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LM5026: How slope compensation implemented in LM5026?

Aneesh TS
Expert 1735 points
Part Number: LM5026
Other Parts Discussed in Thread: LM25037

Hi Friends,

I have the following doubt regarding LM5026.

LM5026 is a current mode controller. I could not find a RAMP or equivalent pin as in LM25037 in LM5026.So how slope compensation is implemented in LM5026?.

Regards

Aneesh

  • 0
    TI__Mastermind 27095 points

    Hi Aneesh,

    Thanks for your interest in LM5026. Slope compensation is implemented internally on LM5026 where an internal ramp is added to the CS signal. This is illustrated in the block diagram on page 14 of the datasheet. For reference, also take a look at section 7.3.5 of the datasheet where the internal slope compensation is explained in greater detail:

    Best Regards,

    Ben alough

  • 0 in reply to Benjamin Lough
    Expert 1735 points

    Hi Ben alough,

    Thanks for your reply. I have the following two doubts regarding slope compensation

    1. Slope compensation can be achieved by either adding a positive slope to the current sense pin(Method1) or adding a negative slope to the error signal(Method2). Most of the standard text books are explaining slope compensation with adding a negative ramp to the error signal. In both LM5026 and LM25037 slope compensation is achieved by adding positive slope to the CS pin. Why TI is preferring the first method (method1)? What is advantage of method1 over method2?

    2. What will be the slope of the the internal ramp in LM5026?

    Regards

    Aneesh

  • 0 in reply to Aneesh TS
    TI__Mastermind 27095 points

    Hi Aneesh,

    Either method will work for slope compensation. When you are adding a negative slope to the error amplifier input you have to be a bit more careful with the gain of the error amplifier in order to maintain stability. This can also be problematic to implement with isolated topologies where an oscillator signal from the IC is not readily available on the secondary side. Adding slope compensation to the current sense signal is what you will find in most modern PWM controllers today.

    LM5026 has an internal current source ramp between 0 and 45uA. This current is fed through an internal 2kOhm resistor to add the slope compensation to the current sense signal. The added slope compensation will be the internal current source ramp multiplied by 2kOhm + external impedance connected to the CS pin.

    Best Regards,

    Ben Lough

  • 0 in reply to Benjamin Lough
    Expert 1735 points

    Hi Ben,

    1. I think generating a positive slope signal is bit easy than generating a negative slope signal. That may be the reason everybody preferring adding positive slope method. Am i right?.

    2. Slope of the ramp signal will be always greater than 0.5 as what i feel. Because as per the theory the slope of the compensation signal should be always greater than one half of the down slope of inductor current.Please correct me if if i am wrong.

    Regards

    Aneesh

  • 0 in reply to Aneesh TS
    TI__Mastermind 27095 points

    Hi Aneesh,

    Yes, one of the benefits is easier implementation.

    You will want the down slope to be greater than or equal to the down slope of inductor current. 

    Best Regards,

    Ben Lough