anybody with Melexis MLX91210 (Hall current sensor) experience? - Page 1
Forum section: electronics / projects

author: KeithBrown
author_rank: Contributor
timestamp: May 02, 2018, 11:59:56 pm
content: I put a couple of these (the 8-pin versions) in a new design and both have (almost) "stuck high" outputs with zero current flowing. I'm wondering if I made a mistake by leaving the "TEST" input pin open during the first power-up (I grounded these later with no change to the output). The data sheet is very sketchy about the function(s) of this pin and there seems to be no ap notes at all. (The pin is also called "MUST" with no explanation regarding the name.) Thereseemsto be some way to calibrate them, but again no details are published. Anybody else used these?

author: ChunkyPastaSauce
author_rank: Unknown
timestamp: May 03, 2018, 12:43:49 am
content: Quote from: KeithBrown on May 02, 2018, 11:59:56 pmhttps://www.melexis.com/en/documents/documentation/datasheets/datasheet-mlx91210See page 5, TEST is recommended but not required to be grounded something to do with improving emcThey come precalibrated, see page 3 for range of calibrations availableFor the high output and no current see page 7, high impedance VOUT_HIZ_PU = 95%vdd and page 11-12 diagnostic modes, if pull up on output

author: KeithBrown
author_rank: Contributor
timestamp: May 03, 2018, 03:27:19 pm
content: So you infer that I'm seeing a fault mode, all of which have the same effect on the output: high Z? I found it strange that one can use either a pull up or a pull down (figures 5 and 7) on what is an analog output, and that there is no way to distinguish which fault it is. My supply is at about 4.8 V but that should not be too low. (And note that the under/over voltage measurements are something below about 3.3 or above at minimum 6.7 V). I'll stick a resistor on the output of one of them and see what happens, no other clues as to what is wrong. On one reading of the data sheet I thought that it was requiring a resistor on the output, but that seems to not be the case. Now I'm thinking that if you want to know if there is a fault you can either pull down or up the output (presumably the choice is based on the expected current range: if you were only measuring positive currents and possibly going close to the max you would use a pull down). And by the way I did see a very small effect of my signal current on the open—except the scope probe—output but the scope had to be ac coupled and at max sensitivity.I went to the Melexis parts after some difficulties on another project with accuracy of an Allegro equivalent, but this seems to have its own quirks.

author: KeithBrown
author_rank: Contributor
timestamp: May 03, 2018, 06:39:46 pm
content: Well 10 k\$\Omega\$ resistors to ground cause the outputs (I tried both after all) to both sit at 3.36 V which I can only assume is the internal supply voltage. This is with any possibility of load current removed physically altogether. If this was an actual reading and referred to 1/2 Vdd it would indicate 10.75 A {(3.36-2.5)/.08 V/A} so the parts appear to not be working. The output amplifier stage is shown in the block diagram as being powered by Vdd not the internal 3.x V. If nobody here comes up with any other ideas, I guess I'll contact Melexis directly.

author: KeithBrown
author_rank: Contributor
timestamp: May 04, 2018, 12:14:49 am
content: Well the problem was the PCB! a via that was supposed to connect both chips down to the ground plane was open!!I went back to the current sensor problem when I discovered another open via... I've never had this happen before, and yes the PCB house will be hearing about it! The blank reference PCB has perfectly conductive vias.