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LM5085: Constant Vin and wide range Vout design. Voltage calculation at RT pin.

Michal Staniak
Prodigy 35 points
Part Number: LM5085

Dear Sir/Madam,

I design a power supply with Vin=70V and Vout settable by control unit to fixed values 5V, 12V, 24V, 36V, 48V and 65V. I've generated separate designs with WEBENCH for all Vout voltages and compared BOMs. The biggest differences are in FB drive voltage and Rt resistor so I decided to control these two values. My question is about control Rt resistance. I used a resistor divider (see attachment) when Rt (R112) is constant 768k (at 65V) and want to regulate voltage at RT pin by connect a "bottom" resistors to GND (R107 to R111). So is there an equation to calculate voltages at RT like those presented on Figure 8 of datasheet? Alternatively I need an internal resistance value of RT pin to calculate bottom resistors values to achieve constant Ton time with variable output voltages.

How do You think, do other components should be changed with Vout change?

TFIT-STD-PowerSupply.pdf

  • +1
    TI__Expert 3766 points

    Hi Michal,

    From the datasheet it's not typical to arrange a resistor divider at the RT and VIN pins. Instead I recommend following the CCM equation 5 in the datasheet to implement a dynamic resistor network on RT without the divider. Have a default RT resistor between RT and VIN for the typical VOUT and then have a resistor and NFET in parallel with the default RT resistor. This way with a high signal on the NFET the effective RT resistance can be lowered and adjusted for the new output voltage value. Your approach to the bottom feedback resistor seems good.

    Regards,

    Henry

  • 0 in reply to Henry Kou
    Prodigy 35 points

    Dear Henry, resistor dividers are less complex but Your suggestions seems more reliable so I will follow them. I will test everything on prototype of course. Thank you for reply!

  • 0 in reply to Henry Kou
    Prodigy 35 points

    Dear Henry, resistor dividers are less complex but Your suggestions seems more reliable so I will follow them. I will test everything on prototype of course. Thank you for reply!