APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739

4

OPERATING

CONSIDERATIONS

PA19 • PA19A

simple for resistive loads but more complex for reactive and

EMF generating loads. The following guidelines may save

extensive analytical efforts:

1. Capacitive and inductive loads up to the following maximums

are safe:

±V

S

CAPACITIVE LOAD

INDUCTIVE LOAD

40V

.1µF

11mH

30V

500µF

24mH

20V

2500µF

75mH

15V

∞

100mH

2. Safe short circuit combinations of voltage and current are

limited to a power level of 100W.

3. The output stage is protected against transient flyback.

However, for protection against sustained, high energy

flyback, external fast-recovery diodes should be used.

SUPPLY CURRENT

The PA19 features a class A/B driver stage to charge and

discharge gate capacitance of Q7 and Q19. As these currents

approach 0.5A, the savings of quiescent current over that of a

class A driver stage is considerable. However, supply current

drawn by the PA19, even with no load, varies with slew rate

of the output signal as shown below.

OUTPUT LEADS

Keep the output leads as short as possible. In the video

frequency range, even a few inches of wire have significant

inductances, raising the interconnection impedance and limit-

ing the output current slew rate. Furthermore, the skin effect

increases the resistance of heavy wires at high frequencies.

Multistrand Litz Wire is recommended to carry large video

currents with low losses.

THERMAL SHUTDOWN

The thermal protection circuit shuts off the amplifier when

the substrate temperature exceeds approximately 150°C. This

allows the heatsink selection to be based on normal operating

conditions while protecting the amplifier against excessive

junction temperature during temporary fault conditions.

Thermal protection is a fairly slow-acting circuit and therefore

does not protect the amplifier against transient SOA violations

(areas outside of the steady state boundary). It is designed to

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 complete Application Notes library; Technical Seminar

Workbook; and Evaluation Kits.

CURRENT LIMIT

Q2 (and Q25) limit output current by turning on and remov-

ing gate drive when voltage on pin 2 (pin 7) exceeds .65V dif-

ferential from the positive (negative) supply rail. With internal

resistors equal to 1.2Ω, current limits are approximately 0.5A

with no external current limit resistors. With the addition of

external resistors current limit will be:

.65V

I

+.54A

R

CL

To determine values of external current limit resistors:

.65V

R

I

PHASE COMPENSATION

At low gain settings, an external compensation capacitor is

required to insure stability. In addition to the resistive feedback

network, roll off or integrating capacitors must also be consid-

ered when determining gain settings. The capacitance values

listed in the external connection diagram, along with good

high frequency layout practice, will insure stability. Interpolate

values for intermediate gain settings.

SAFE OPERATING AREA (SOA)

The MOSFET output stage of this power operational ampli-

fier has two distinct limitations:

1. The current handling capability of the MOSFET geometry

and the wire bonds.

2. The junction temperature of the output MOSFETs.

The SOA curves combine the effect of these limits and allow

for internal thermal delays. For a given application, the direc-

tion and magnitude of the output current should be calculated

or measured and checked against the SOA curves. This is

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