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4

OPERATING

CONSIDERATIONS

PA96

GENERAL

Please read Application Note 1 “General Operating Con-

siderations” which covers stability, 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 Application Notes library, Technical Seminar Workbook,

and Evaluation Kits.

SPECIAL PRECAUTIONS

The PA96 operates with up 300V rail to rail voltage, and

delivers amperes of current. Precautions should be taken for

the safety of the user and the amplifier.

Although the non-operating common mode input range is rail

to rail, the differential input voltage must not exceed ±15 V.

Therefore; if the feedback ratio is less than 10, even if

caused by disconnecting a signal source , typical power turn

on transients can destroy the amplifier.

Similarly in a voltage follower application a large differential

transient can be generated if the slew rate of the input is greater

than that of the voltage follower.

Therefore it is prudent to clamp the input with series back

to back diodes as shown below.

If experimentally optimizing the compensation capacitor,

turn off the supplies and let them bleed to low voltage before

installing each new value. Otherwise internal current pulses of

up to 3 amps can be induced. Also, do you want your fingers

around 300V?

Essentially the full rail to rail power supply voltage may be

applied to the compensation capacitor. A 400V COG or Mica

capacitor is recommended.

POWER BANDWIDTH

The power bandwidth is 1/(π x the negative edge slew time).

The slew time is determined by the compensation capacitor,

load, and internal device capacitance; it is independent of

closed loop gain. The uncompensated power bandwidth is

cally increases to above 300KHz with no load.

COMPENSATION TABLE

The following table tabulates recommended compensation

capacitor values vs. gain. These values will typically result in

less than 2% overshoot and a -3db small signal bandwidth of

greater than 1MHz, except under operating conditions where

uncompensated gain bandwidth is too low to support a 1MHz

bandwidth. (See gain bandwidth vs. Plus power supply curves).

Note that other factors such as capacitance in parallel with

the feedback resistor may reduce circuit bandwidth from that

determined from the gain bandwidth curve.

Cc

Inverting Gain

From

To

150pf

1

2

51pf

2

5

33pf

5

10

22pf

10

20

10pf

20

50

5pf

50

100

None

100

up

Cc

Non-Inverting Gain

From

To

330 pf

1

2

150pf

2

3

51pf

3

6

33pf

6

10

22pf

10

20

10pf

20

50

5pf

50

100

None

100

up

CURRENT LIMIT

For proper operation the current limit resistor, Rcl, must

be connected as shown in the external connections diagram.

The minimum value is 0.2Ω, with a maximum practical value of

100Ω. For optimum reliability the resistor should be set as high

as possible. The value is calculated as I

the 0.68V is reduced by 2mV every °C rise in temperature.

Also note that the current limit can be set such that the SOA

is exceeded on a continuous basis. As an example if the current

limit was set at 1.5A and the supply was at 150V, a short to

ground would produce 225 watts internal dissipation, greatly

exceeding the 83 watt steady state SOA rating.

Under some conditions of load and compensation the am-

plifier may oscillate at a low level when current limit is active,

even though the amplifier is stable otherwise. The current

will be limited to the programmed value in this situation. To

minimize such occurrences, use a non-reactive resistor to

program current limit.

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.

PA96U REV B DECEMBER 2005 © 2005 Apex Microtechnology Corp.

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