Evaluation of OPA569 Bridge-Tied-Load Errol Leon and Thomas Kuehl Precision Linear Analog Applications February 3,

OPA569 bridge-tied-load analysis outline 2 1)Simulation with R set of 2.5kΩ and R load of 10Ω 2)I-monitor pin limit as feedback 3)TINA-TI model verification with traditional feedback in place of I MONITOR with an R load of 10Ω 4)Test set-up of customer’s circuit with traditional feedback with an R load of 9.9Ω 5)Test set-up of customer’s circuit with traditional feedback with an R load of 10.2Ω 6)Summary of analysis and recommendation

Simulation with R set at 2.5kΩ and R load at 10Ω 3

TINA-TI schematic of customer circuit with R set 2.5kΩ and R load 10Ω 4

TINA-TI simulation of customer circuit with R set 2.5kΩ and R load 10Ω 5 Note that even though the simulation doesn’t show the I-flag condition, the actual circuit does due to the I MONITOR limit

I MONITOR pin limit as feedback 6

Analysis of I-monitor pin limitations of OPA569 7 Even with R set at 2.5kΩ, the limit of the I MONITOR pin still causes the I-flag condition. From page 13 in the “current monitor” section of the data states: “Additionally, the swing on the I MONITOR 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 I load. To overcome the I MONITOR pin limit, a traditional voltage feedback configuration using a 2.5kΩ resistor was tested.

Simulation using traditional voltage feedback with R set and R f at 2.5kΩ, and R load at 10Ω 8

TINA-TI schematic of customer circuit with R set and R f are 2.5kΩ and R load is 10Ω using traditional voltage feedback 9

TINA-TI simulation of customer circuit with R set and R f are 2.5kΩ and R load 10Ω using traditional voltage feedback 10 Note symmetry of I load

Test setup of customer circuit with PCB 11 REF5020 voltage regulator was used to generate a V ref of 2V. R load is 10Ω, R cl1 and R cl2 are 14kΩ, R set is 2.5kΩ. Feedback resistor is 2.5kΩ. REF5020 Feedback resistor

Observed “current limit flag” pin and I-load in traditional feedback configuration with an R load of 9.9Ω 12 V in V load + V load - Current Limit Flag “Current limit flag” does not trigger below specified limit and no clipping occurs

Observed “current limit flag” pin and I-load in traditional feedback configuration with an R load of 10.2Ω 13 “Current limit flag” does not trigger below specified limit and no clipping occurs V in V load + V load - Current Limit Flag

Summary of analysis and recommendation 14 I MONITOR must be 200mV from supply as specified on page 13 of datasheet. If violated, I MONITOR no longer holds a linear relationship with I load. 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 I MONITOR 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 I MONITOR configuration. A feedback resistor value of 2.5kΩ was used for the verification.