AMC23C15DWVR datasheet(25/42 Pages) TI

25 / 42 page

7.3.3 Isolation Channel Signal Transmission

The AMC23C15 uses an on-off keying (OOK) modulation scheme, as shown in Figure 7-4, to transmit the

comparator output states across the SiO2-based isolation barrier. The transmit driver (TX) shown in the

Functional Block Diagram transmits an internally-generated, high-frequency carrier across the isolation barrier to

represent a digital one and does not send a signal to represent a digital zero.

The receiver (RX) on the other side of the isolation barrier recovers and demodulates the signal and provides the

data for the logic that drives the open-drain output buffers. The AMC23C15 transmission channel is optimized

to achieve the highest level of common-mode transient immunity (CMTI) and lowest level of radiated emissions

caused by the high-frequency carrier and RX/TX buffer switching.

Data

on High-side

Signal Across Isolation Barrier

Recovered Data

on Low-side

Figure 7-4. OOK-Based Modulation Scheme

7.3.4 Open-Drain Digital Outputs

The AMC23C15 has two open-drain outputs, one for each window comparator. As illustrated in Figure 7-1,

OUT1 is actively pulled low when |VIN| exceeds the threshold values defined by the voltage on the REF pin.

OUT2 is actively pulled low when |VIN| exceeds the threshold values defined by the internal 60-mV reference.

The open-drain outputs are diode-connected to the VDD2 supply, see the Functional Block Diagram, which

means the outputs cannot be pulled more than 500 mV above the VDD2 supply before significant current begins

to flow into the OUTx pins. In particular, the open-drain outputs are clamped to one diode voltage above ground

if VDD2 is at the GND2 level. This behavior is indicated by the gray shadings in Figure 7-5 through Figure 7-10.

On a system level, the CMTI performance of an open-drain signal line depends on the value of the pullup

resistor. During a common-mode transient event with a high slew rate (high dV/dt), the open-drain signal line

can be pulled low because of parasitic capacitive coupling between the high-side and the low-side of the printed

circuit board (PCB). The effect of the parasitic coupling on the signal level is a function of the pullup strength

and a lower value pullup resistor results in better CMTI performance. The AMC23C15 is characterized with a

relatively weak pullup resistor value of 10 kΩ to make sure that the specified CMTI performance is met in a

typical application with a 4.7 kΩ or lower pullup resistor.

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AMC23C15

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AMC23C15DWVR Datasheet(PDF) 25 Page - Texas Instruments