LM3424: A few questions about block diagrams in LM3424 datasheet.

Kazuya Nakai59

Part Number: LM3424

Hello guys,

One of my customers is considering using LM3424 for their new products. Now they have the following questions about the device datasheet.

Could you please give me your answers for the customer questions?

Q1. What is the reason that 0.9V in figure 22 on page 16 of the datasheet is added to GND?

Q2. What is the reason that equation 22 uses 25uA for tCMP calculation?

Your answers would be much appreciated.

Best regards,

Kazuya Nakai.

over 6 years ago

0 Tuan Tran74 over 6 years ago

TI__Mastermind 18890 points

Hi Kazuya,

Q1. 0.9V is uses to DC offset the current sense to be in the right range to compare to the COMP voltage.

Q2. The 25uA the nominal source current of error amplifier COMP pin. This current is used to calculate the time that it take for the COMP pin to reach 0.9V with a given Ccmp

Thanks Tuan

0 Kazuya Nakai59 over 6 years ago in reply to Tuan Tran74

Guru 21575 points

Hi Tuan,

Thank you for you reply. I could understand.

Could I ask you an additional question?

Q1. What is the reason that 1.7kohm resistor is inserted between + input pin of CURRENT LIMIT comparator and Artficial Ramp signal?

Thank you again and best regards,
Kazuya Nakai.

0 Tuan Tran74 over 6 years ago in reply to Kazuya Nakai59

TI__Mastermind 18890 points

Hi Kazuya,

The purpose of the 1.7K resistor is to show in the block diagram how we sum in the artifical ramp (Slope compensation) into the switch node current. This is done for stability in current mode control with greater than 50% duty cycle.

Thanks Tuan

0 Kazuya Nakai59 over 6 years ago in reply to Tuan Tran74

Guru 21575 points

Hi Tuan,

Thank you for your reply.
I understood that the total voltage of IS terminal voltage (= external FET drain current) + Artificial ramp + 0.9V is given
to PWM comparator plus terminal. Is my understanding correct?
Also could you have your answer to the following question?
Q. I think the output of the oscillator block in the block diagram on page 11 of LM3424 is current output not voltage output.
Is this correct?

Thank you again and best regards,
Kazuya Nakai.

0 Tuan Tran74 over 6 years ago in reply to Kazuya Nakai59

TI__Mastermind 18890 points

Hi Kazuya,

I will get back to you on this next Monday since I am out of the office.

Thanks Tuan

0 Tuan Tran74 over 6 years ago in reply to Tuan Tran74

TI__Mastermind 18890 points

Hi Kazuya,

I am sorry we will not answer to some of your questions...You are asking a lot of questions about how the internal circuits are designed and some of that is proprietery to our company. We will happily reply to questions of applications of the part but not how we design the part internally.

0 Kazuya Nakai59 over 6 years ago in reply to Tuan Tran74

Guru 21575 points

Hi Tuan,

Thank you for your reply. I understood what you said.

Best regards,
Kazuya Nakai.

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LM3424: A few questions about block diagrams in LM3424 datasheet.