PCB Design Considerations When Using Bi-Directional Translators - News

In the previous article, we discussed

. In addition to the working principle, these converters must be carefully placed in the circuit.

The equipment connected by the converter greatly affects design considerations when placing them on the PCB. According to ON Semiconductor’s application note,

: Power supply decoupling, RF susceptibility/radiated emission, and required ESD and EMI protection.

In this article, we will review some design guidelines when using voltage converters, which are becoming more and more common in electronic applications.

First, power supply noise can destroy the function of the bidirectional converter. A sharp change may indicate a logical change in the bus-holding circuit and incorrectly switch the direction of input and output.

Moreover, digital power and power supply noise will never mix well, because power supply noise will affect the digital circuit and cause command/response distortion. The best way to solve this problem is to place a decoupling capacitor, especially a ceramic capacitor in the range of 0.01uF to 1uF, to bypass the AC noise on the power line.

These capacitors may be the most useful solution due to their high frequency characteristics. They act as a ground short circuit for these AC signals, thus leaving only the DC supply voltage. The designer must place these capacitors as close as possible to the chip and connect them to ground to ensure maximum efficiency, as shown below:

As can be seen from the figure above, the capacitor is grounded through the ground plane. Perhaps this is one of the most effective grounding methods to avoid grounding, which can minimize secondary grounding.

.

In addition, this application note states that these converters must be connected to traces that minimize loops. The loop is formed by signal/power traces and ground traces. The larger the loop, the more antenna characteristics the antenna exhibits. The following figure shows the loop formed due to poor PCB layout design.

The cycle in the figure can be minimized by

By moving the converter as far as possible to the I/O connector, and connecting the I/O connector and converter to the ground through the ground plane. These changes will reduce the loop area, thereby maximizing efficiency while reducing RF emissions. The resulting layout might look similar to this:

Since the converter can be connected to internal and external equipment, safety is essential for its operation. Since many applications that require voltage converters involve the connection of external signals, these devices are usually designed to withstand certain surge levels, including some ESD protection.

Usually, these functions are not enough for external connection applications, so some additional protection is required.

Transient voltage suppressor (TVS) diodes can be solved. According to Semtech, TVS diodes are

. Under normal operation, they act as an open circuit, reducing some leakage. When a certain threshold voltage is detected, it will avalanche and short-circuit.

These devices can be one-way or two-way in nature, and two-way devices are often used. The TVS diode should be connected to the EARTH / CHASSIS ground connection, not the signal ground signal of the circuit. This implementation is useful to avoid any "ground bumps" because these surges will interfere with your ground signal, which will have a destructive effect on the operation of the entire circuit.

In addition, Semtech recommends that designers place TVS diodes close to the external signal source entering the converter. They must be grounded through a plane (ideally), or have a very short "short stub" connection with their grounding side to maximize their effect. Examples of best-case layouts for these diodes are shown below:

If these guidelines are followed, the operation of the translator will be almost guaranteed, and the safety of the equipment will be maximized.

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