I was looking at the Zynq-7000 SOC startup sequence designed in the Zynq Evaluation and Development board (ZedBoard), which requires specific voltage level regulation sequence (1.0V, 1.8V, 3.3V and 1.5V precisely). To achieve such sequence, the designers used a quad comparator LM339 to ensure the previous voltage level is properly regulated and settled before start regulating the next voltage level.

The LM339 output is a transistor which goes HIGH impedance when Vin+>Vin- and ties to REF when Vin+

Since the output is already pulled-up to 5.0V, what is the purpose of the Schottky barrier diode? (Precisely D23 and D24)

And, since the LM339 inputs are transistor, don't they require input resistors?

Attached image

Datasheets of components here:

LM339 quad comparator

RB751S40 Schottky barrier diode for D23 and D24

Xilinx Zynq7000 DC&AC switching characteristics (sequencing info on page 8)

ZedBoard schematics, from where I grabbed the circuit of the attached screenshot

EDIT: Added datasheet for the Zynq-7000 and link to the Zedboard schematics for more info

  • \$\begingroup\$ Good question. Where do PG-1V0 and PG-ALL go? Maybe there's a hint there. \$\endgroup\$ – JRE Oct 24 '19 at 11:11
  • \$\begingroup\$ @JRE The PG-ALL is just used for comparator 3 (IN3+) and the PG-1V0 is the ENABLE signal for the 1V8 regulator which has an input leakage current of 0.025uA, so I guess it has nothing to do with it? Maybe it has something to do when the comparator output is tied to GND? \$\endgroup\$ – BamsBamx Oct 24 '19 at 11:16

These diodes ensure that the capacitors C36 and C368 are quickly discharged when the VCC5V0 supply voltage falls to ground. This might have been done to prevent damage to whatever circuits are connected to these two comparator outputs.

  • \$\begingroup\$ It makes total sense. The Zynq-7000 in addition to the required power-on sequence, there is a recommended power-off sequence, which is the reverse of the power off sequence. If one of these capacitors holds the voltage longer than it should then possible damage might be caused to the SOC. Answer accepted \$\endgroup\$ – BamsBamx Oct 24 '19 at 13:20

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