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Evaluation of OPA569
Bridge-Tied-Load
Errol Leon and Thomas Kuehl
Precision Linear Analog Applications
February 3, 2016
1
OPA569 bridge-tied-load analysis outline
1) Simulation with Rset of 2.5kΩ and Rload of 10Ω
2) I-monitor pin limit as feedback
3) TINA-TI model verification with traditional feedback in place of IMONITOR with an Rload of 10Ω
4) Test set-up of customer’s circuit with traditional feedback with an Rload of 9.9Ω
5) Test set-up of customer’s circuit with traditional feedback with an Rload of 10.2Ω
6) Summary of analysis and recommendation
2
Simulation with Rset at 2.5kΩ and Rload at 10Ω
3
TINA-TI schematic of customer circuit with Rset 2.5kΩ and
Rload 10Ω
4
TINA-TI simulation of customer circuit with Rset 2.5kΩ and
Rload 10Ω
• Note that even though the simulation doesn’t show the I-flag condition, the
actual circuit does due to the IMONITOR limit
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IMONITOR pin limit as feedback
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Analysis of I-monitor pin limitations of OPA569
•
Even with Rset at 2.5kΩ, the limit of the IMONITOR pin still causes the I-flag condition.
•
From page 13 in the “current monitor” section of the data states:
“Additionally, the swing on the IMONITOR pin is smaller than the output swing.
When the amplifier is sourcing current, the voltage of the Current Monitor
pin must be two hundred millivolts less than the output voltage of the
amplifier. Conversely, when the amplifier is sinking current, the voltage of
the Current Monitor pin must be at least two hundred millivolts greater than
the output voltage of the amplifier.”
•
When condition is violated the current is no longer a linear representation of 1:475
Iload.
•
To overcome the IMONITOR pin limit, a traditional voltage feedback configuration
using a 2.5kΩ resistor was tested.
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Simulation using traditional voltage feedback
with Rset and Rf at 2.5kΩ, and Rload at 10Ω
8
TINA-TI schematic of customer circuit with Rset and Rf are
2.5kΩ and Rload is 10Ω using traditional voltage feedback
9
TINA-TI simulation of customer circuit with Rset and Rf are
2.5kΩ and Rload 10Ω using traditional voltage feedback
Note symmetry of Iload
10
Test setup of customer circuit with PCB
•
•
•
REF5020 voltage regulator was used to generate a Vref of 2V.
Rload is 10Ω, Rcl1 and Rcl2 are 14kΩ, Rset is 2.5kΩ.
Feedback resistor is 2.5kΩ.
Feedback
resistor
REF5020
11
Observed “current limit flag” pin and I-load in traditional
feedback configuration with an Rload of 9.9Ω
• “Current limit flag” does not trigger below specified limit and no clipping occurs
Vin
Vload +
Vload Current Limit
Flag
12
Observed “current limit flag” pin and I-load in traditional
feedback configuration with an Rload of 10.2Ω
• “Current limit flag” does not trigger below specified limit and no clipping occurs
Vin
Vload +
Vload Current Limit
Flag
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Summary of analysis and recommendation
•
IMONITOR must be 200mV from supply as specified on page 13 of datasheet. If violated,
IMONITOR no longer holds a linear relationship with Iload. This can cause the flag to trigger
early and may cause the output to latch at a supply rail.
•
The output is clamping when Vin approaches 0V in the customer’s application circuit.
This is due to exceeding the OPA569 output swing limit described on page 3 of the
datasheet.
•
Modifying the application circuit to use a traditional voltage feedback configuration
resolves the issue encountered when the IMONITOR swing limit is exceeded.
•
It is recommended for the intended input voltage range that a traditional voltage
feedback configuration be used in place of the IMONITOR configuration. A feedback
resistor value of 2.5kΩ was used for the verification.
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