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Part Number: LM22675
I am using LM22675, and I connect EN pin to 24V Vin with a 100kR pull up resistor, and also connect EN pin to GND with a 4.7kR pull down resistor.
Then I test the voltage at EN pin, and get 6.4V test result.
However, the maxim spec of EN voltage refer to datasheet is 6V, can you help me clear about why I get a 6.4V at EN pin? Is the pull up and pull down resistor value incorrect?
The datasheet shows the internal block diagram with an internal pullup to VIN. The datasheet also explains "The current [in this en pin] must be limited to less than 100 µA. A resistor of 470 kΩ will limit the current to a safe value for input voltages as high 42 V."
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In reply to Samuel Jaffe:
I have already read these sections in the datasheet, but did not get clear understanding, that's why I ask the question. Can you make me clear, then I can be able to solve my issue. Thank you!
1. As the EN pin have internal pull up to VIN, so in order to remove the effect of external pull up/pull down resistor, I just left EN floating but I still get a 6.4V voltage at EN pin. Is it normal that V_EN > 6V when EN pin is floating?
2.Then I connect a external 100kR to pull EN up to VIN, and a 4.7kR to pull EN down to GND. Still get 6.4V V_EN. Can you share the internal structure of EN pin? Is it like below figure?
3. If internal structure is like above picture,why external resistors(100kR pull up, 4.7kR pull down) cannot pull down the EN voltage?
A 100kR pull up will make the current in zerner less than 100uA, and 4.7kR pull down is much smaller than 20kR inside the range of datasheet R_ENB requirments.
In reply to user5350137:
I will check with our team to find the tolerances on the internal zener to see if we would expect 6.4V on an open EN pin. I'll have an answer in a day or two.
The 6V abs max is for the voltage applied externally. It's a bit conservative which is why we see a voltage higher than this on the zener. If the zener breakdown was less than 6V, applying 6V would draw lots of current so there's a bit of margin to account for zener tolerances. It is normal to see 6.4V on this pin when an external voltage is not applied.
It is strange that with a 4k pulldown you still see 6.4V on EN. Can you please probe the pad of that resistor to check that it's connected?
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