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LTC2871 DC/DC Boost Convertor Control Mechanism

Category: Hardware
Product Number: LTC2871
Software Version: N/A

Hello ADI

A colleague of mine is having issues with the rail generation of the VDD and VEE rails on the LTC2871 transceiver. The design guidelines have been followed for component selection of the capacitors and inductor for the DC/DC convertor. The the Fast Enable (FE) pin is pulled high and the Loop Back (LB) pin is low. Outboard logic drives the state of the RS485/RS232 enable pins (DX232/RX232 & DX485/RX485) and CH2 is pulled high to disable it. Does the part look at anything beyond the state of the RS485/RS232 enable logic when deciding to enable the RS232 rails (VDD & VEE) as it appears to not get enabled as we observe almost 100% duty cycle on the SW pin and 3.3V and 0V on the VDD/VEE pin respectively? 

Sincerely,

Craig MacKinnon

Christie Digital Systems

Kitchener, ON

Canada

   

Thread Notes

Moved from Switches/Multiplexers to Interface and Isolation on Monday, November 27, 2023 8:24:23 PM by CraigM@CDS

Parents
  • Hi Craig,

    The LTC2871 needs a booster converter and a charge pump to generate VDD (+7V) and VEE (-6.3V) for RS232 rails. Beside the switching components for the boost converter, a 0.22uF cap between SW and CAP pins, and 1uF caps closes to VDD and VEE pins to ground are also required.  You should be able to see VDD and VEE voltages regardless which protocol you are using. Are you able to get the correct VDD and VEE voltage after you power up the part?

    Best,

    Jane

  • All of the recommended component values were used. The only place from specification standpoint we don't meet the recommended components is the ESR of the inductor is greater 1.3 Ohms (it's actually 2.1 Ohms). What impact will this have on the boost convertor's performance. We do not get the correct voltage for either VDD or VEE. The convertor appears to be switching at almost 100% duty cycle if we look at the switch node if we power our board on.

    cheers,

    Craig M

Reply
  • All of the recommended component values were used. The only place from specification standpoint we don't meet the recommended components is the ESR of the inductor is greater 1.3 Ohms (it's actually 2.1 Ohms). What impact will this have on the boost convertor's performance. We do not get the correct voltage for either VDD or VEE. The convertor appears to be switching at almost 100% duty cycle if we look at the switch node if we power our board on.

    cheers,

    Craig M

Children
  • Hi Craig,

    The VDD and VEE voltages should be presented in all modes expect in shutdown mode. The inductor is the most critical part for a DC/DC converter. It could make the converter switches incorrectly if an inductor doesn’t meet the requirements. The typical DRC value on the LTC2871 demo board is 0.5ohm (max. = 0.8ohm). We highly recommend using the components from the datasheet. The on-chip DC/DC converter is a pulse-skipping boost regulator at 1.2MHz. You should be able to see the discontinuous pulse switching at SW after you power up the part without connecting any signal to I/Os.

    Best,

    Jane

  • Hi Jane

    You can close this issue as we found a schematic error that was the source of the problem.

    cheers,

    Craig