How to sync multiple L7987 DC/DC Switching Converters

[Central SUPPORT]

We are planning to use three L7987 DC/DC switching regulators on our interface module. All L7987 should be synchronized and should operate with the same frequency set with an external resistor on the pin FSW.

In datasheet https://www.st.com/resource/en/datasheet/l7987.pdf (chapter 4.1, page 13) it is mentioned that the same resistor on pin FSW should be used for multiple L7987. It says: “In case two or more (up to five) L7987 SYNCH pins are tied together, the L7987 IC with higher programmed switching frequency is typically the master device; however, the SYNCH circuit is also able to synchronize with a slightly lower external frequency, so the frequency pre-adjustment with the same resistor on the FSW pin, as suggested above, is required for a proper operation.�?

So ,what does that mean? Should each L7987 should connected to one resistor with the same resistor value (R1=R2=R3=20kOhm) or should only one resistor R1=20kOhm connected to the pins FSW of the three L7897 ?

 NOTE: This question and answer originated from a customer support case which was handled by us. We regularly review support cases and add any helpful ones here for all to benefit from.

[Central SUPPORT · Accepted solution]

The text "so the frequency pre-adjustment with the same resistor on the FSW pin, as suggested above, is required for a proper operation" refers to the preceding paragraph that describes how to compensate the attenuation of the sawtooth ramp and thus the PWM gain by using an external resistor to fix the gain. "The same resistor" means that the same technique as previously described still applies, but you should not share the same resistor per all devices to set a common switching frequency.

As the Slave devices assume the frequency of the faster switching Master, the switching frequency of each Slave device should be set by separate resistors so that they operate slower than the Master device (also taking account of a +/-10% spread distribution per die).

In your case I suggest to use

Master: R1 = 56kOhm (473kHz) or 59kOhm (461kHz)
Slave1: R2 = 51kOhm (i.e. slower than 500kHz)
Slave2: R2 = 51kOhm

These values would maintain a larger margin and ensure a more consistent Master/Slave operation.