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4
OPERATING
CONSIDERATIONS
PA05 • PA05A
GENERAL
Please read Application Note 1 "General Operating Con-
siderations" which covers stability, supplies, heat sinking,
mounting, current limit, SOA interpretation, and specication
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
The two current limit sense lines are to be connected directly
across the current limit sense resistor. For the current limit to
work correctly, pin 11 must be connected to the amplifier out-
put side and pin 10 connected to the load side of the current
limit resistor, RCL, as shown in Figure 1. This connection will
bypass any parasitic resistances, RP formed by sockets and
solder joints as well as internal amplier losses. The current
limiting resistor may not be placed anywhere in the output
circuit except where shown in Figure 1. If current limiting is
not used, pins 10 and 11 must be tied to pin 7.
The value of the current limit resistor can be calculated as
follows:
SAFE OPERATING AREA (SOA)
The MOSFET output stage of this power operational ampli-
er 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.
NOTE: The output stage is protected against transient y-
back. However, for protection against sustained, high energy
yback, external fast-recovery diodes should be used.
The output stage thermal protection circuit engages when
junction temperatures reach approximately 175C. If the con-
dition remains that caused the shutdown, the amplier may
oscillate in and out of shutdown, creating high peak power
stresses reducing the reliability of the device.
SHUTDOWN OPERATION
To disable the output stage, pin 12 is connected to ground
via relay contacts or via an electronic switch. The switching
device must be capable of sinking 2mA to complete shutdown
and capable of standing off the supply voltage +VS. See Figure
2 for suggested circuits.
From an internal circuitry standpoint, shutdown is just a
special case of current limit where the allowed output current
is zero. As with current limit, however, a small current does
ow in the output during shutdown. A load impedance of 100
ohms or less is required to insure the output transistors are
turned off. Note that even though the output transistors are off
the output pin is not open circuited because of the shutdown
operating current.
BOOST OPERATION
With the VBOOST feature, the small signal stages of the ampli-
er are operated at higher supply voltages than the amplier’s
high current output stage. +VBOOST (pin 9), and –VBOOST (pin
5) are connected to the small signal circuitry of the amplier.
+VS (pin 8) and –VS (pin 6) are connected to the high current
output stage. An additional 5V on the VBOOST pins is sufcient
to allow the small signal stages to drive the output transistors
into saturation and improve the output voltage swing for extra
efcient operation when required. When close swings to the
supply rails is not required the +VBOOST and +VS pins must be
strapped together as well as the –VBOOST and –VS pins. The
boost voltage pins must not be at a voltage lower than the
VS pins.
COMPENSATION
The external compensation components CC and RC are con-
nected to pins 3 and 4. Unity gain stability can be achieved at
any compensation capacitance greater than 470 pF with at
least 60 degrees of phase margin. At higher gains, more phase
shift can be tolerated in most designs and the compensation
capacitance can accordingly be reduced, resulting in higher
bandwidth and slew rate. Use the typical operating curves as
a guide to select CC and RC for the application.
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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.
PA05U REV H AUGUST 2005 © 2005 Apex Microtechnology Corp.