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How to use the sync pin of the LTC3805

Hello People,

I'm using a LTC3805 in a sepic design and I have some questions regarding the sync input.

I have a Vin of 19V to 24V, Vout of 20V and -20V at Iout(max) of 260mA and -200mA. I use two coupled inductors of 330uH and the sense resistor is 56mOhm. I can supply a schematic but I have to clean it up a bit before posting.

When I leave the sync pin floating the device boots and switches at the set frequency by the resistor (68k) on the RFS pin, the frequency is about 361kHz as expected. I can increase and decrease the load etc.

When I solder a 30cm wire to the sync input the device won't boot anymore, if I connect this wire to ground it does. If I apply a signal within the specifications (370kHz square wave 3,3Vpp) to the sync input the device boots again.

I use a function generator to make the sync signal for now and I see that the device is critical about the signal shape and size. When I generate a sine or triangular wave the device doesn't boot. If I decrease the amplitude the output voltage changes. The requirements in the datasheet tells me that the amplitude of the signal has to be between 2.9V and 6V and that the frequency has to be within the +/-30% range of the set frequency.

I have multiple devices that I want to synchronize to one clock signal and I don't want the device to be very critical about the signal shape. The driving source does not have a 0Ohm output impedance and the signal goes from one board to another.

Is there more information available about the sync input? Input impedance, input capacity and requirements regarding the rising edge of the signal?

How can it be that the amplitude of the sync signal has influence on the DC output level?

Is it possible to include the sync input in the ltspice model?

Thank you in advance, if a schematic is required please let me know.

Justin Vis

  • The sync input has to be within the spec at the input pin. Also, the sync signal has to be within the specified abs max limits.

    Using long leads or long traces can result in excessive noise pickup that could result in erroneous operation of LTC3805.

    This problem can be minimized by careful PCB layout and by adding a small RC filter right at the input pin. An RC filter value of 100 ohm and 100pF would be effective against high frequency voltage spikes that may be coming down the sync line.

    I hope that this helps.

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