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PA12
(Rev.:2005)

POWER OPERATIONAL AMPLIFIERS

Apex Microtechnology

PA12 Datasheet PDF : 4 Pages

1 2 3 4

OPERATING

CONSIDERATIONS

PA12 • PA12A

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.

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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

±VS

ILIM = 5A

ILIM = 10A ILIM = 5A ILIM = 10A

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 8V below the

supply rail with ILIM = 15A or 25V below the supply rail with ILIM

= 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

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

current limits are set as follows at TC = 25°C:

SHORT TO ±VS

SHORT TO

±VS

C, L, OR EMF LOAD

COMMON

50V

.30A

2.4A

40V

.58A

2.9A

35V

.87A

3.7A

30V

1.5A

4.1A

25V

2.4A

4.9A

20V

2.9A

6.3A

15V

4.2A

8.0A

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 the Power

Design spreadsheet (Excel) 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 7 open and use equations

1 and 2.

RCL = 0.65/LCL

(1)

ICL = 0.65/RCL

(2)

Where:

ICL is the current limit in amperes.

RCL is the current limit resistor in ohms.

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 7 and use equations 3 and 4.

0.65 + (Vo * 0.014)

ICL =

RCL

(3)

0.65 + (Vo * 0.014)

RCL =

ICL

(4)

Where:

Vo is the output voltage in volts.

Most designers start with either equation 1 to set RCL for the

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

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 (RFO) between pin

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

in the circuit.

0.65 + Vo * 0.14

ICL =

10.14 + RFO

RCL

(5)

RCL =

0.65 +

Vo * 0.14

10.14 + RFO

ICL

(6)

Where:

RFO is in K ohms.

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