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LM5116 - diode emulation mode and the failure of Vout

Hirotaka Matsumoto
Guru 27585 points
Other Parts Discussed in Thread: LM5116

Hi all

Would you mind if we ask LM5116?

<Question1>
Could you let us know diode emulation mode?
-During diode emulation mode, does low side FET operate normally?
-This mode is to improve the efficiency in case of light load, and it operates discontinuity, isn't it?
-Forcing diode emulation during soft-start allows the LM5116 to start up into a pre-biased output without unnecessarily discharging the output capacitor.

<Question2>
Our customer designs the circuit as the attachment file.
6675.20151105_LM5116.pdf
They used "LM5116_quickstart" to calculate constants of the circuit.
They mentioned that C34(2200uF)  is added and from power ON to OFF.
After that Power on, Vout is 3.7V. It was not Vout 13.5V
So, if you have some advice, could you let us know?

Kind regards,

Hirotaka Matsumoto

  • 0
    TI__Guru 58171 points

    Hi Matsumoto-san 

    Q1) During diode emulation mode, does low side FET operate normally?

    A1)  Yes

    Q2) This mode is to improve the efficiency in case of light load, and it operates discontinuity, isn't it?

    A2) Light load efficiency in diode emulation mode is higher than FPWM mode's. 

    Q3) Forcing diode emulation during soft-start allows the LM5116 to start up into a pre-biased output without unnecessarily discharging the output capacitor.

    A3) Yes

    There is no attachment.


    Regards,

    EL

  • 0 in reply to EricLee
    TI__Guru 58171 points
    Hi Matsumoto-san

    I found the attachment.

    Can you explain in detail what this means "They mentioned that C34(2200uF) is added and from power ON to OFF. After that Power on, Vout is 3.7V. It was not Vout 13.5V"

    Did it work well before adding C34?
    Was C34 added after power on ?

    Regards,
    EL
  • 0 in reply to EricLee
    Guru 27585 points

    Eric san,

    Thank you for your reply!

    Did it work well before adding C34? 
    -> Yes, it worked well.

    Was C34 added after power on ?
    -> Yes, it was correct.

    Furthermore, our customer calculated "compensation Network" using bandwidth Fc = 1/10 * Switching frequency.
    As our recognition, our customer must calculate bandwidth Fc = 1/50 * Switching frequency. So, we guess that it is relation to this failure.
    Is our recognition correct?(Please refer to the "20151106_LM5116_quickstart.xls".)
    20151106_LM5116_quickstart.xls

    And then, we have additional question about diode emulation.

    We found followings on the datasheet
    -Full synchronous operation is obtained if the DEMB pin is always biased to a higher potential than the SW pin when LO is high.R DEMB = 10 kΩ will bias the DEMB pin to 0.45V minimum, which is adequate for most applications.
    -When RDEMB = 0 Ω, the LM5116 will always run in diode emulation.

    Certainly, our customer confirmed  RDEMB = 0 Ω - diode emulation mode and  RDEMB = 10 kΩ - full synchronous operation.
    Our customer confirmed using  RDEMB  from 51kΩ to 510kΩ. The result was the following file.
    20151106_LM5116_DiodeEmulation.pdf

    So, in case of RDEMB  from 51kΩ to 510kΩ, is diode emulation mode enable or disable or indefinite?

    We appreciate for your help always.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    Guru 27585 points

    Eric san,

    We are sorry to rush you, but if you have some information, could you let us know?

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 58171 points
    Hi Matsumoto-san
    Q1) in case of RDEMB from 51kΩ to 510kΩ, is diode emulation mode enable or disable ?
    A1) Please check 'recommended operating range' in the datasheet. V(DEMB to GND) should be less than 2V. 40uA x 50kOhm = 2000mV.
    Q2) They mentioned that C34(2200uF) is added and from power ON to OFF. After that Power on, Vout is 3.7V. It was not Vout 13.5V
    Was C34 added after power on ? Yes, it was correct.
    A2) It is not recommended adding such a large capacitor (2200uF) during operation. The surge current which charges 2200uF can make an abnormal operation unless a hot-swap used. Also, your are right. Loop compensation should be designed to be able to handle both cases.
    Regards,
    EL
  • 0 in reply to EricLee
    Guru 27585 points

    Eric san,

    Thank you for your reply!

