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LM5116WG: Variable power supply with LM5116: diode emulation mode and phase margin help.

Giuseppe Busi
Prodigy 70 points
Part Number: LM5116WG
Other Parts Discussed in Thread: LM5116

Hi,

we have designed a variable power supply (Vin = 48Vdc; Vout = 4.5V up to 34.5V) with an LM5116, my questions are the below ones:

1) how we can verify the phase margin at the various output voltages?

2) we have changed the resistor placed at the DEMB input, now it's 2k2, because we want the LM5116 to work ONLY in synchronous mode, but we have verified that with light  load, the LM5116 starts to work in diode emulation mode, so we ask to you about where we are in wrong.

Can you support us?

Thank you.

Regards,

 Giuseppe Busi

 

  • 0
    TI__Genius 11040 points
    Hello,

    You can find the below description on page 19 of the datasheet. You will want to make sure this resistance is sized correctly. What load range is the device operating in diode emulation mode?

    "The DEMB bias potential should always be kept below 2V. At very light loads with larger values of output inductance and MOSFET capacitance, the switch voltage may fall slowly. If the SW voltage does not fall below the DEMB threshold before the end of the HO fall to LO rise dead-time, switching will default to diode emulation mode. When RDEMB = 0 Ω, the LM5116 will always run in diode emulation."

    Thank you,
    Katelyn
  • 0 in reply to Katelyn Wiggenhorn
    Prodigy 70 points
    Hello Katekyn,
    we have read the LM5116's datasheet a lot of times, in order to well understand what is the behavior of the resistor connected to the DEMB pin, we also made some tests modifing the value of R47: with values of 3k3, 2k2, 330, but we had verified that the best value is 2k2 one. With such value, when the output voltage at the minimum (around 4.5 Volt), if the output current is below of 200 mA, then the LM5116 enters in diode emulation mode.
    The datasheet doesn't specified how to completely disable the diode emulation mode!
    Thank you.
    Giuseppe
  • 0 in reply to Giuseppe Busi
    TI__Guru 62987 points

    Hi Giuseppe,

    Here is the applicable section (7.4.1) in the datasheet:

    "As the SS capacitor continues to charge beyond 1.215 V to 3 V, the DEMB bias current will increase from
    0 μA up to 40 μA. With the use of an external DEMB resistor (RDEMB), the current sense threshold for diode emulation will increase resulting in the gradual transition to synchronous operation. Forcing diode emulation during soft-start allows the LM5116 to start up into a pre-biased output without unnecessarily discharging the output capacitor. Full synchronous operation is obtained if the DEMB pin is always biased to a higher potential than the SW pin when LO is high. RDEMB = 10 kΩ will bias the DEMB pin to 0.45V minimum, which is adequate for most applications."


    I notice the FDMS86105 FET has a high Rdson, so you may need to increase the DEMB resistor value. Alternatively, use a lower Rdson FET (this may be required anyway depending on the max output current spec).

    PS: I recommend using a 0.1uF boot cap, not 1uF. Thus, the VCC cap is 10 times the boot cap, so VCC is not discharged too much upon first boot cap charge.

    Regards,
    Tim