LM5122 DC2DC

Dear customer support

i see the evaluation board the next instruction :

5.3.1 Startup

• Set load current to 0 A and turn the load on

• Set power supply current limit to 25 A

• Turn on the power supply and increase voltage slowly up to 20 V

• Increase load current slowly up to 7 A

5.3.2 Shutdown

• Turn off the load

• Decrease the input voltage down to 0 V

• Turn on the load and discharge output capacitor

why ?

Hi Sharga,

Thanks for using LM5122.

It may experience the current limit at startup transient. What is the SS capacitor you use?

Thanks,

Yinsong

Hello

The value is : 27nF GRM155R71A273KA01D

I can add the schema if you need

shraga

Hi shraga,

27nF looks ok unless you have very large output capacitance. Yes, please show me the schematic and I can help review it.

Thanks,

Yinsong

7181.LM5122.pdf

Hi shraga,

Please try bigger SS cap like 82nF. Your output cap is very big.

Thanks,

Yinsong

 Hello Yinsong

When you say the output cup very big how much capacitance you think that it is good?

Shraga

Hi Shraga,

Sorry I did not make it clear. I was saying that the output cap was bigger than normal cases. There is no real "good" value and it is fully dependent on the customer requirement including steady state ripple, transient performance, etc.

Thanks,

Yinsong

 Hello Yinsong

1. I did the capacitor change and it is not help

2. we still has problem that when we turn on the board with out load we see 29.5 volt on the output

3. we did add load of 200 ohm and the output was dropping to 29.11 volt

4. when we turn on the board with load the output voltage was close to input voltage 

5. we need help ASAP we have a big delay with the project.

Shraga

Hi Shraga,

What is the input voltage you run the test?

It sounds like a new issue now. There are two ways to startup the converter.

1. Turn on the power supply with the load on. You have mentioned at the beginning that this does not work, the output voltage becomes low.

2. Turn on the power supply with no load. You mentioned the output voltage was good. And then turn on the load. Does the output voltage drop or not? If it also drops, it is not the start time issue (more like the cycle by cycle current limit). If it does not drop, it is the start time issue.

Thanks,

Yinsong

Good morning Yinsong

we see two problems.

1. when we turn on with load the output voltage is almost input voltage, for example: when the input voltage is 15V the output almost 15V and so on

2, when we turn on without load, the output voltage is 29.47V (almost what we did design ), then when we put low load like 200 ohm the output voltage fail to 29.11.

we really don't know what to do, we did the design according to the WEBENCH® program recommendation - by the way we dont see that the WEBENCH® use this IC any more why ?

Shraga

Hi Shraga,

I see. Could you show the waveforms at the following pins?

VIN, VCC, COMP, FB, SW, RES, and also the output voltage waveform

Thanks,

Yinsong

good morning

test1 results :

Vin: 15V

Vcc : 7.48

Comp: 1.438

FB: 1.198

Res: 0.014

Vout 29.47

Note ! without load

test2 results :

Vin: 15V

Vcc : 7.45

Comp: 1.67

FB: 1.198

Res: 0.02

Vout 29.14

Load : 26 Ohm

i did try to turn on the power with 16 Ohm load and the board was burnout (short the input voltage )

The picture is SW pin

Hi Sharga,

Thanks for the information. But it seems the data are at the good condition? (Vout=29.5V)

Could you please show the information under the bad condition? For example, increase the load so that Vout has a obvious drop.

Thanks,

Yinsong