LTC3112 undefined behaviour when power supply reaches current limit

Thread Starter

__Jana__

Joined Aug 7, 2019
4
I am getting a behaviour of LTC3112 that I do not understand. I am using it very close to the schematic recommendations in the datasheet. It works perfectly fine until my power supply (TOE 7610-60) goes into current limiting (at 2.5 Amps). The current limit is only reached due to switching peaks in the current, but not actually used by the circuitry. Then the LT3112 seems to get stuck and constantly draws 2.5 Amps (and gets really hot).

I have done a simulation with LT Spice, that does not show this behaviour. Note the voltage source V2 (going from 0.5 V to 0V in steps) and the voltage source V (with current limiting for 2.5 Amps).

In short: the power supply reaches its current limit and even if I decrease the output current of LTC3112, the power supply keeps outputting 2.5 Amps.
 

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ericgibbs

Joined Jan 29, 2010
8,723
hi Jana,
Welcome to AAC.
Running your LTS asc file, I see the same as you, ie: correct operation.
What is the inductor specification, is it saturating.??

E
 

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Thread Starter

__Jana__

Joined Aug 7, 2019
4
Hi!
The saturating current for the inductor is 7 Amps, so no.
Thanks for the suggestion though!
 

ericgibbs

Joined Jan 29, 2010
8,723
hi,
I see on the datasheet that Sync and RUN are set for 5V operation..?
They are linked to Vin on your app.
E
 

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Thread Starter

__Jana__

Joined Aug 7, 2019
4
Yes Sync > 1.5 V commands 750kHz fixed frequencymode.
If RUN < 0.75 V LTC3112 is disabled.

Those two inputs are actually regulated by an MCU, that is also powered by Vin.
So maybe when the current limiting is activated and the voltage (Vin) drops the MCU might go to its initial state which means RUN will be disabled.
The question still remains why the power source never leaves the state of outputting 2.5 Amps.

Sorry for not positing the whole schematic. I am not allowed to do so.
 

TeeKay6

Joined Apr 20, 2019
381
I am getting a behaviour of LTC3112 that I do not understand. I am using it very close to the schematic recommendations in the datasheet. It works perfectly fine until my power supply (TOE 7610-60) goes into current limiting (at 2.5 Amps). The current limit is only reached due to switching peaks in the current, but not actually used by the circuitry. Then the LT3112 seems to get stuck and constantly draws 2.5 Amps (and gets really hot).

I have done a simulation with LT Spice, that does not show this behaviour. Note the voltage source V2 (going from 0.5 V to 0V in steps) and the voltage source V (with current limiting for 2.5 Amps).

In short: the power supply reaches its current limit and even if I decrease the output current of LTC3112, the power supply keeps outputting 2.5 Amps.
@__Jana__
I don't understand why you would expect normal operation of the LTC3112 while the power supply (I assume that means the source at the far left in your schematic) is experiencing current limiting. Who knows what that supply does or looks like (electronically) while in current limiting. I feel sure that the LTC3112, like all other IC voltage regulators, expects its DC input to be from well-bypassed voltage source, not from a current source. You could try putting a huge cap (1000uF or more) in parallel with C1, as an attempt to keep your power supply from entering current limit mode during load spikes. Note: It's possible that the 1000uF cap will slow the power supply voltage rise so much that the LTC3112 will not start properly.
 

Thread Starter

__Jana__

Joined Aug 7, 2019
4
Hi TeeKay6,
I do not expect normal operation of the LTC3112.
BUT: I do not understand why the power supply never exits the state of outputting 2.5 Amps, even though that is just caused by current peaks.

So this is what I expect: (in green: working fine, in red: not the case)
power supply entering current limit due to load peaks -> LTC3112 does whatever
no load peaks anymore -> power supply exits current limit ----- LTC3112 should not constantly draw 2.5 Amps but does draw 2.5 Amps
-->> LTC3112 seems to get stuck
 

TeeKay6

Joined Apr 20, 2019
381
Hi TeeKay6,
I do not expect normal operation of the LTC3112.
BUT: I do not understand why the power supply never exits the state of outputting 2.5 Amps, even though that is just caused by current peaks.

So this is what I expect: (in green: working fine, in red: not the case)
power supply entering current limit due to load peaks -> LTC3112 does whatever
no load peaks anymore -> power supply exits current limit ----- LTC3112 should not constantly draw 2.5 Amps but does draw 2.5 Amps
-->> LTC3112 seems to get stuck
@__Jana__
After you smash an egg with a hammer, do you expect the egg to reform after you remove the hammer? The LTC3112 has entered an abnormal state of operation due to the initial current limiting of the power supply. The LTC3112 was not designed to work, nor to recover from that abnormal state. If you are lucky, the LTC3112 may recover when all power is completely removed and it has cooled. Or the LTC3112 may by now be permanently damaged and will not recover. The power supply's current limiting scheme/method may also be part of the problem.
 
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