Figure 3-4 shows the schematic of the phase-current sense subsystem using the functionally isolated modulator AMC0106M05 (U8) with a ±50mV linear input voltage range, and a 1mΩ, 3W shunt (R39). The 1mΩ shunt value determines that the linear input range is ±50A. The AMC0106M05 has a ±64mV input clipping range; therefore, the maximum current range is ±64A. The power dissipation in the shunt at 35A_RMS is 1.25W.

Figure 3-4 Inline Phase Current Sense Schematic with AMC0106M05 for Phase C

The differential anti-aliasing low-pass filter (R41 = 10Ω, R45 = 10Ω, C61 = 10nF) in front of the isolated modulator has a cutoff frequency of 795kHz and helps to improve the signal-to-noise performance of the signal path. The purpose of the low-pass filter is to attenuate high-frequency input noise below the desired noise level of the measurement. Without the input filter, noise close the sampling frequency (f_CLKIN), or multiples of the sampling frequency, are aliased to low-frequencies by the delta-sigma modulator and are passed through the digital low-pass filter. The capacitors C65 = 1nF and C66 = 1nF are optional and improve common-mode input voltage rejection at frequencies above 10MHz. C65 and C66 are sized 10 × smaller than C61. For best performance, make sure C65 and C66 values match better than 5%. Mismatches between C65 and C66 cause differential input errors during common-mode transients. NP0-type capacitors offer low temperature drift and are preferred for common-mode filtering. The analog supply AVDD is decoupled with a 100nF capacitor, C56. AVDD is supplied by one of two bootstrap supply options. The default option leverages the LMG2100R044 bootstrap supply with C40 = 4.7µF and a current-limit resistor R15 = 3Ω. The bootstrap diode is integrated into the LMG2100R044 GaN-FET. The AMC0106M05 typically draws 6.6mA from the AVDD supply. This configuration allows for operating at PWM frequencies from 10kHz to 100kHz with a maximum continuous duty cycle of a round 95%. See also the test results in the High Resolution, Small Form Factor Phase Current Sense for 48V Robotics and Servo Drives application note. The resistor R14 = 0Ω is a configuration option to use a separate bootstrap supply. The resistor consists of an ultra-fast rectifier diode D1, a 4.7µF capacitor C57 and a 3Ω current limit resistor R34, not populated with the default option. The digital supply DVDD is decoupled with the capacitors C58 = 2.2µF and C59 = 100nF. A series 0Ω resistor (R37) is a placeholder for an optional ferrite bead. Ferrite beads help reduce coupling of transient load current spikes into the 3.3V plane; therefore, improving EMI performance. A 50Ω series line termination resistor R40 at the AMC0106M05 DOUT pin improves signal integrity. An optional capacitor C62 = 33pF allows for slew rate reduction of the modulator output bit-stream signal to further reduce EMI. For more information on improving the digital interface from an isolated modulator to a microcontroller, see also the Achieving Better Signal Integrity with Isolated Delta-Sigma Modulators in Motor Drives and Clock Edge Delay Compensation With Isolated Modulators Digital Interface to MCUs application notes.