    OK, we got about diode-emulation.Thank you for your kind explanation.

    Furthermore, our customer tested following conditions;
    (They choose resistance and capacitance constants using "LM5116_quickstart".xls)
    7711.20151106_LM5116_quickstart.xls
    <Procedure>
    1. C34 is added.
    2. Power on
    3. Power off(They made pre-bias situation)
    4. Power on(Pre-bias start with diode emulation mode DEMB=0ohm)
    5. If the Vout(Pre-bias) is 11.7V - 13.5, it was possible to start up. However Vout(Pre-bias) is less than 11.7V, it was not possible to start up.

    So, with 2200uF + pre-bias situation and diode-emulaftion, we guess that it seems that it can't start with pre-bias due to a large capacitor(heavy charge).
    And then, our customer tested with 200ohm load with 2200uF. The result was good. We guess that it can be discharged using load 100ohm.
    By adding 200ohm, we think that pre-bias situation was disappeared.
    As the conclusion, 2200uF is too big to start-up with prebias, isn't it?(Or, according to input capacitance 1000uF, does this situatiuon improve?) 
    Should our customer use 100ohm for output dischage?

    If you have some advice, could you let us know?
    We are sorry to bother you, however we appreciate your help always.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    Guru 27585 points

    Furthermore,

    Our customer tested the following situation;
    -No load
    -DEMB=10kohm(Full synchronous operation)

    As the result, sometimes Low-Gate output is short, so it was not  Full synchronous operation.
    It seems discontinuous mode operation.
    When DEMB=10kohm is asserted, is there discontinuous mode operation?

    kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 58171 points

    Hi Matsumoto-san 

    It is mentioned in the datasheet 'Soft-start and Diode Emulation' section. If the SW voltage does not fall down below the DEMB threshold before the end of the HO fall to LO rise dead-time, switching will default to diode emulation mode. 

    When this situation happens, usually LO turns on for a short time (like as you mentioned) and then turns off. If the LO turn-on time is too short, HB capacitor cannot be replenished enough to maintain its V(HB-SW).  If the V(HB-SW) is less than 3V HB UVLO threshold, the device stops switching. 

    Please check whether V(HB-SW) is enough high or not. 

    Regards,

    EL

  • 0 in reply to EricLee
    Guru 27585 points

    Eric san,

    We would like to confirm additional question.
    Our customer will use 100ohm external resistance only in case of discharge.(2200uF is too much to discharge.)
    So, when they add the 100ohm external resistance, it is not problem, isn't it?

    We appreciate for your help.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru 58171 points

    Hi Matsumoto-san 

    Please make it sure that the power rating of the 100 resistor is enough high and no undershoot after fully discharged. This undershoot happens by parasitic inductance of the 100 resistot and/or its wires. 

    Regards,

    EL

  • 0 in reply to EricLee
    Guru 27585 points

    Eric san

    Thank you for your cooperation always.
    We are sorry that we can't explain diode emulation mode perfectly.
    So, could you let us know followings again?

    <Queston1>
    When the device enters diode specifically, what will the device do specifically?
     
    <Question2>
    If you have some details about diode emulation mode, could you give us it?

    Your comment will be appreciated.

    Kind regards,

    Hirotaka Matsumoto

  • 0
    TI__Guru 58171 points

    Hi Matsumoto-san 

    A1) In diode emulation mode, the low-side MOSFET will turn off for the remainder of a cycle if the sensed inductor current becomes negative. The inductor current is sensed by monitoring the voltage between SW and DEMB. 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.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

    Yes, the inductor current flow is discontinuous in diode emulation mode.

    A2) I guess it is in hiccup current limit, please check UVLO pin.

    Regards,

    EL

  • 0 in reply to EricLee
    Intellectual 670 points

    Dear Eric

    I have same pre bias startup issue as in my application, i have connected 12V battery at output of converter. since i am operating at 65Khz, almost all of the values are similar to circuit refereed in to this post. I am trying not to use ideal diode controller at output of this DC DC Converter which i can only do when LM5116 can start in to prebias mode. let me know if you have some workaround that makes it start with battery connected to output without use of ideal diode solution.

    regards

    vikas