Introduction

A NOVOSENSE isolator or isolated amplifier delivers its specified performance only when the layout across the barrier is designed with care. The device itself provides the capacitive barrier and the high common-mode transient immunity, but the creepage, the clearance and the decoupling decide whether the isolation holds and the signal stays clean. This application note explains the practical rules for applying a NOVOSENSE isolator or amplifier on a real product board.

Capacitive Isolation

NOVOSENSE builds its isolators on a capacitive barrier. The digital signal is modulated with a radio-frequency carrier at the transmitter and demodulated at the receiver, so the signal crosses the barrier without an optical LED and without a magnetic transformer. This gives a high noise immunity, a low radiated emission and a high common-mode transient immunity, and it removes the LED that ages and dims in an opto-coupled device, so the life of the barrier is not limited by an optical element.

The Common-Mode Transient Immunity

The CMTI is the rate of common-mode voltage change the isolator tolerates before the data is corrupted. In a motor drive or an inverter the grounds move by tens or hundreds of kilovolts per microsecond, and a low CMTI would turn that movement into an error, so the NOVOSENSE family provides a high CMTI that keeps the interface reliable in a fast-switching system. Confirm the CMTI against the worst-case rate in your application.

Layout Across the Barrier

The isolation is only as good as the layout, so provide the required creepage and clearance under the package and around every pin that crosses the barrier. Keep the signal traces short on both sides, place the decoupling capacitor close to the supply pins of each side and use a ground plane on each side. Do not run a trace across the barrier, even a low-voltage one, because a single trace defeats the isolation, and do not place a component that bridges the barrier.

Creepage and Clearance

The creepage is the shortest path along the surface between the two domains and the clearance the shortest path through the air, and both are set by the voltage and the standard. A wider package body gives more distance, so a higher-voltage application uses a wider body such as a 16-SOIC or a SOW16. Confirm the values against the standard you must meet, and keep a slot in the board under the barrier where the standard allows it to increase the creepage.

Power and Decoupling

Each side of the isolator needs its own decoupling close to the supply pins, because the fast edges draw a current that a distant capacitor cannot supply. Where the isolated side needs its own supply, a part with an integrated isolated DC-DC carries the power across the barrier, and the isolated supply still needs its own decoupling and, where the load is large, its own reservoir capacitor. Confirm the isolated supply is stable under the load.

Gate-Drive and Current-Sense Layout

For a gate driver keep the gate loop short and place the driver close to the gate, because the loop inductance is what turns a fast edge into a ringing gate waveform. For an isolated amplifier keep the shunt and the input filter close with a Kelvin connection and keep the low-voltage side away from the switching nodes. In both cases the layout decides the result as much as the device.

Verification

Validate the design on the bench by passing the signal at the worst-case rate, by imposing a common-mode step that matches the application and by checking the output for an error or a glitch. Verify the isolation with a hipot test at the rated voltage and confirm the creepage and the clearance against the standard. Measure the current on each side and confirm the isolated supply is stable. Our FAE team can review your layout and your measurements and help you interpret them, so the isolation performs in the product as it does on the datasheet.