SBAS951B August 2019 – April 2020 AMC1336
Use Ohm's Law to calculate the minimum total resistance of the resistive dividers to limit the cross current to the desired values:
Consider the following two restrictions to choose the proper value of the resistors RDC3 and RAC3:
Use similar approach for calculation of the shunt resistor values RSHUNT and see the AMC1306M25 data sheet for further details.
Table 12 lists examples of nominal E96-series (1% accuracy) resistor values for systems using 600 V and 800 V on the DC bus.
|PARAMETER||600-V DC BUS||800-V DC BUS|
|Resistive divider resistor RDC1||3.01 MΩ||4.22 MΩ|
|Resistive divider resistor RDC2||3.01 MΩ||4.22 MΩ|
|Sense resistor RDC3||10 kΩ||10.5 kΩ|
|Resulting current through resistive divider IDC||99.5 µA||94.7 µA|
|Resulting voltage drop on sense resistor VRDC3||0.995 V||0.994 V|
Table 13 lists examples of nominal E96-series (1% accuracy) resistor values for systems using 230 V and 690 V on the input or output phases.
|PARAMETER||±400-VAC PHASE||±690-VAC PHASE|
|Resistive divider resistor RAC1||2.0 MΩ||3.48 MΩ|
|Resistive divider resistor RAC2||2.0 MΩ||3.48 MΩ|
|Sense resistor RAC3||10.0 kΩ||10.0 kΩ|
|Resulting current through resistive divider IAC||99.8 µA||99.0 µA|
|Resulting voltage drop on sense resistor VRAC3||±0.998 V||±0.990 V|
Use a power supply with a nominal voltage of 3.3 V for DVDD to directly connect all modulators to the microcontroller.
For modulator output bitstream filtering, a device from TI's TMS320F2807x family of low-cost microcontrollers (MCUs) or TMS320F2837x family of dual-core MCUs is recommended. These MCU families support up to eight channels of dedicated hardwired filter structures called sigma-delta filter modules (SDFMs) that significantly simplify system level design by offering two filtering paths per channel: one providing high accuracy results for the control loop and one that offers a fast response path for overcurrent detection. Use one of the pulse-width modulation (PWM) sources inside the MCU to generate the clock for the modulators and for easy synchronization of all feedback signals and the switching control of the gate drivers.
The application examples in Figure 50 and Figure 51 use a clock buffer to distribute the clock reference signal generated on one of the PWMx outputs of the MCU to all modulators used in the circuit and as a reference for the digital filters in the MCU. In this example, TI's CDCLVC1106 is used for this purpose. Each CDCLVC1106 output can drive a load of 8 pF that is sufficient to drive up to two modulator and up to four SDFM clock inputs.