LM51561: About internal error amplifire charecteristic

Hi Teritama,

Thanks for using the E2E forum.

The COMP sourcing current is 180 uA.

Best Regards,

Hassan 

Hi Hassan,

Thank you for reply.

180uA is wrote in datasheet characteristic as min spec at Vcomp=1.2V.

Is it okey that we understand 180uA is typ value?

Best regards,

teritama

Hi teritama,

Yes you can use 180uA as a typ value.

Best Regards,

Hassan 

HI Hassan,

Thank you for reply.

In addition, This 180uA seems very smaller value than Gm=2mA/V.

Is this because that this source current is defined from difference of SS voltage or Vref and FB pin voltage?

Best regards,

teritama

Hi Teritama,

I have talk with the designer and they have informed me that the typ. value is 240uA and the max. value is 300uA. 

Gm does not influence the source current when the comp is control externally.

Best Regards,

Hassan 

Hi Hassan

Is it okey taht the true value is typ. value is 240uA and the max. value is 300uA not typ 180uA?

Hassan Jamal said:

Gm does not influence the source current when the comp is control externally.

In my understanding, the source current is internal transconductans Amp output(Io). So source current value is defined by below equation.

Io = Gm x (Vss or Vref -Vfb)

Is this wrong? Could you please tell us that how define source current value at COMP pin?

In the customer use case, FB is connect to GND. So if follow above equation, Io is around 2mA and there is gap between max value 300uA and 2mA.

The customer is confusing with above gap.

It is helpful that help to solve this question.

Best regards,

teritama

Hi Teritama,

Thanks for your interest in the topic.

Regarding the Source current for the COMP pin, the information provided by the designer is explained below. 

According to the attached waveform of gm versus FB-VREF, you can see, gm is not constant, but parabolic and reaches a maximum value for VREF = FB. In the implementation of LM5156, there is also a temperature dependency. This is not the case in every device.

Always, gm is limited by the maximum current the circuit can provide, which here is 240 uA +/- 25%.

For a small delta between VFB and VREF, gm is proportional to (VFB - VREF)**2. Is due to the characteristics of the MOSFET (ID~(VGS-VTH)**2).

When VFB-VREF becomes larger, the current is limited by the bias applied to the circuit. That curve you can also find attached. 

Best Regards,

Hassan 

Hi Hassan,

Thank you share the designer's explanation.

I understood that when small delta between Vfb and Vref, Gm is gm is proportional to (VFB - VREF)**2 from MOSFET characteristic and when vfb=0V, the source current is limited typ 240uA +/- 25%.

In the second curve, It seems X-axis is Vfb-Vref, Y-axis is the source current from internal transconductans Amp output(Io). Is my understanding correct?

And the limited current value(240uA +/-25%) is from bias operating current? I guess the difference of value is from BIAS supply to other block tin same time.

Best regards,

teritama

Hi Hassan

Thank you so much for your kindly supports.

It make sence.

Best regards,

teritama