LM25118: Resistor between Ramp and VCC

Danilo A.

Part Number: LM25118

Hi Team,

In the Webench simulation of LM25118 there is a resistor between Ramp and VCC pins. However, the typical application circuit shows this resistor is unpopulated. The customer wants to know the purpose of the resistor and how to compute its value. Below is the Webench simulation of LM25118 showing the resistor between Ramp and VCC. The initial value is 20k but the customer changed it to 1M to simulate an open circuit.

Regards,

Danilo

over 4 years ago

0 Brigitte over 4 years ago

TI__Guru 65170 points

Hello Danilo,

As far as I know this resistor is optional. Please let me check the details.

0 Brigitte over 4 years ago in reply to Brigitte

TI__Guru 65170 points

Hello Danilo,

This resistor is optional and we work on marking it accordingly in Webench.

0 Danilo A. over 4 years ago in reply to Brigitte

TI__Mastermind 28560 points

Hi Brigitte,

Thank you for your response. If the resistor between Ramp and VCC is optional, what is the condition that will require the customer to use it and how do we compute for its value?

Regards,

Danilo

0 Brigitte over 4 years ago in reply to Danilo A.

TI__Guru 65170 points

Hello Danilo,

This resistor is only necessary if the created ramp does not fulfill the requirements for high output voltage designs. Normally it is not necessary. If it is necessary, the additional current into the RAMP capacitor defines the resistor value based on Ohm's law.

0 Romans Poltarjonoks over 4 years ago in reply to Brigitte

Prodigy 10 points

Hello Brigitte,

Thank you for your reply. This is what I suspected, but just wanted to clarify.

Kind Regards,

Romans

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LM25118: Resistor between Ramp and VCC