Schematic design

AN3978

6/19

Doc ID 022179 Rev 1

Section 1.1 to Section 1.5 show how the L6370Q chip features are configured, which

external components are used, and how they are dimensioned.

1.1

Input interface

The aim of the design is to provide a galvanic isolated output module. It requires the use of

isolators on the control / feedback signals. In the case of STEVAL-IFP0020V1, cost

effective, compact size optocouplers (TLP281) have been selected.

Important parameters for the application design are:

●

Collector-emitter voltage higher than 80 V - which exceeds the L6370Q absolute

maximum ratings and therefore doesn't limit supply voltage range

●

Isolation voltage higher than 2500 Vrms - usually sufficient for this kind of application

●

Current transfer ratio (depends on LED current) - LED and output phototransistor

currents must be adjusted accordingly.

Figure 2.

Input interface circuit

The input interface connection can be seen in Figure 2. LED current ranges between

approximately 2.4 mA, considering a 3.3 V signal level, and 4.3 mA for 5 V logic level.

Symmetrical LED resistor connection should positively influence EMC immunity. To achieve

the maximum tolerance of IN+ threshold (2 V according to the datasheet, considering that

IN- = 0 V), the required minimum phototransistor current is approximately 0.11 mA (2 V /

18 k

Ω). This means that CTR of the optocoupler must be at least ~5% (0.11 mA / 2.4 mA).

This is in accordance with the TLP281 datasheet which shows a much higher CTR.