase

margin of 45 degrees. At higher gains more phase shift can

usually be tolerated in most designs and the compensation

capacitor value can be reduced resulting in higher bandwidth

and slew rate. Use the typical operating curves as a guide to

select CC for the application. An NPO (COG) type capacitor

is required rated for the full supply voltage (100V).

OVERVOLTAGE PROTECTION

Although the MP111 can withstand differential input voltages

up to ±25V, additional external protection is recommended. In

most applications 1N4148 signal diodes connected anti-parallel

across the input pins is sufﬁcient. In more demanding applica-

tions where bias current is important diode connected JFETs

such as 2N4416 will be required. See Q1 and Q2 in Figure

1. In either case the differential input voltage will be clamped

to ±0.7V. This is usually sufﬁcient overdrive to produce the

maximum power bandwidth. Some applications will also need

over voltage protection devices connected to the power supply

rails. Unidirectional zener diode transient suppressors are rec-

ommended. The zeners clamp transients to voltages within the

power supply rating and also clamp power supply reversals to

ground. Whether the zeners are used or not the system power

supply should be evaluated for transient performance including

power-on overshoot and power-off polarity reversals as well

as line regulation. See Z1 and Z2 in Figure 1.

POWER SUPPLY BYPASSING

Bypass capacitors to power supply terminals +VS and -VS

must be connected physically close to the pins to prevent local

parasitic oscillation in the output stage of the MP111. Use

electrolytic capacitors at least 10µF per output amp required.

Bypass the electrolytic capacitors with high quality ceramic

capacitors (X7R) 0.1µF or greater. In most applications power

supply terminals +Vb and -Vb will be connected to +VS and

-VS respectively. Supply voltages +Vb and -Vb are bypassed

internally but both ground pins 3 and 32 must be connected to

the system signal ground to be effective. In all cases power to

the buffer ampliﬁer stage of the MP111 at pins 8 and 25 must

be connected to +Vb and

-Vb at pins 4 and 30 respectively. Provide local bypass

capacitors at pins 8 and 25. See the external connections

diagram on page 1.

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FIGURE 1

FIGURE 2

OVERVOLTAGE PROTECTION 4 WIRE CURRENT LIMIT

CURRENT LIMIT

The two current limit sense lines are to be connected directly

across the current limit sense resistor. For the current limit to

work correctly pin 28 must be connected to the ampliﬁer output

side and pin 27 connected to the load side of the current limit

resistor RLIM as shown in Figure 2. This connection will bypass

any parasitic resistances RP, formed by socket and solder joints

as well as internal ampliﬁer losses. The current limiting resistor

may not be placed anywhere in the output circuit except where

shown in Figure 2. The value of the current limit resistor can

be calculated as follows: RLIM = .65/ILIMIT

BOOST OPERATION

With the boost feature the small signal stages of the ampliﬁer

are operated at a higher supply voltages than the ampliﬁer's

high current output stage. +Vb (pins 4,8) and -Vb (pins 25,30)

are connected to the small signal stages and +VS (pins 14-16)

and -VS (pins 17-19) are connected to the high current output

stage. An additional 10V on the +Vb and -Vb pins is sufﬁcient

to allow the small signal stages to drive the output stage into

the triode region and improve the output voltage swing for extra

efﬁcient operation when required. When the boost feature is

not needed +VS and -VS are connected to the +Vb and -Vb

pins respectively. The +Vb and -Vb pins must not be operated

at supply voltages less than +VS and -VS respectively.

BACKPLATE GROUNDING

The substrate of the MP111 is an insulated metal substrate.

It is required that it be connected to signal ground. Connect pin

2 (back plate) to signal ground. The back plate will then be AC

grounded to signal ground through a 1µF capacitor.

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