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4

OPERATING

CONSIDERATIONS

MP108FD

This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.

MP108 REV B AUGUST 2004 © 2004 Apex Microtechnology Corp.

GENERAL

Please read Application Note 1 "General Operating

Considerations" 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 MP108FD has two ground pins (pins 3, 32). These pins

provide a return for the internal capacitive bypassing of the

small signal portions of the MP108FD. 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 MP108FD 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

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

required rated for the full supply voltage (200V).

OVERVOLTAGE PROTECTION

Although the MP108FD 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

applications 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 recommended. 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

must be connected physically close to the pins to prevent local

parasitic oscillation in the output stage of the MP108FD. 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

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 MP108FD at pins 8 and 25

Provide local bypass capacitors at pins 8 and 25. See the

external connections diagram on page 1.

34

33

+Vs

-Vs

OUT

3

32

GND

GND

+Vb

+Vs

-Vb

-Vs

Z1

Z2

-IN

+IN

Q1

Q2

34

OUT

11-13

20-22

33

27

28

IN

FIGURE 1

OVERVOLTAGE PROTECTION

FIGURE 2

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

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

BOOST OPERATION

With the boost feature the small signal stages of the amplifier

are operated at a higher supply voltages than the amplifierís

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

BACKPLATE GROUNDING

The substrate of the MP108FD 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.