LP5907-Q1: LP5907-Q1

Hey Shreeharsha, 

Thank you for your question! Please allow 2-3 business days to help answer your questions

Thanks, 

McKyla

Hello Shreeharsha,

1) What current are you planning to use for your system?

The resistance between IN and OUT is dependent on the current used in the system.

2) The resistance value will vary with temperature, but we first must know what current is being used in your system. 

Please let me know if you have any additional questions.

Best Regards,

Hannah

Hello

please find my answer in line

1) What current are you planning to use for your system?  ---100mA. the reason for asking resistance value is to calculate the inrush current value.  how do we calculate the inrush current if the Vout, Cout are known?

The resistance between IN and OUT is dependent on the current used in the system. 

2) The resistance value will vary with temperature, but we first must know what current is being used in your system. 

Please let me know if you have any additional questions.

Hello Shreeharsha, 

I will be answering your thread soon.

Best Regards,

Hannah

Hello Shreeharsha, 

If the part is in dropout, then the FETon resistance will be 5m Ohm, as shown in figure 17 below.

To calculate the inrush current, please use the equation I=Cload*dv/dt. Please see Managing Inrush Current Application Report to learn more about the effects of inrush current on your LDO. 

Typically, when temperature increases the resistance will increase. Please also see LDO basics to learn more about dropout for your LDO. 

Please let me know if you have any additional questions or concerns.

Best Regards,

Hannah

Hello Nelson,

To use I=Cload*dv/dt equation, we know Cload, dv. However I and dt are not known. Hence asked for support from your end.

Cload = output capacitor of LDO

dv = output voltage set for LDO.- in this case it is 3.3V.

However dt and I are not known. Hence we cant calculate.

please support.

Hello,

Typical rise time used for dt is 80uS as shown in the datasheet below.

This value can be used for dt to solve for your system's inrush current.

Please let me know if you have any follow up questions.

Best Regards,

Hannah

Hello Hannah,

thanks for quick response.

i have another related question :

if the output capacitor is more than the recommended value, the inrush current will be more. To control this, can we introduce a series resistor to LDO input? i know this is bit crude way, but do you foresee any other issue due to this?

Hello Shreeharsha,

With the larger than recommended output cap, you could risk an increase in the inrush current from charging the increased capacitance. 

Page 5 of this application note: Managing Inrush Current (Rev. A) (ti.com) will highlight some solutions to decreasing the inrush current. 

Though by decreasing output cap, the stability of the voltage rail may also decrease during transient current events. 

By introducing a series resistor, you increase the inductance of the solution. There could also be a voltage drop at the part if a resistor would be added to the input. We do not recommend adding a resistor at the input, because we believe that it will not solve the inrush current problem.

Another way to decrease the inrush current would be to increase the input cap. We recommend matching the input cap to the output cap.

Here is another application note that may help with better understanding an LDO when implementing one in your design: LDO Basics (Rev. A) (ti.com)

Please let me know if you have any additional questions. 

Best Regards,

Hannah