Datasheet ADuM3190S (Analog Devices) - 9

THEORY OF OPERATION In the test circuits of the ADuM3190 (see Figure 3 through Figure 5), external supply voltages from 3 V to 20 V are provided to the VDD1 and VDD2 pins, and internal regulators provide 3.0 V to operate the internal circuits of each side of the ADuM3190. An internal precision 1.225 V reference provides the reference for the ±1% accuracy of the isolated error amplifier. UVLO circuits monitor the VDDx supplies to turn on the internal circuits when the 2.96 V rising threshold is met and to turn off the error amplifier outputs to a high impedance state when VDDx falls below 2.5 V. The op amp on the right side of the device has a noninverting +IN pin and an inverting −IN pin available for connecting a feedback voltage in an isolated dc-to-dc converter output, usually through a voltage divider. The COMP pin is the op amp output, which can be used to attach resistor and capacitor components in a compensation network. The COMP pin internally drives the Tx transmitter block, which converts the op amp output voltage into an encoded output that is used to drive the digital isolator transformer. On the left side of the ADuM3190, the transformer output PWM signal is decoded by the Rx block, which converts the signal into a voltage that drives an amplifier block; the amplifier block produces the error amplifier output available at the EAOUT pin. The EAOUT pin can deliver ±3 mA and has a voltage level between 0.4 V and 2.4 V, which is typically used to drive the input of a PWM controller in a dc-to-dc circuit. For applications that need more output voltage to drive their controllers, Figure 4 illustrates the use of the EAOUT2 pin output, which delivers up to ±1 mA with an output voltage of 0.6 V to 4.8 V for an output that has a pull-up resistor to a 5 V supply. If the EAOUT2 pull-up resistor connects to a 10 V to 20 V supply, the output is specified to a minimum of 5.0 V to allow use with a PWM controller requiring a minimum input operation of 5V. ACCURACY CIRCUIT OPERATION See Figure 3 and Figure 4 for stability of the accuracy circuits. The op amp on the right side of the ADuM3190, from the −IN pin to the COMP pin, has a unity-gain bandwidth (UGBW) of10 MHz. Figure 6, Bode Plot 1, shows a dashed line for the op amp alone and its 10 MHz pole. Figure 6 also shows the linear isolator alone (the blocks from the op amp output to the ADuM3190 output, labeled as the linear isolator), which introduces a pole at approximately 400 kHz. This total Bode plot of the op amp and linear isolator shows that the phase shift is approximately −180° from the −IN pin to the EAOUT pin before the crossover frequency. Because a −180° phase shift can make the system unstable, adding an integrator configuration, as shown in the test circuits in Figure 3 and Figure 4, consisting of a 2.2 nF capacitor and a 680 Ω resistor helps to make the system stable. In Figure 7, Bode Plot 2 with an integrator configuration added, the system crosses over 0 dB at approximately 100 kHz, but the circuit is more stable with a phase shift of approximately −120°, which yields a stable 60° phase margin. This circuit is used for accuracy tests only, not for real-world applications, because it has a 680 Ω resistor across the isolation barrier to close the loop for the error amplifier; this resistor causes leakage current to flow across the isolation barrier. For this test circuit only, GND1 must be connected to GND2 to create a return for the leakage current created by the 680 Ω resistor connection.