LM5141: No output

Matt Bruensteiner67

Prodigy 220 points

Part Number: LM5141
Other Parts Discussed in Thread: CSD18563Q5A

I have built a circuit based on a WebBench design for 24-48 V input to 5 V / 5 A output using LM5141.

However, when I test it, I get no output.

Some observations:

* The low-side FET gate is driven to 5 V. No switching activity is seen. 0 V on the high-side FET's gate.

* When the EN input is asserted, the SS pin initially ramps up to 5 V (takes about 25 ms), but then drops very quickly to ~0.19 V.

* I verified 3.3 V is present on EN pin.

* I verified 5 V is present on VCC pin.

* ~4.9 V present on the boost diode (D8) cathode.

* So far I'm testing with 24 V at the VCC_48 node, and light load (no load or 56 ohms load).

I noticed this text in the LM5141 datasheet and wonder if it could be related (based on the behavior at the SS pin):

At the beginning of the soft-start sequence when SS = 0 V, the internal 20 μA softstart
current source gradually increases the voltage on an external soft-start capacitor connected to the SS pin,
resulting in a gradual rise of the FB and output voltages. The controller is in the forced PWM (FPWM) mode
when the DEMB pin is connected to VDDA. In this mode, the SS pin is clamped at 200 mV above the feedback
voltage. This ensures that SS will be pulled low quickly when FB falls during brief over-current events to prevent
overshoot of VOUT during recovery. SS can be pulled low with an external circuit to stop switching, but this is not
recommended. Pulling SS low will result in COMP being pulled down internally as well. If the controller is
operating in FPWM mode (DEMB = VDDA), LO will remain on and the low-side MOSFET will discharge the
VOUT capacitor resulting in large negative inductor current. In contrast when the LM5141 pulls SS low internally
due to a fault condition, the LO gate driver is disabled.

Any thoughts on possible root cause or next debugging steps is appreciated.

Thanks

Matt

over 5 years ago

0 Timothy Hegarty over 5 years ago

TI__Guru 59957 points

Hi Matt,

The CSD19534 100V FET is not a logic-level threshold device and thus not suitable for 5V gate drive amplitude. A FET rated with Rds(on) at Vgs = 4.5V is required.

Consider using the following FET for both the high and low-side positions:
CSD18563Q5A 60V NexFET with Rds(on) = 8.6mOhm at Vgs = 4.5V.

Here are some other recommendations:

1. A 70V/0.1A boot diode is sufficient, e.g 1PS79SB70 Schottky in SOD523 package.
2. A 0.1uF/100V/0603 ceramic cap at the VIN pin of the IC is adequate for decoupling. Use two 1uF/100V caps near the FETs.
3. Remove gate resistors on the low-side FET.
4. Consider reducing buck inductor L3 to 6.8uH to obtain 30% ripple current.
5. Loop crossover frequency looks low (it depends on the ESR of the output caps). If better transient response is required, increase compensation resistor R59 and decrease caps C97 and C98. Use the quickstart calculator to assist with component selection here.

Regards,
Tim

0 Matt Bruensteiner67 over 5 years ago in reply to Timothy Hegarty

Prodigy 220 points

Hi Tim,

Thanks for your reply.

I will try out the FET you recommended, and keep in mind the possible tuning of the control loop.

Can you give a link to the quickstart calculator? I haven't heard of that tool before.

As for the boot diode and bypass capacitors, do you think these could really be part of my problem?
I'm using those part numbers because they're used elsewhere in my design. By re-using the same part I reduce the number of reels on the pick & place machine, reduce the number of items purchasing has to track, reduce the number of items that might need engineering attention in the future due to obsolesence, increase the order volume (and lower the price) per part number, etc.

Thanks & best regards,

Matt

0 Timothy Hegarty over 5 years ago in reply to Matt Bruensteiner67

TI__Guru 59957 points

HI Matt,

Here is the link http://www.ti.com/product/LM5141/toolssoftware#softTools

The FETs are the issue here, the other comments are just general circuit optimization recommendations.

Regards,

Tim

0 Matt Bruensteiner67 over 5 years ago in reply to Timothy Hegarty

Prodigy 220 points

Hi Tim,

I have reworked the board with the CSD18563 FETs.

I saw no change in the behavior.

I'd appreciate any further thoughts.

Thanks,

Matt

0 Matt Bruensteiner67 over 5 years ago in reply to Timothy Hegarty

Prodigy 220 points

I reworked the board with the recommended FET and the behavior did not change.

0 Timothy Hegarty over 5 years ago in reply to Matt Bruensteiner67

TI__Guru 59957 points

Hi Matt, please send the layout and a completed quickstart file for this design.

Regards,

Tim

0 Matt Bruensteiner67 over 5 years ago in reply to Timothy Hegarty

Prodigy 220 points

Here is the Quickstart file.

* I wasn't sure how to enter the feedback resistors. I have the FB pin tied directly to AGND to get fixed 5 V output. There seems to be a big discrepency between the "Rfb" and "Rfb_used" values in the spreadsheet (cells C62 & C63).

* Two schematics on the spreadsheet show different compensation networks. The entries seem to assume the simpler one (one R, one C).
My actual design uses the other one ( one R, two C's) as recommended by WebBench.

* My REStart capacitor seems to be far out of line with what the spreadsheet expects (1 uF vs 80 pF) could this cause failure to power up? I'll investigate off-line.

I will send the layout shortly.

Regards & thanks,

Matt

LM5141ADESIGN-CALC_Filled.xlsx

0 Matt Bruensteiner67 over 5 years ago in reply to Timothy Hegarty

Prodigy 220 points

Hi Tim,

I have found the issue. The COMP pin was shorted to one of the unnamed leadframe corner pads, which I imagine is connected to ground.

Will need to investigate why this was not caught by DRC in the PCB layout tool.

Thanks for your help with this.

Matt

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LM5141: No output