P r o d u c t I n n o v a t i o n F r o m

PA07 • PA07A

4

PA07U

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.Cirrus.com for design tools that help

automate tasks such as calculations for stability, internal power

dissipation, current limit; heat sink selection; Apex Precision

Power’s completeApplication Notes library;Technical Seminar

Workbook; and Evaluation Kits.

saFeoPeratinGarea(soa)

The output stage of most power amplifiers has three distinct

limitations:

1. The current handling capability of the wire bonds.

2. The second breakdown effect which occurs whenever the

simultaneous collector current and collector-emitter voltage

exceed specified limits.

3. The junction temperature of the output transistors.

saFeoPeratinGareacurVes

The SOA curves combine the effect of these limits. For a

given application, the direction and magnitude of the output

current should be calculated or measured and checked against

the SOA curves. This is simple for resistive loads but more

complex for reactive and EMF generating loads. However,

the following guidelines may save extensive analytical efforts.

1. For DC outputs, especially those resulting from fault condi-

tions, check worst case stress levels against the new SOA

graph.

line. Make sure the load line does not cross the 0.5ms limit

and that excursions beyond any other second breakdown

line do not exceed the time label, and have a duty cycle of

no more than 10%.

For other waveform outputs, manual load line plotting is

recommended. Applications Note 22, SOA AND LOAD

LINES, will be helpful. A Spice type analysis can be very

useful in that a hardware setup often calls for instruments

or amplifiers with wide common mode rejection ranges.

Tc =

125°

C

THE

RMA

L

ste

ady

sta

te

SE

CO

ND

BR

EA

KD

OW

N

t =

1m

s

t =

5m

s

t =

0.5

ms

.2

.3

.4

.6

.8

1.0

1.5

2.0

3.0

4.0

5.0

10

15

20

25 30 35 40

50 60 70 80

100

Tc =

85°C

2. The amplifier can handle any reactive or EMF generating

load and short circuits to the supply rail or common if the

current limits are set as follows at T

SHORT TO ±V

S

SHORT TO

±V

S

C, L, OR EMF LOAD

COMMON

50V

.21A

.61A

40V

.3A

.87A

30V

.46A

1.4A

20V

.87A

2.5A

15V

1.4A

4.0A

These simplified limits may be exceeded with further analysis

using the operating conditions for a specific application.

3. The output stage is protected against transient flyback.

However, for protection against sustained, high energy

flyback, external fast-recovery diodes should be used.

tHerMaLsHutdownProtection

The thermal protection circuit shuts off the amplifier when

the substrate temperature exceeds approximately 150°C.This

allows 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 T

protect against short-term fault conditions that result in high

power dissipation within the amplifier. If the conditions that

cause thermal shutdown are not removed, the amplifier will

oscillate in and out of shutdown. This will result in high peak

power stresses, will destroy signal integrity and reduce the

reliability of the device.

currentLiMit

Proper operation requires the use of two current limit resis-

tors, connected as shown in the external connections diagram.

The minimum value for R

reliability it should be set as high as possible. Refer to the

“General Operating Considerations” section of the handbook

for current limit adjust details.

1

Note 1. Power Design is a self-extracting Excel spreadsheet

available free from www.Cirrus.com