Hello all,
We've been using LMR62014 in one of our products which is in production now, however, with the new batch of manufactured units, we are observing strange behavior in Lmr62014 where it loses track of the regulated voltage for like 100ms creating 20V at its output and then goes back to regulation again with 12V, in our circuit Lmr62014 is supposed to create a fixed 12V/100mA output out of a 5V/2A input, the Lmr62014 here is used for some mosfet gate drivers, thus 20V output is very dangerous and can damage the MOSFETs and the whole board.
Here is the circuit:
Here are all the tests I've done so far without being able to have a stable result between three different samples:
|
R94 [Kohm] |
C82 (feed-forward) |
Dead Time (DSP counts) |
L3 |
C78 |
Output voltage |
Notes
|
|
118 |
2200 pF |
60 to 100 |
10uH |
47uF |
12V |
is not stable |
|
118 |
0.1 uF |
60 to 100 |
10uH |
47uF |
12V |
is not stable |
|
100 |
2200 pF / 25V |
100 |
10uH |
47uF |
10.5 |
Stable on 2 boards but Not on the third board |
|
100 |
6800 pF /16V |
100 |
10uH |
47uF |
10.5 |
Not stable on the third board |
|
100 |
330 pF /16V |
100 |
10uH |
47uF |
10.5 |
unstable |
|
100 |
39 nF /16V |
100 |
10uH |
47uF |
10.5 |
Mostly stable, some occasional instability -- regulation is a bit noisy during Steady State |
|
100 |
1 nF /50V |
100 |
10uH |
47uF |
10.5 |
Unstable on the third board |
|
100 |
220 pF |
100 |
47uH (74404032470) |
47uF |
10.5 |
Unstable ( triangular shape saturating at 20V and going down to 10V) |
|
100 |
2200 pF |
100 |
47uH (74404032470) |
47uF |
10.5 |
Unstable staying at 20V (good regulation) |
|
100 |
2200 pF |
100 |
10uH |
10uF/75V |
10.5 |
Initial instability -stable after 200ms |
|
100 |
330 pF |
100 |
10uH |
10uF/75V |
10.5 |
Unstable ( back and forth to 20V) |
|
100 |
0.1 uF/25V |
100 |
10uH |
10uF/75V |
10.5 |
Stable on all the 3 boards ( saw a couple of times instability with one of the boards once was floating in the air) |
|
118 |
0.1 uF/25V |
100 |
10uH |
10uF/75V |
10.5 |
unstable |
|
100 |
0.047 uF/25V |
100 |
10uH |
10uF/75V |
10.5 |
Stable on the board that was having issue with 0.1uF, stable on the 2nd board, unstable on the 3rd board |
|
100 |
0.022 uF/25V |
100 |
10uH |
10uF/75V |
10.5 |
Stable on the 1st board, stable on the 2nd board, unstable on the 3rd board |
|
100 |
6800 pF/25V |
100 |
10uH |
10uF/75V |
10.5 |
unstable |
Here is what happens with unstable Boosts:
The situation on Stable boards:
of course, the problem is this issue only happens in 1 out of three boards almost randomly depending on the value of C82, but so far I couldn't find any good value for that, another issue is on the boards with instability, when I change the physical position of the board ( floating in the air or touching a surface) the board can get unstable which can show some effect of parasitic effect, but to me, that is very strange, based on the table above that the sensitivity of the C82 is not that much ( in range of pico farad differences).
I'll await your thoughts on this.
John
