LMZ14201: concern with EN pin driving by LM3880 still open

Hello David, 

I ordered LM3880 and LMZ14201 evaluation boards to test this out. I removed the resistor divider on the EN pin on the LMZ14201 board so that the EN pin is floating. 

Here is the LMZ14201 evaluation board startup with the EN floating:

CH1 = VIN (12V)

CH2 = EN (floating), internally pulled up by a 6uA current source and internally clamped to 5V minus a diode drop. I have shared the internal clamp diagram before. As you can see, the voltage on the EN pin when left floating does not exceed 5V.

CH3 = VOUT (3.3V) 

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Here is now the LM3880 by itself:

CH1 = VCC = 5V supply

CH2 = external logic input from function generator

CH3 = FLAG1 (pulled up to the 5V supply)

So far so good. 

Here are the two circuits working together - the LM3880 Flag 1 driving the EN pin of the LMZ14201:

CH1 = The output voltage of the LMZ14201 (3.3V in this case)

CH2 = input to the LM3880 sequencer from a function generator. 

CH3 = FLAG1 (output of the LM3880) = EN pin of the LMZ14201 device. 

I ran the two ICs together (LM3880 driving the LMZ14201 directly) for 90 minutes, with a 2Hz pulse without any issues. This is more than 10k cycles without a problem. Again, this is the LM3880 driving the LMZ14201 directly, without any additional circuitry for level shifting, etc.

I am not sure why you would get a failure on your side with this configuration. The voltage on the EN pin is clearly not exceeding the 6.5V max. 

Is it possible for you to capture waveforms on your scope to compare to what I see? Please monitor the following nodes:

1. VCC of the LM3880 device

2. VIN of the LMZ device

3. The logic input to the LM3880 sequencer

4. The flag output of the LM3880 sequencer driving the LMZ EN pin

Regards, 

Denislav

Hello David,

For the sake of completeness, I also put together the circuit with the two n-ch FETs between the LM3880 and the LMZ14201. Here is a diagram of what I tested. You can adjust the resistor divider so that the EN voltage is not exceeded when the bottom FET is OFF.

Here is a waveform:

CH2= logic from a function generator driving the LM3880

CH3=FLAG from LM3880

CH1=EN pin of the LMZ14201 (output of the n-ch FET circuit above)

CH4= output voltage of the LMZ14201

Again, I don’t think you should need the external FET circuit, but this should help if you think the 5V on LM3880 is getting exceeded for some reason.

Sincerely,

Denislav