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Dec 04, 2016, 01:52 PM
AKA jflyper
Mini-HowTo

OMNIBUS F4 LDO woes


OMNIBUS F4 and F4SD (aka F4 V2) has a 5V LDO which converts VBAT to 5V. However, because of the current draw of these boards, the LDO gets extremely hot even with 3S..

NOTE: This blog entry does not apply to OMNIBUS F4 Pro.

And it is officially advised to use an external 5V source and feed it through one of the ESC connectors 5V row.
Quote:
Originally Posted by myairbot.com
Yes, it is useless, it is 100% safe if you remove it.
Connecting External 5V Source: How does it work?

Figure.1 shows the brief schematic of this circuit (source: post #226 of the Omnibus F4 AIO). Diodes are used to select the source with highest voltage and feed it to VCC5. They also prevent reverse current to unselected sources.

By connecting an external 5V source to ESC_5V that is higher than the VCC5_LDO, the diode triplet let current from ESC_5V to flow though the board. Removing the connection to VBAT (VCC5_LDO) and connecting 5V to ESC 5V will accomplish this effect.

Do I loose my VBAT sensor?

Figure 2 Shows Figure 1 amended with some more relevant entities, and answers the first FAQ:
Q1. If I connect an external 5V source to ESC 5V, do I loose VBAT sensor?
A1. Yes and No. To retain the VBAT sensor functionality, VBAT must also be connected.
Disabling LDO
However, connecting VBAT will activate the LDO, and if the VCC5_LDO is higher than ESC_5V, then VCC5_LDO will still be used and the LDO gets hot.

To solve this problem, the LDO must be disabled as shown in Figure 3.

Then the second FAQ:
Q2. If I remove the LDO, do I loose VBAT sensor?
A2. The VBAT sensor comes before the LDO, so the VBAT sensor function is retained as long as the VBAT is connected.

Figure 4 through 7 shows the removal process.
Figure 4 shows the original condition.
Figure 5 shows input and output pins of the LDO cut. The corner at the output pin was accidentally got broken as I tried to make sure the output has a clear cut.

Removing the LDO
The goal of excluding LDO out of the circuit has been achieved at this point. Rest is optional.

Figure 6 shows the LDO removed. Notice the big ground pad underneath the chip.
Figure 7 shows the pads being cleaned with solder wick.

After removing the LDO completely and examining the area for few minutes, I decided to put a small 5V regulator (Pololu 5V 500mA) in place of the LDO. The Pololu was adhered to the board with a piece of thin mounting tape which also serves as an insulation.

Figure 8 shows the completed board.

EDIT
De-soldering the disabled LDO
There's a big ground pad underneath the LDO (large heat capacity), so it needs some technique to remove it. What I did was:
(1) Soldering iron heated to 250 deg, keep touching the corner of LDO ground 'ear' and the pad. Keep it for 1min or so.
(2) Stop touching, raise the temperature to 400 deg, put a solder blob at the tip, touch it against the corner again.
(3) As soon as the solder around the 'ear' liquify, gently slide the tip towards the edge of the board. The LDO should move along with the tip and fall of the edge.
(4) Clean pads with solder wicks.
Be careful not to wipe out small components around the LDO while applying heat.
Last edited by teralift; Jan 21, 2017 at 09:46 AM.
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Jan 13, 2017, 01:05 AM
Crash specialist
pFrancisco's Avatar
Is a 5v 500mah polulu enough to power the board and Tx ?
Jan 13, 2017, 01:20 AM
AKA jflyper
Quote:
Originally Posted by pFrancisco
Is a 5v 500mah polulu enough to power the board and Tx ?
If you mean Rx, yes, but there's not much head room left. I would suggest 1A.

If you meant VTX, then it depends on your RF power; +400mA for 200mW, +800mA for 600mW.
Jan 14, 2017, 11:55 PM
Crash specialist
pFrancisco's Avatar
Just finished the LDO mod using a 5v 500mah Polulu. This will only be powering the board and a Frsky XM+ Rx. (The black stuff is liquid electrical tape)
Jan 14, 2017, 11:59 PM
AKA jflyper
Quote:
Originally Posted by pFrancisco
Just finished the LDO mod using a 5v 500mah Polulu. This will only be powering the board and a Frsky XM+ Rx. (The black stuff is liquid electrical tape)
Nice.

Side note:
Please be careful that silk prints on through-holes are reversed inside-out.
Jan 19, 2017, 12:55 PM
Crash specialist
pFrancisco's Avatar
Another question. I have a Racestar 30x4 ESC with BEC. Is it possible to apply 5v through J11 to power the board instead of the LDO? I dont want the polulu and 5v BEC from the ESC fighting with eachother. It would be nice if the ESC with its 3A BEC could provide the power.
Jan 19, 2017, 04:00 PM
AKA jflyper
Quote:
Originally Posted by pFrancisco
Another question. I have a Racestar 30x4 ESC with BEC. Is it possible to apply 5v through J11 to power the board instead of the LDO? It would be nice if the ESC with its 3A BEC could provide the power.
I believe you can connect your ESC output 5V to J11. Actually, this pin is prepared for the purpose.

