erations" which covers stability, supplies, heat sinking, mount-
ing, current limit, SOA interpretation, and specification inter-
pretation. Visit www.apexmicrotech.com for design tools that
help automate tasks such as calculations for stability, internal
power dissipation, current limit and heat sink selection. The
"Application Notes" and "Technical Seminar" sections contain
a wealth of information on specific types of applications. Pack-
age outlines, heat sinks, mounting hardware and other acces-
sories are located in the "Packages and Accessories" section.
Evaluation Kits are available for most Apex product models,
consult the "Evaluation Kit" section for details. For the most
current version of all Apex product data sheets, visit
www.apexmicrotech.com.
OPERATING
CONSIDERATIONS
THERMAL CONSIDERATIONS
Although R
θJC
is the same for PA28/29 there are differences
in the thermal interface between case and heatsink which will
limit power dissipation capability. Thermal grease or an Apex
TW03 thermal washer, R
θCS
= .1-.2°C/W, is the only recom-
mended interface for the PA28. The PA29 may require a
thermal washer which is electrically insulating since the tab is
tied to –V
S
. This can result in thermal impedances for R
θCS
of
up to 1°C/W or greater.
ADDITIONAL PA29 PIN FUNCTIONS
V
BOOST
SOA
OUTPUT CURRENT, Io(A)
5
4
3
2
ED
AD
ED
LO
AD
NE
LO
H
,O
OT
CH
,B
EA
CH
EA
+V
S
s
1m
– IN
+IN
OUT
I
SENSE
R
S
1
1
2
3
4
5
10
20
30 40
– V
S
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, Vs – Vo(V)
FIGURE 2. PA29 EQUIVALENT SCHEMATIC (ONE CHANNEL)
V
BOOST
SAFE OPERATING AREA (SOA)
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.
NOTE: For protection against sustained, high energy flyback,
external fast-recovery diodes should be used.
The V
BOOST
pin is the positive terminal for the load of the
second stage of the amplifier. When that terminal is connected
to a voltage greater than +V
S
it will provide more drive to the
upper output transistor, which is a darlington connected emit-
ter follower. This will better saturate the output transistor.
When V
BOOST
is about 5 Volts greater than +V
S
the positive
output can swing 0.5 Volts closer to the rail. This is as much
improvement as is possible.
V
BOOST
pin requires approximately 4-6mA of current. Dy-
namically it represents 1K
Ω
impedance. The maximum volt-
age that can be applied to V
BOOST
is 40 volts with respect to –
V
S
. There is no limit to the difference between +V
S
and V
BOOST
.
MONOLITHIC AMPLIFIER
STABILITY CONSIDERATIONS
All monolithic power op amps use output stage topologies
that present special stability problems. This is primarily due to
non-complementary (both devices are NPN) output stages
with a mismatch in gain and phase response for different
polarities of output current. It is difficult for the op amp manu-
facturer to optimize compensation for all operating conditions.
The recommended R-C network of 1 ohm in series with
0.1µF from output to AC common (ground or a supply rail, with
adequate bypass capacitors) will prevent local output stage
oscillations.
APEX MICROTECHNOLOGY CORPORATION
• 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739
OPERATING
CONSIDERATIONS
PA28/29 • PA28A/29A
D
B2
+VS
PA29
+V
S
≤
20V
D
B1
7
3
10
C
B1
PA29A
5
C
B2
PA29B
8
SPEAKER
VREF
VIN
B
R
RL
A
RFB
RIN
IL
R
FIGURE 3. SIMPLE BOOTSTRAPPING IMPROVES POSITIVE
OUTPUT SWING. CONNECT PINS 3 AND 10 TO V
S
IF NOT
USED. TYPICAL CURRENTS ARE 12
m
A EACH.
RS
RS
RIN
RS
Figure 3 shows a bootstrap which dynamically couples the
output waveform onto the V
BOOST
pin. This causes V
BOOST
to
swing positive from it's initial value, which is equal to +V
S
-0.7 V
(one diode drop), an amount equal to the output. In other
words, if V
BOOST
was initially 19.3, and the output swings
positive 18 Volts, the voltage on the V
BOOST
pin will swing to 19.3
-0.7 + 18 or 36.6. The capacitor needs to be sized based on a
1K
Ω
impedance and the lowest frequency required by the
circuit. For example, 20Hz will require > 8uF.
–VS OR GND
VREF
FIGURE 4. I
SENSE
TRANSCONDUCTANCE BRIDGING
AMPLIFIER
I
SENSE
The I
SENSE
pin is in series with the negative half of the output
stage only. Current will flow through this pin only when nega-
tive current is being outputted. The current that flows in this pin
is the same current that flows in the output (if –1A flows in the
output, the I
SENSE
pin will have 1A of current flow, if +1A flows
in the output the I
SENSE
pin will have 0 current flow).
The resistor choice is arbitrary and is selected to provide
whatever voltage drop the engineer desires, up to a maximum
of 1.0 volt. However, any voltage dropped across the resistor
will subract from the swing to rail. For instance, assume a +/–
12 volt power supply and a load that requires +/–1A. With no
current sense resistor the output could swing +/–10.2 volts. If
a 1
Ω
resistor is used for current sense (which will drop 1 Volt
at 1 Amp) then the output could swing +10.2, –9.2 Volts.
Figure 4 shows the PA29 I
SENSE
feature being used to obtain
a Transconductance function. In this example, amplifier "A" is
the master and amplifier "B" is the slave. Feedback from
sensing resistors R
S
is applied to the summing network and
scaled to the inverting input of amplifier "A" where it is com-
pared to the input voltage. The current sensing feedback
imparts a Transconductance feature to the amplifiers transfer
function. In other words, the voltage developed across the
sensing resistors is directly proportional to the output current.
Using this voltage as a feedback source allows expressing the
gain of the circuit in amperes vs input voltage. The transfer
funcion is approximately:
I
L
+ (V
IN
– V
REF
) =
R
IN
R
FB
*
R
S
In the illustration, resistors R
IN
, R
FB
and R
S
determine gain.
MOUNTING PRECAUTIONS
1. Always use a heat sink. Even unloaded, the PA29 can
dissipate up to 3.6 watts. A thermal washer or thermal
grease should always be used.
2. Avoid bending the leads. Such action can lead to internal
damage.
3. Always fasten the tab to the heat sink before the leads are
soldered to fixed terminals.
4. Strain relief must be provided if there is any probability of
axial stress to the leads.
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.
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