Evaluation of OPA569 Bridge-Tied-Load Errol Leon and Thomas Kuehl Precision Linear Analog Applications January 26,

OPA569 bridge-tied-load analysis outline 2 1)Statement of customer issue 2)Comparison of TIPD103 and customer circuit 3)I-monitor pin limitations as feedback and R max 4)TinaTI model verification of output swing limitation and R load 5)Test set-up of customer’s circuit varying R load 6)Summary of Analysis and Recommendations

Statement of customer issue 3

4 Customer is using OPA569 in applications configuration used in TIPD103, “Bridge-Tied-Load (BTL) Voltage-to -current (VI) Converter.” The following changes in the TIPD103 were made to fit customers needs:  Rload is 10Ω  Rcl1 and Rcl2 are 14kΩ (sets “current limit flag” to trigger at 800mA)  Rset is equivalent to 2kΩ (sets maximum output current ±400mA)  Vref is 2.048V  Vin range from 0V-4.096V When an input voltage generates close to ± 360mA, the “current limit flag” on the OPA569 triggers. When an input voltage generates the maximum value ± 400mA the “current limit flag” is continuous triggered.

Comparison of TIPD103 and customer configuration 5

Comparison of different circuit configuration 6 The following three configurations were built:  Customer circuit in bread board configuration with 10Ω load  Customer circuit in PCB configuration with 10Ω load  TIPD103 configuration with 2.3Ω load

Bread board configuration of customer setup 7

Observed “current limit flag” pin and I-load in bread board circuit configuration 8 I-limit flag triggering at Vin approaches ground. V in V load + V load - Current Limit Flag Triggers at 120mV input

Observed “current limit flag” pin and I-load in bread board circuit configuration- Zoomed in 9 “Current limit flag” triggering for 38us when input signal approaches ground (0 volts). Trigger time of “current limit flag” V in V load + V load - Current Limit Flag

Test setup of customer circuit with PCB 10 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 2kΩ. REF5020

Analysis of “current limit flag” pin and I-load of customer’s circuit with PCB 11 Triggers at 120mV input V in V load + V load - Current Limit Flag

Analysis of “current limit flag” pin and I-load of customer circuit – Zoomed in 12 Trigger time of “current limit flag” V in V load + V load - Current Limit Flag

Test setup of original TIPD103 circuit with PCB 13

Analysis of “current limit flag” pin and I-load of TIPD Note R load is 2.1Ω as specified in TIPD103 application “Current limit flag” does not trigger at currents below its specified limit of 2A V in V load + V load - Current Limit Flag

I-monitor pin limitations as feedback and R load (max) 15

Analysis of I-monitor pin limitations of OPA 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. This is why TIPD103 voltage range is from 0.5V-4.5V. Output is clamping when Vin approaches 0V. This is due to output swing limit of the OPA569 from page 3 of the datasheet. The result is a higher I load current of 480mA when V in is 0V and 400mA when V in is 4.098V. Using max current of 480mA and V comp of 4.75V in equation 12 in TIPD103, the maximum R load value calculated is approximately 9.8Ω. A 10Ω load violates the R max based on the maximum load compliance voltage.

TINA-TI model verification of output swing limit and R load 17

DC analysis of TINA-TI schematic using customer circuit with 10Ω load 18 The input voltage 4.096VDC I load is approximately 404mA

DC analysis of TINA-TI schematic using customer circuit with 10Ω load 19 The input voltage 0VDC I load is approximately 480mA

TINA-TI schematic of customer circuit with 10Ω load 20 The input voltage 0V-4.096V R load is 10Ω

TINA-TI simulation of customer circuit with 10Ω load 21

TINA-TI schematic of customer circuit with 8Ω load 22 The input voltage range from 0V V R load is 8Ω which is within R max of 9.8Ω

TINA-TI simulation of customer circuit with 8Ω load 23

TinaTI schematic of customer circuit with 8.5Ω load 24 The input voltage range from 0V V R load is 8.5Ω which is within R max of 9.8Ω

TINA-TI simulation of customer circuit with 8.5Ω load 25

Test setup of customer’s circuit varying R load 26

Analysis of output swing and “current limit flag” for customer’s circuit with a R load of 9.4Ω 27 This condition violates output swing limit and clipping occurs.

Analysis of output swing and “current limit flag” for customer’s circuit with a R load of 9.1Ω 28 This condition just violates output swing limit and slight clipping occurs.

Analysis of output swing and “current limit flag” for customer’s circuit with a Rload of 8.1Ω 29 This condition does not violate output swing limits and no clipping occurs.

Summary of analysis 30

Summary 31 Verified TIPD103 “current limit flag”, the pin triggers at the correct current set by R cl1 and R cl2. Early trigger was identified on the customers configuration with a 10Ω load where the “current limit flag” would trigger low for 30-60us. 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 the OPA569 output swing limit described on page 3 of the datasheet. The result of doing so is a higher I load current of 480mA when V in is 0V, and 400mA when V in is 4.098V. Applying a max current value of 480mA and V comp of 4.75V in equation 12 in TIPD103, results in a calculated maximum R load value of approximately 9.8Ω. When using a recommended R max of 8.1Ω, the peak-to-peak V load waveforms do not exhibit any clipping.