The ISOW784x family of devices comprises a high-efficiency, low-emissions isolated DC-DC converter and four high-speed isolated data channels. Figure 34 shows the functional block diagram of the ISOW784x family of devices.
The integrated DC-DC converter uses switched mode operation and proprietary circuit techniques to reduce power losses and boost efficiency. Specialized control mechanisms, clocking schemes, and the use of a high-Q on-chip transformer provide high efficiency and low radiated emissions. The integrated transformer uses thin film polymer as the insulation barrier.
The VCC supply is provided to the primary power controller that switches the power stage connected to the integrated transformer. Power is transferred to the secondary side, rectified and regulated to either 3.3 V or 5 V, depending on the SEL pin. The output voltage, VISO, is monitored and feedback information is conveyed to the primary side through a dedicated isolation channel. The duty cycle of the primary switching stage is adjusted accordingly. The fast feedback control loop of the power converter ensures low overshoots and undershoots during load transients. Undervoltage lockout (UVLO) with hysteresis is integrated on the VCC and VISO supplies which ensures robust system performance under noisy conditions. An integrated soft-start mechanism ensures controlled inrush current and avoids any overshoot on the output during power up.
The integrated signal-isolation channels employ an ON-OFF keying (OOK) modulation scheme to transmit the digital data across a silicon-dioxide based isolation barrier. The transmitter sends a high-frequency carrier across the barrier to represent one state and sends no signal to represent the other state. The receiver demodulates the signal after signal conditioning and produces the output through a buffer stage. The signal-isolation channels incorporate advanced circuit techniques to maximize the CMTI performance and minimize the radiated emissions from the high frequency carrier and IO buffer switching. Figure 35 shows a functional block diagram of a typical signal isolation channel.
The ISOW784x family of devices is suitable for applications that have limited board space and require more integration. These devices are also suitable for very-high voltage applications, where power transformers meeting the required isolation specifications are bulky and expensive.
Figure 36 shows a conceptual detail of how the OOK scheme works.
Table 1 provides an overview of the device features.
|PART NUMBER(1)||CHANNEL DIRECTION||MAXIMUM DATA RATE||DEFAULT OUTPUT STATE||RATED ISOLATION(2)|
|ISOW7840||4 forward, 0 reverse||100 Mbps||High||5 kVRMS / 7071 VPK|
|ISOW7841||3 forward, 1 reverse||High|
|ISOW7842||2 forward, 2 reverse||High|
|ISOW7843||1 forward, 3 reverse||High|
|ISOW7844||0 forward, 4 reverse||High|
The ISOW784x family of devices use emissions reduction schemes for the internal oscillator and advanced internal layout scheme to minimize radiated emissions at the system level.
Many applications in harsh industrial environment are sensitive to disturbances such as electrostatic discharge (ESD), electrical fast transient (EFT), surge and electromagnetic emissions. These electromagnetic disturbances are regulated by international standards such as IEC 61000-4-x and CISPR 22. Although system-level performance and reliability depends, to a large extent, on the application board design and layout, the ISOW784x family of devices incorporates many chip-level design improvements for overall system robustness. Some of these improvements include:
The ISOW784x family of devices has built-in UVLO on the VCC and VISO supplies with positive-going and negative-going thresholds and hysteresis. When the VCC voltage crosses the positive-going UVLO threshold during power-up, the DC-DC converter initializes and the power converter duty cycle is increased in a controlled manner. This soft-start scheme limits primary peak currents drawn from the VCC supply and charges the VISO output in a controlled manner, avoiding overshoots. Outputs of the isolated data channels are in an indeterminate state until the VCC or VISO voltage crosses the positive-going UVLO threshold. When the UVLO positive-going threshold is crossed on the secondary side VISO pin, the feedback data channel starts providing feedback to the primary controller. The regulation loop takes over and the isolated data channels go to the normal state defined by the respective input channels or their default states. Design should consider a sufficient time margin (typically 10 ms with 10-µF load capacitance) to allow this power up sequence before valid data channels are accounted for system functionality.
When VCC power is lost, the primary side DC-DC controller turns off when the UVLO lower threshold is reached. The VISO capacitor then discharges depending on the external load. The isolated data outputs on the VISO side are returned to the default state for the brief time that the VISO voltage takes to discharge to zero.
The ISOW784x family of devices is protected against output overload and short circuit. Output voltage starts dropping when the power converter is not able to deliver the current demanded during overload conditions. For a VISO short-circuit to ground, the duty cycle of the converter is limited to help protect against any damage.
Thermal protection is also integrated to help prevent the device from getting damaged during overload and short-circuit conditions on the isolated output. Under these conditions, the device temperature starts to increase. When the temperature goes above 180°C, thermal shutdown activates and the primary controller turns off which removes the energy supplied to the VISO load, which causes the device to cool off. When the junction temperature goes below 150°C, the device starts to function normally. If an overload or output short-circuit condition prevails, this protection cycle is repeated. Care should be taken in the design to prevent the device junction temperatures from reaching such high values.
Table 2 lists the supply configurations for these devices.
|Shorted to VISO||5 V||5 V|
|Shorted to GND2 or floating||5 V||3.3 V|
|Shorted to GND2 or floating||3.3 V(1)||3.3 V(2)|
Table 3 lists the functional modes for ISOW784x devices.
|PU||H||H||Output channel assumes the logic state of its input|
|Open||Default||Default mode(2): When INx is open, the corresponding output channel assumes logic based on default output mode of selected version|