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What is the internal pull up resistor value in LM3489-Q1. How to decide the external resistor?
I can only assume you are referring to the internal pull up on the Enable pin. Is this correct?
The internal pull up is a current source that pulls up to Vin. If I have this correct and you are attempting to establish a UVLO with a single external resistor, the predictability and repeatability will be poor. The actual value of current varies between devices and is not specified.
The input trip voltages of Enable are specified as:
But you must not exceed the maximum rating of the input:
Thus you can use a voltage divider of two external resistors, not a single resistor to ground.
I should also point out that this is a hysteretic device and most switching parameters rely on external variables such as inductor value and delays through the MOSFET and driver path. The device can handle a wide range of inputs and seems quite versatile. However, it is difficult to create single applications for this device that have a wide input voltage range. So apply this device with the understanding it should be driven by a well regulated source, not one of wide range. Additionally hysteretic regulators benefit from having some ripple injected from the switch node into the feedback node. Be sure to find an application where this is implemented.
There are now many fully integrated regulators that would be a more modern choice than this device that requires a comparatively expensive external PFET.
I highly recommend Webench from the TI.com site to assist in selecting other products.
Alan Martin - Wide Vin Applications Wrangler (retired)
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