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LM5022: Can I control SS pin externally so that to control the duty cycle?

Ye Zhao37
Prodigy 40 points
Part Number: LM5022

Hello,

I am working on a boost converter using LM5022 on the bench. No input power. Feedback loop is connected by Vout remains at 0V.

Objective: I would like to control SS pin externally so that to control the duty cycle. The info is very limited in the datasheet.  Need your help.

 What I did is: use a voltage divider with Vcc pin (Vcc=7.5V measured by DMM) to control SS pin voltage. Vin = 12V is given by bias voltage power supply.

When Vin=12V, Vcc = 7.5V, SS pin is found by DMM about 3.1V (by changing the trimmer pot in the voltage divider with Vcc). However, OUT pin of IC remains low. No gate signal even at start up.

At start up, I can only see SS ramp up by about 3.1V. No gate signal.

Since there is no real input voltage into boost,1) I notice that CS pin remains low. I didn't add any external resistor at CS pin to create slope compensation (not yet). According to TI-TINA, I would add 1kohm for stability.

My questions:

1. Can I control SS pin externally so that to control the duty cycle? (actually to control peak-current command)

2. If we cannot, is there any way we can do this? How about control COMP pin directly so that peak-current command under control. It may be susceptible to noise. Is this recommended to do so?

3. How does SS pin control the internal EA? Please give more info.

4. If no real input voltage into boost and switch current remains zero, and COMP pin voltage remains pretty high, like 3.7V. In this case, will output duty cycle becomes maximum, given CS remains around 0V? 

5. Can SS pin overwrite EA no matter what?  Or SS pin only works for certain time window during start up?

Thank you in advance

YZ

.

  • 0
    TI__Guru* 78430 points
    In order to operate the IC, UVLO must be >1.25V, COMP must be >1.4V, and CS must be <0.5V, in addition to SS is released from external pull down. Check all these pins and see if any of them keep the IC off.

    To answer your question about SS, it is equivalent to having a diode from COMP to SS, and when SS will clamp COMP through that diode. That means, COMP voltage can never be a diode drop higher than SS voltage. However, COMP can be lower than SS, this is why SS only controls the soft START but not normal operation after SS is charge up.

    PWM is actually determined by the CS and COMP. To control duty cycle directly, you'd better use COMP instead of SS. Ground FB, and apply a voltage at COMP, then you can control the duty cycle. Note that the control is only doable after SS is charged up, namely it is no longer clamp the COMP.
  • 0 in reply to Youhao Xi
    Prodigy 40 points

    Thanks to Youhao for the explanation.

    It turned out that the problem is because of bad soldering at IC.  How frustrating it is!

    Now the duty cycle can be modified directly by SS pin voltage. It works now.

    The duty cycle is dependent on quite a few factors, such as external slope compensation, CS pin current sensing. and SS pin voltage.