Quote:
I dont want the polulu and 5v BEC from the ESC fighting with eachother.
It doesn't happen even when you have both, because which ever supplies higher voltage would power the entire board through diode triplets.
Jan 23, 2017, 06:43 PM
Registered User
Hi
I have the Omnibus F4 Pro Flight Controller V2 coming, is this mod needed on this board ?

It dose say,,,

The v2 Pro version now includes a built in BEC so you can power this board directly from your flight battery (up to 4S)
5v3a SBEC
On-Board Video Filter (only can supply 5V to VTX and Camera)

I will be using a mico 5v cam and a 5v unify pro.

Thanks.
Jan 23, 2017, 09:47 PM
AKA jflyper
Quote:
Originally Posted by dna001
Hi
I have the Omnibus F4 Pro Flight Controller V2 coming, is this mod needed on this board ?

It dose say,,,

The v2 Pro version now includes a built in BEC so you can power this board directly from your flight battery (up to 4S)
5v3a SBEC
On-Board Video Filter (only can supply 5V to VTX and Camera)

I will be using a mico 5v cam and a 5v unify pro.

Thanks.
F4 Pro doesn't have a heating problem thanks to the BEC, but unfortunately, there is another problem on 5V-RAM path just identified few days ago. Due to the components on 5V-RAM jumper to RAM pin path, there will be a substantial voltage drop on the RAM, and will not be able to drive 5V CAM (people say unify is olay). To avoid this, don't use the 5V-RAM configuration (thus loosing LC filter, but it really doesn't heart because 5V already has a BEC on the path), but use regular 5V output onboard to power the CAM and VTX.
Jan 24, 2017, 04:57 AM
Registered User
Quote:
Originally Posted by teralift
F4 Pro doesn't have a heating problem thanks to the BEC, but unfortunately, there is another problem on 5V-RAM path just identified few days ago. Due to the components on 5V-RAM jumper to RAM pin path, there will be a substantial voltage drop on the RAM, and will not be able to drive 5V CAM (people say unify is olay). To avoid this, don't use the 5V-RAM configuration (thus loosing LC filter, but it really doesn't heart because 5V already has a BEC on the path), but use regular 5V output onboard to power the CAM and VTX.
Excellent
Thank you.
Jan 27, 2017, 05:35 AM
Registered User
Reading through this post, and waiting for my Omnibus F4 to arrive I came to this conclusions:
  • included LDO is not capable of runing FC, RX, VTX and camera at once (all running 5V)
  • all the above is true, cause LDO can supply max 1.5A with great amount of heat dissipation
Therefore, my plan is to supply only FC and RX using included LDO configuration (RX is Frsky X4R with 100mA current usage). This setup gives more than enough space not to stress LDO.
VTX and camera could be powered either by RAM voltage, or from regulated PDB (+12V).
Any thoughts??
Last edited by Majk_CRO; Jan 30, 2017 at 03:07 AM.
Jan 27, 2017, 06:49 AM
Registered User
Well My Omnibus F4 V2 with the 5V jumper shorted give 6.7V and slowly drops to around 4V, But with out the jumper shorted the rest of the boards 5V are 4.5-4.6V
Jan 27, 2017, 08:17 AM
AKA jflyper
Quote:
Originally Posted by dna001
Well My Omnibus F4 V2 with the 5V jumper shorted give 6.7V and slowly drops to around 4V, But with out the jumper shorted the rest of the boards 5V are 4.5-4.6V
What's happening???
Jan 30, 2017, 07:57 PM
DroneShotFPV
arcaine25's Avatar
If I don't power anything other than the FC via direct vbat, will the heat be too bad still? I power camera and VTX from a PDB always, and usually Connect a VCC pad to my Omnibus FC's.

Why would they continue to make these FC's with this fault? What is the issue with those that do get too hot? Do they reset, shut down, or what?
I have yet to experience an issue with mine, but maybe it's due to not having any other load on it other than the FC directly as I even power the Rx from the PDB.
Jan 30, 2017, 08:19 PM
AKA jflyper
Quote:
Originally Posted by arcaine25
If I don't power anything other than the FC via direct vbat, will the heat be too bad still? I power camera and VTX from a PDB always, and usually Connect a VCC pad to my Omnibus FC's.
It depends on the VBAT voltage if it gets really hot or not.
As you know, LDO simply "burns" excess voltage. If you are drawing 5V 0.5A (500mA) from 3S (12.6V), then the LDO is a 3.8W heater ((12.6 - 5.0) * 0.5 = 3.80). With 4S, it will be 5.9W, and 8W with 5S.
Thinking that there is a micro soldering iron rated at 10W, these are lots of heat.

myairbot.com said 3S is marginal with board + receiver, but he also mentioned the LDO to be removed for security.

Quote:
Why would they continue to make these FC's with this fault?
That, I don't know. But they will be coming up with a revision. I thought it was mentioned somewhere sometime in the main threads.

Quote:
What is the issue with those that do get too hot? Do they reset, shut down, or what?
People have been experiencing the MCU will keep rebooting.

Quote:
I have yet to experience an issue with mine, but maybe it's due to not having any other load on it other than the FC directly as I even power the Rx from the PDB.
Probaby yes.

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