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4

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

45V

.43A

3.0A

40V

.65A

3.4A

35V

1.0A

3.9A

30V

1.7A

4.5A

25V

2.7A

5.4A

20V

3.4A

6.7A

15V

4.5A

9.0A

These simplified limits may be exceeded with further analysis

using the operating conditions for a specific application.

CURRENT LIMITING

Refer to Application Note 9, "Current Limiting", for details of

both fixed and foldover current limit operation. Visit the Apex web

site at www.apexmicrotech.com for a copy of Power_design.

exe which plots current limits vs. steady state SOA. Beware that

current limit should be thought of as a +/–20% function initially

and varies about 2:1 over the range of –55°C to 125°C.

For fixed current limit, leave pin 4 open and use equations

1 and 2.

R

(1)

I

(2)

Where:

I

R

For certain applications, foldover current limit adds a slope

to the current limit which allows more power to be delivered

to the load without violating the SOA. For maximum foldover

slope, ground pin 4 and use equations 3 and 4.

0.65 + (Vo * 0.014)

I

(3)

R

CL

0.65 + (Vo * 0.014)

R

(4)

I

CL

Where:

Vo is the output voltage in volts.

Most designers start with either equation 1 to set R

desired current at 0v out, or with equation 4 to set R

maximum output voltage. Equation 3 should then be used to

plot the resulting foldover limits on the SOA graph. If equa-

tion 3 results in a negative current limit, foldover slope must

be reduced. This can happen when the output voltage is the

opposite polarity of the supply conducting the current.

In applications where a reduced foldover slope is desired,

this can be achieved by adding a resistor (R

4 and ground. Use equations 4 and 5 with this new resistor

in the circuit.

Vo * 0.14

0.65 +

10.14 + R

FO

I

(5)

R

CL

Vo * 0.14

0.65 +

10.14 + R

FO

R

(6)

I

CL

Where:

R

OPERATING

CONSIDERATIONS

PA13

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.

SAFE OPERATING AREA (SOA)

The output stage of most power amplifiers has three distinct

limitations:

1. The current handling capability of the transistor geometry

and the wire bonds.

2. The second breakdown effect which occurs whenever the

simultaneous collector current and collector-emitter voltage

exceeds specified limits.

3. The junction temperature of the output transistors.

The SOA curves combine the effect of all limits for this Power

Op Amp. 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. Capacitive and dynamic* inductive loads up to the following

maximum are safe with the current limits set as specified.

CAPACITIVE LOAD

INDUCTIVE LOAD

±V

S

I

I

I

I

50V

200µF

125µF

5mH

2.0mH

40V

500µF

350µF

15mH

3.0mH

35V

2.0mF

850µF

50mH

5.0mH

30V

7.0mF

2.5mF

150mH

10mH

25V

25mF

10mF

500mH

20mH

20V

60mF

20mF

1,000mH

30mH

15V

150mF

60mF

2,500mH

50mH

*If the inductive load is driven near steady state conditions,

allowing the output voltage to drop more than 12.5V below the

supply rail with I

= 5A while the amplifier is current limiting, the inductor must

be capacitively coupled or the current limit must be lowered

to meet SOA criteria.

2. The amplifier can handle any EMF generating or reactive

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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.

PA13U REV K NOVEMBER 2003 © 2003 Apex Microtechnology Corp.