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PRELIMINARY

POWER SUPPLY BYPASSING

Bypass capacitors to power supply terminals +Vs and –Vs

must be connected physically close to the pins to prevent lo-

cal parasitic oscillation in the output stage of the PA79. Use

electrolytic capacitors at least 1μF. Bypass the electrolytic

capacitors with high quality ceramic capacitors (X7R) 0.1μF

or greater.

SUPPLY CURRENT

The PA79 features a class A/B driver stage to drive the output

MOSFETs and an innovative input stage to achieve very high

slew rates. The supply current drawn by the PA79, even with

no load, varies with the slew rate of the output signal.

HEATSINKING

TBD

STABILITY

The PA79 is externally compensated and performance

can be tailored to the application. Use the graphs of small

signal response and power response as a guide. Due to the

innovative design of the PA79, two compensation networks

are required. The values of these components should be the

same to provide symmetric slew rate characteristics. The

compensation capacitor Cc must be rated at 500V working

voltage. NPO capacitors are recommended. The compensa-

tion networks CcRc must be mounted closely to the amplifier

pins x & y and z & w to avoid spurious oscillation.

The PA79 may require an external 33 pF capacitor (mini-

mum breakdown of 350V) between Cc- and –Vs to prevent

oscillations in the falling edge of the output. This capacitor

is provided with the evaluation kit. Please refer to EK60U

datasheet for details.

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.

SPECIAL CONSIDERATIONS

Itisveryimportanttorealizethatinordertoachieve

highfrequencyperformancetheheatsinktabhastobe

tiedtoastable,lowimpedancereference,i.e.powersup-

plyorGND.AnACconnectionthrougha0.1µFcapacitor

isalsosufficient.InternaltothePA79,theheatsinktab

iselectricallyisolatedtomorethan350V.Thismayhelp

allaysomeelectricalisolationconcernsintyingtheheat

sinktoVsorGND.

CURRENT LIMIT

For proper operation, the current limit resistor, Rlim, must

be connected as shown in the external connections diagram.

For maximum reliability and protection, the resistor should

be set as high as possible. The value of the resistor is calcu-

lated as follows, with Ilim in A; the maximum practical value

is 1500Ω.

Rlim = 0.7 / Ilim

SAFE OPERATING AREA

The MOSFET output stage of the PA79 is not limited by

second breakdown considerations as in bipolar output stages.

However there are still three distinct limitations:

1. Voltage with stand capability of the transistors.

2. Current handling capability of the die metallization.

3. Temperature of the output MOSFETS.

These limitations can be seen in the SOA (see Safe Operat-

ing Area graphs). Note that each pulse capability line shows

a constant power level (unlike second breakdown limitations

where power varies with voltage stress). These lines are

shown for a case temperature of 25°C and correspond to

thermal resistances of TBD°C/W for the PA79DK. Pulse stress

levels for other case temperatures can be calculated in the

same manner as DC power levels at different temperatures.

The output stage is protected against transient fly back by

the parasitic diodes of the output stage MOSFET structure.

However, for protection against sustained high energy fly back

external fast-recovery diodes must be used.

OPERATING

CONSIDERATIONS

PA79

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.

PA79U REV 2 FEBRUARY 2006 © 2006 Apex Microtechnology Corp.