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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
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.
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 4 and use equations 3 and 4.
0.65 + (Vo * 0.014)
ICL = (3)
RCL
0.65 + (Vo * 0.014)
RCL = (4)
ICL
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
4 and ground. Use equations 4 and 5 with this new resistor
in the circuit.
Vo * 0.14
0.65 +
10.14 + RFO
ICL = (5)
RCL
Vo * 0.14
0.65 +
10.14 + RFO
RCL = (6)
ICL
Where:
RFO is in K ohms.
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
±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 12.5V below the
supply rail with ILIM = 10A or 27V 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
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.