There are two types of bridge mode operation that will be
covered in this section; dual (or split) supply and single supply.
The PA74 is well suited for both types of bridge mode operation.
If another vendor’s pin compatible part is to be compared to the
PA74, a close look at output swing and input common mode range
is in order. The features that make the PA74 an excellent choice
for bridge operation are not included in most other amplifiers. A
lack of common mode range may cause permanent damage to
other pin compatible parts and the inability of other amplifiers
to swing close to the supply rails may cause a lack of available
output voltage at the load as well as increase internal dissipation.
The circuit shown in Figure 4 is a dual supply bridge using
the “master-slave” configuration. Resistors R 6,7,8,9,14,15
and J1 should be shorts. The available output voltage swing
is Vss–(2*Vsat). If operating a PA74A at 3 Amps and 30 Volts
total supply this translates to:
V
AB
(max) = 30–(2*3.5) = 23
Of course this 23 volts may be applied in either direction across
Figure 4
Dual
Supply
Bridge
J1
1
R1
R5
C4 C5
V
IN
D1
R6
3
R14
D3
C11
C12
D4
A
R7
A
B
R8
B
R15
+V
S
R3
R2
the load. To set the gain of the circuit you must determine the
desired voltage across the load at Vin = full scale. Inserting these
values into the following equation will yield the ratio of R1 to R5.
(V
AB
/(2*Vin)) = R1/R5
The values of R 1,2,3, and 5 should be chosen such that
input bias current will not cause an error voltage that is unac-
ceptable. Set R2 equal to R3 to configure the slave amplifier
as a unity gain inverter.
Figure 5 shows a typical single supply bridge circuit for an AC
coupled input signal. DC coupled inputs may require a different
topology to accommodate proper gain and offset terms for a
desired transfer function.
The gain and output voltage capability for the single supply
bridge are determined the same way as the dual supply bridge
(see AN#2). The difference is the bias requirement for the slave
amplifier. The noninverting input of the slave amplifier should
be biased at mid supply, and must be bypassed.
+V
S
D2
R9
9
–V
S
–V
S
Figure 5
Single
Supply
Bridge
2
1
C1
R5
R1
C4 C5
+V
S
J1
R2
+V
S
+V
S
13
3
C15
4
R12
C16
R14
R17
D3
D4
D1
R6
A
A
R7
B
R8
B
D2
R3
R9
R15
EK21U
3
EK21
HS11 HEATSINK NOTE
P r o d u c t I n n o v a t i o nF r o m
The HS11 Heatsink is provided in this evaluation kit to
guar-
antee
adequate
thermal
design through heat removal from
the part under evaluation. Once maximum power dissipation
for the application is determined (refer to “General Operating
Considerations” and Application Note 11 in the Apex Precision
Power DATA BOOK), the final mechanical design will probably
require substantially less heatsinking.
Apex Precision Power makes no representation that the use
or interconnection of the circuits described herein will not in-
fringe on existing or future patent rights, nor do the descriptions
contained herein imply the granting of licenses to make, use or
sell equipment constructed in accordance therewith.
NOTES:Refer
to the following sections of the Apex Precision Power DATA BOOK as noted.
1. See Stability section of “General Operating Considerations.”
2. See “Gen. Operating Considerations,” and AN3 “Bridge Circuit Drives.”
3. See Power Supplies section of “General Operating Considerations.”
4. See “Parameter Definitions and Test Methods.”
5. See Amplifier Protection section of “Gen. Operating Considerations.”
ContACting CiRRUs LogiC sUPPoRt
For all Apex Precision Power product questions and inquiries, call toll free 800-546-2739 in North America.
For inquiries via email, please contact apex.support@cirrus.com.
International customers can also request support by contacting their local Cirrus Logic Sales Representative.
To find the one nearest to you, go to www.cirrus.com
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