OPERATING
CONSIDERATIONS
MP240
clamped to ±0.7V. This is sufficient overdrive to produce the
maximum power bandwidth. Some applications will also need
over-voltageprotectiondevicesconnectedtothepowersupply
rails. Unidirectional zener diode transient suppressors are
recommended. Thezenersclamptransientstovoltageswithin
thepowersupplyratingandalsoclamppowersupplyreversals
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.
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
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The MP240 has two ground pins (pins 2, 40). These pins
provide a return for the internal capacitive bypassing of the
small signal stages of the MP240. 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.
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BALANCING RESISTOR SELECTION (RS1-RS4)
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TheMP240usesparallelsetsofoutputtransistors.Toensure
that the load current is evenly shared among the transistors
externalbalancingresistorsRS1-RS4 arerequired. Tocalculate
the required value for each of the resistors use: R = 4.5/I2
,whereIisthemaximumexpectedoutputcurrent.Forexample,
with a maximum output current of 10A each balancing resistor
should be 0.045 ohms. Each resistor dissipates 1.125W at the
maximum current. Use a non-inductive 2W rated resistor. A
ready source for such resistors is the IRC resistor series LR
available from Mouser Electronics.
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SAFE OPERATING AREA
POWER SUPPLY BYPASSING
The MOSFET output stage of the MP240 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 diodes of the output stage MOSFET structure.
However, for protection against sustained high energy flyback
external fast-recovery diodes must be used.
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 MP240. Use
electrolytic capacitors at least 10µF per output amp required.
Bypass the electrolytic capacitors with high quality ceramic
capacitors 0.1µF or greater. In most applications power
supply terminals +VB and -VB will be connected to +VS and -VS
respectively. Although +VB and -VB are bypassed internally it
is recommended to bypass +VB and -VB with 0.1µF externally.
Additionally ground pins 2 and 40 must be connected to the
system signal ground.
COMPENSATION
The external compensation capacitor CC is connected to
pins 4 and 6. Unity gain stability can be achieved with CC =
680pF for a minimum phase margin of 60 degrees. At higher
gains more phase shift can usually be tolerated and CC can be
reduced resulting in higher bandwidth and slew rate. Use the
typical operating curves as a guide to select CC. A 200V NPO
(COG)typecapacitorisrequired.Boostoperationrequiresmore
compensation or higher gains than with normal operation due
to the increased capacitance of the output transistors when
the output signal swings close to the supply rails.
CURRENT LIMIT
Thetwocurrentlimitsenselinesaretobeconnecteddirectly
across the current limit sense resistor. For the current limit to
workcorrectlypin36mustbeconnectedtotheamplifieroutput
side and pin 35 connected to the load side of the current limit
resistor RLIM as shown in Figure 2. This connection will bypass
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OVERVOLTAGE PROTECTION
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AlthoughtheMP240canwithstanddifferentialinputvoltages
up to ±25V, in some applications additional external protection
may be needed. 1N4148 signal diodes connected anti-parallel
across the input pins is usually 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
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