LMQ61460: Drop Vout when Vin is above 21V

Hi Michal,

I would increase the input capacitance to the minimum 10uF of ceramic following the datasheet recommendations. At higher input voltages the MLCC will derate significantly so having larger input caps will help manage the ripple per Section 10.2.2.5. 

What is your ambient operating temperature? As for the basic operation of 24Vin|5Vout|5A|400kHz, if your circuit follows similar PCB and thermal layout considerations like the EVM, you should be able to push 5A even at elevated temperature of 110degC (Figure 10-2). 

I would start out at 24Vin|5Vout and no load. Then start slowly ramping up the load current to see when the output voltage drops. Right now the schematic looks okay and there isn't any clear signs of why you wouldn't be able to regulate at higher input voltage.

Regards.

Jimmy

Hi Jimmy, thanks for quick reply!

Now input caps are 10uF + 2,2uF

Ambient temperature is 20°C, I've measured IC temperature at 4A load: 55°C. f=400kHz

I'm not able to set 24V and no load because on that conditions output is 0V :-(

I took some measurements - look at photos.app.goo.gl/bFjxUJBUysnVQVPd7

Photo001 and 002: Under 21V even load is 4A output voltage still remains 5V (almost independent from load current). Ripple voltage is within 50mV. It seems that turning off output voltage only depends from input voltage, not load current.

Photo003 and 004: When I turn ON input voltage (24V) at no load(!) then output voltage first go to 5V and next slow rises to zero (-1V/s), when load is 100mA output voltage goes very fast to zero. Looks like output cap discharging at OFF state of IC power stage.

Photo 004: See what is happen when I slowly rise input voltage and then fall it

Photo 005: Same as 004 but at 4A load and detailed. There are oscillations between 21,2V and 22,8V. Under 22,8V oscillations disappear and output voltage is 0V.

See also my layout - I have got a lot of space on board so power planes are big enough to carry high currents.

I wonder if reason may be overheating chip during manual soldering (with hot air set to 240°C). Maybe I should replace the chip

On the same board and power supply are present two TPS5430DDAR, both works fine.

Hi Michal,

Thanks for the detailed scope images and yes I can see that the at input voltages above 21V there seems to be issues with regulation for whatever reason.

1. Can you provide me the datasheet of the external power inductor that you used? 
2. Can you try to reflow and replace the chip with a new device and retest functionality? It is possible that overheating the part may cause internal damage that could impact functional operation.
3. The device should be able to regulate 24Vin|5Vout|400kHz for the full operating current as shown in Figure 10-18 load regulation. It sounds like it's only an input voltage issue and not a load current issue. What input supply are you using and is it stable? 

Regards,

Jimmy