Evaluation of OPA569
Bridge-Tied-Load
Errol Leon and Thomas Kuehl
Precision Linear Analog Applications
January 26, 2016
1
OPA569 bridge-tied-load analysis outline
1)
Statement of customer issue
2)
Comparison of TIPD103 and customer circuit
3)
I-monitor pin limitations as feedback and Rmax
4)
TinaTI model verification of output swing limitation and Rload
5)
Test set-up of customer’s circuit varying Rload
6)
Summary of Analysis and Recommendations
2
Statement of customer issue
3
Statement of customer issue
•
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.
4
Comparison of TIPD103 and customer
configuration
5
Comparison of different circuit configuration
•
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
6
Bread board configuration of customer setup
7
Observed “current limit flag” pin and I-load in bread board
circuit configuration
• I-limit flag triggering at Vin approaches ground.
Vin
Vload +
Vload -
Current Limit
Flag
Triggers at
120mV input
8
Observed “current limit flag” pin and I-load in bread board
circuit configuration- Zoomed in
• “Current limit flag” triggering for 38us when input signal approaches
ground (0 volts) .
Vin
Trigger time
of “current
limit flag”
Vload +
Vload Current Limit
Flag
9
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 2kΩ.
REF5020
10
Analysis of “current limit flag” pin and I-load of customer’s
circuit with PCB
Vin
Vload +
Vload -
Current Limit
Flag
Triggers at
120mV input
11
Analysis of “current limit flag” pin and I-load of customer
circuit – Zoomed in
Vin
Trigger time
of “current
limit flag”
Vload +
Vload Current Limit
Flag
12
Test setup of original TIPD103 circuit with PCB
13
Analysis of “current limit flag” pin and I-load of TIPD103
• Note Rload is 2.1Ω as specified in TIPD103 application
• “Current limit flag” does not trigger at currents below its specified limit of 2A
Vin
Vload +
Vload -
Current Limit
Flag
14
I-monitor pin limitations as feedback and
Rload(max)
15
Analysis of I-monitor pin limitations of OPA569
•
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.
•
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 Iload current of 480mA when Vin is 0V and
400mA when Vin is 4.098V.
•
Using max current of 480mA and Vcomp of 4.75V in equation 12 in TIPD103, the maximum Rload
value calculated is approximately 9.8Ω.
•
A 10Ω load violates the Rmax based on the maximum load compliance voltage.
16
TINA-TI model verification of output swing limit
and Rload
17
DC analysis of TINA-TI schematic using customer circuit with
10Ω load
•
The input voltage 4.096VDC
•
Iload is approximately 404mA
18
DC analysis of TINA-TI schematic using customer circuit with
10Ω load
•
The input voltage 0VDC
•
Iload is approximately 480mA
19
TINA-TI schematic of customer circuit with 10Ω load
•
The input voltage 0V-4.096V
•
Rload is 10Ω
20
TINA-TI simulation of customer circuit with 10Ω load
21
TINA-TI schematic of customer circuit with 8Ω load
•
The input voltage range from 0V- 4.096V
•
Rload is 8Ω which is within Rmax of 9.8Ω
22
TINA-TI simulation of customer circuit with 8Ω load
23
TinaTI schematic of customer circuit with 8.5Ω load
•
The input voltage range from 0V- 4.096V
•
Rload is 8.5Ω which is within Rmax of 9.8Ω
24
TINA-TI simulation of customer circuit with 8.5Ω load
25
Test setup of customer’s circuit varying Rload
26
Analysis of output swing and “current limit flag” for
customer’s circuit with a Rload of 9.4Ω
• This condition violates output swing limit and clipping occurs.
27
Analysis of output swing and “current limit flag” for
customer’s circuit with a Rload of 9.1Ω
• This condition just violates output swing limit and slight clipping occurs.
28
Analysis of output swing and “current limit flag” for
customer’s circuit with a Rload of 8.1Ω
• This condition does not violate output swing limits and no clipping occurs.
29
Summary of analysis
30
Summary
•
Verified TIPD103 “current limit flag”, the pin triggers at the correct current set by Rcl1 and Rcl2.
•
Early trigger was identified on the customers configuration with a 10Ω load where the “current limit
flag” would trigger low for 30-60us.
•
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
the OPA569 output swing limit described on page 3 of the datasheet. The result of doing so is a
higher Iload current of 480mA when Vin is 0V, and 400mA when Vin is 4.098V.
•
Applying a max current value of 480mA and Vcomp of 4.75V in equation 12 in TIPD103, results in a
calculated maximum Rload value of approximately 9.8Ω.
•
When using a recommended Rmax of 8.1Ω, the peak-to-peak Vload waveforms do not exhibit any
clipping.
31