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Strange problem with LT1912 step-down DC-DC regulator

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EDA_hg81

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I am debugging LT1912 switching step down DC-DC regulator.

I have checked schematic, PCB layout, part marking and soldering connections, they all looks fine.

But it is not working; the voltage at FB pin is only 0.09V instead of 0.79V, which is mentioned on datasheet.

Run/SS pin is powered with 3.6V.

What the problem with this simple LT1912?

Please help.
Thank you so much!!!
 

Re: Problem with LT1912

Could you pls send your schematic? Did you choose resistor divider in such way that FB should have 0.79V?

Added after 3 minutes:

Could you pls send your schematic? Did you choose resistor network in such way that FB should have 0.79V? It is defined in datasheet page 9.
 

    E

    EDA_hg81

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Problem with LT1912

Hi,
What about the Synch, BP & Exposed Pad pins(backside contact must be on GND!) pls?
You can change the resistors on FB "or a check"...
K.
 

    E

    EDA_hg81

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Re: Problem with LT1912

here is the schematic.

I have changed resistor divider, got no luck to make it work.

L11, C53 and C54 are for reducing the ripple voltage.

C50 should be 470PF, I have corrected this value on PCB.

even I took L11, C53 and C54 off, LT1912 still not work.
:cry::cry::cry::cry::cry::cry:

Thank you so much for your any help.
 

Problem with LT1912

Hi & tnx!
No;
C53 is not (only)to filtering the ripples needed, but more, and to first (with L10) is your only energy storager if the "switch is changing over"...
It MUST BE tenth of uF, practically 47uF & a good, low ESR type_I think these is yours Nr.1 failure! Than the C50 the second failure: its value is too high! :-( Select pls. one at 470pF instead drowed 0.47uF! Pls refer to pages 11 & 21.
Good progress!
K.
 

    E

    EDA_hg81

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Re: Problem with LT1912

I have taken L11, C53 and C54, only connected C55 with L11.

It still didn't work.

:cry::cry::cry::cry::cry::cry::cry:
 

Problem with LT1912

Hmm;
Do you have some Sw/Booster Signals_pins 2,3&10?
Whats on RT?...
K.
 

    E

    EDA_hg81

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Re: Problem with LT1912

No, I don't have those.

I used the same reference design as on the first page of LT1912 datasheet.

It is so .....:cry:
 

Re: Problem with LT1912

I've got the same problem.
What footprint and date code does Your chips have?
 

    E

    EDA_hg81

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Problem with LT1912

Hi Youngs,
Interesting! :-(
Than is maybe LT`s support to call...
Otherwise I would test it in steps but yet Im not on my PC_I dont have a Schema..
As I remember on the schema: it must be possible to check it as an oscillator/ Switch driver and so on, or is it possible that the pin functions are falsh documented?
For me is LT a very serious/accepted producer, but failures are over all possible.
K.
 

    E

    EDA_hg81

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Problem with LT1912

You have grounded the NC pin 7. I don't know what it does, but it may be a test mode pin. Try disconnecting it.

Keith.

Added after 2 minutes:

One other thing - as Karesz pointed out, C53 must be large, not small - 47uF. You said "I have taken L11, C53 and C54, only connected C55 with L11" but that will make it worse! Remove L11 and short it out.

Keith.
 

    E

    EDA_hg81

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Problem with LT1912

Hi,
Keith has fully right_I have L11 in the answer overseen-sorry! Tnx!
Datasheet tolds us over pin7: "N/C (Pin 7): This pin should be tied to ground."
In all case_it is eventually a good idea to experiment with them to take it from GND...
K.

Added after 12 minutes:

Keith,
maybe you have really right with pin 7_I found a "Ref design", that has it NC!!
Second difference to datasheets circuit:
Run pin is over a 100K on the control voltage...
http://cds.linear.com/docs/Reference Design/dc1313A.pdf
K.
 

    E

    EDA_hg81

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Problem with LT1912

karesz,

Yes, that is where I saw N/C not connected - on the "ref design". N/C does mean "no connection" after all, not ground!

Keith
 

    E

    EDA_hg81

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Re: Problem with LT1912

the marking on the Chip is:

LTDJS0A30E3

and footprint is MSOP.

we bought directly from digikey.

I tried disconnected NC pin7, it still don't work.

I have used the Spice model for LT1912 from linear, it is working fine with 3.6V powered on RUN pin without 100K resistor connected.

I think that 100K ohm resistor and cap is for soft start.

I have sent letter to linear two days ago, get no feedback yet.

Thank you for your help.
 

Problem with LT1912

Have you removed L11? It will not work with L11 there if C53 is small as has been stated several times now.

Keith
 

    E

    EDA_hg81

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Problem with LT1912

Hi
@EDA_hg81,
:),tnx!
@Keith & all,
its a good question, & as I wrote some earlier; C50?
Its 1000x higher value on the drowing is a possible problem for the amp too.
Otherweise was a news from "EDA_Hansi" over same problems :-(
@EDA_Hansi;
Whats your circuit Hansi?_ in details pls.
K.
 

Re: Problem with LT1912

I have removed L11, C53 and C54 and only 47uF C55 is left.

Nothing works.

I have used LTspice IV to confirm the value of C50 is not the big issue.

with 0.47uF C50, the output should be around 3.6V based on LTSpice IV simulation.


:cry::cry::cry::cry::cry::cry::cry:
 

Problem with LT1912

What is the specification of the inductor (L10) you are using?

You need to be probing the circuit with an oscilloscope, not using LTspice.

Keith.
 

Re: Problem with LT1912

I am using a 1.2A and 6.8uH power inductor.

Thank you again.
 

Problem with LT1912

I think you really need to be probing the circuit with an oscilloscope. Look for signs of switching the MOSFET gate. Look at the feedback pin.

What load do you have?

Are you using a bench power supply to power it? If so, does it have a current limit and what is it set to? Sometimes the surge current on DC-DC converters is quite high on switch-on so they can current limit if you set it too low.

Keith.
 

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