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# LM25117-Q1: Formulas for calculate Vin minimum voltage when Vout is 5V or 3.3V

Part Number: LM25117-Q1
Other Parts Discussed in Thread: LM25117, LM25117-5117QUICK-CALC,

Hi TI Engineer,

My question is what is minimum input voltage can allow LM25117 to output 5V or 3.3V?

I have tried to use "LM25117-5117QUICK-CALC — LM(2)5117 Quick Start Calculator", but it doesn't work.

Target output voltage, VOUT [V] = 5

Desired minimum start-up voltage, VIN(STARTUP) [V] = 4.5

It won't have any error message. Is it correct?

Actually, I am looking for formula/method can calculate Vin minimum voltage when LM25117 output voltage is 5V or 3.3V.

Nick Huang

• Hello Nick

You can start up the device at 4.5V input voltage but the output will be out of the 5V regulation target. Also please make it sure that your MOSFET can be fully enhanced at 4.5V over temperature otherwise the MOSFET can be damaged.

In order to regulate the output voltage properly, the input voltage should be higher than ;

Min VIN = VOUT_target + IOUT_max * ( Rdcr + Rdson) / (1 - 440ns * Fsw_max)

- Eric Lee ( Applications Engineering)

• Hi Eric,

Is Rdcr related to inductor?

Is Rds-on related to MOSFET? also how can I prevent MOSFET damage?

Could you explain the the theory of the formula?

BR,

Nick Huang

• Hi TI AE team,

How can I calculate LM25117-Q1 fsw (switch frequency range(min and max))?

BR,

Nick Huang

• Hello Nick

Max/Min switching frequency is 10% higher /lower than the RT programmed switching frequency which is calculated by the equation(3)

- Eric Lee (Applications Engineering)