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Part Number: TPS51200
My question is related to the contents of this thread;
My customer apply TPS51200 for DDR3. SO, they was facing the inrush
input current on VLDOIN & outout on VO pins. They apply TPS51200 for DDR3.
They tried to add 1 ohm resistor between VLDOIN and VREF pin, then
the input peak was reduced from 2.75A to 0.688A.
So, I would like to confirm followings;
1. Is it no problem on this kind of usage for TPS51200?
2. If no problem, How much voltage drop should be estimated on VLDOIN
3. Personally, I think Trcking Start-up and shutdown is not so good for
DDR. Because DDR has the initialization time inside.
S3 and pseudo S5 is better to escape from unknown VTT setting
inside of DDR. How do you think about my thinking?
I an hoping to get your advice.
1. In typical DDR application, TPS51200 VLDOIN pin is connected to VDDQ, REFIN is connected to VDDQ via resistor divider. So there is no problem to add 1ohm in the circuit.
2. What voltage drop? Because VLDOIN pin is connected to VDDQ, the VLODIN pin voltage should be VDDQ voltage.
3. It depends on DDR requirement. I would suggest to check with DDR vendor. If non-tracking startup works for DDR, then it is fine to use non-tracking startup.
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In reply to Qian Chen:
Thank you so much for your advice as;
I would like to double-confirm.
No impact for REFIN voltage to add 1ohm resistor? Right?
In reply to hideyuki sakai:
VLDOIN pin should be connected to VDDQ directly, because VLDOIN current is large.
R1 in your figure can be connected to VDDQ via 1ohm resistor, because the current going through 1ohm resistor is very small.
Thank you so much for all of your advice & sorry to disturb your jon again and again.
I have confirmed the real schematic of the customer. Sorry my mistake on previous schematic.
They connect VLDOIN to VDDQ through 1okm resistor. The voltage drop by load current
at least will be happened. They said it is effective to reduce the input current peak.
Is it no problem to take this kind of circuit?
What's the concern of 2.75A VLDOIN inrush current?
Will it cause OC on VDDQ rail?
What's maximum VTT DC current?
1A current is the spec limit of 1.5V LDO as the voltage source and they don't measure
VTT outputcurrent. There are just input current.
Below is the before to add 1ohm resister between REFIN and VLDOIN.
Also, below is after to add 1ohm resister between REFIN and VLDOIN.
VLDOIN voltage during inrush current flow will be 0.182V(1.5 - 1*0.688).
I would like to make sure this kind of usage is acceptable or not.
If acceptable, I would like to know the acceptable voltage range of dropped down VLDOIN.
There are two concerns with 1ohm resistor between VDDQ and VLDOIN.
1. the power loss on 1ohm resistor
2. With the voltage drop on the 1ohm resistor, the VLDOIN voltage may too low.
With VLDOIN shorted to VDDQ, can you turn on VDDQ and 3.3Vin first, then turn on EN pin to see if the inrush current is lower.
Thank you so much for all of your advice.
They tried to With VLDOIN shorted to VDDQ, turn on VDDQ and 3.3Vin first, then turn on EN pin to
utilize /RESET signal of DDR3, but it is not successful because the inrush is still higher.
So, I would like to reconsider the method of adding 1 ohm resistor VDDQ & VLDOIN.
You pointed out 2 items.
They will put more higher power dissipation 1 ohm resistor, #1 could be cleared.
Regrading #2, How much minimum voltage must be maintained on VLDOIN for VO=0.75V?
Sorry again& again, but Please give your advice.
Before we look into the impact of voltage drop on 1ohm resistor, can you ask customer to use input filter inductor instead of 1ohm resistor, and check if the initial inrush current is reduced?
The inductor should help to limit the inrush current. Also the voltage drop on the inductor during stead state is very small.
Thank you so much for your advice of the inductor solution instead of 1 ohm resistor.
I would like to make sure following.
1) I think the inductor solution would be expected the peak current capability of the inductor.
I wonder the inductance value would be higher to set lower peak current?
2) Please introduce more suitable product to reduce the inrush current instead of
TPS51200 if you have.
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