LM5642X: Not Getting any output voltage

Hi Anshu,

SS when enabled should be pulled up internally to a diode drop below 5V (internal LDO voltage), so around 4.3V.  Can you send your SW node waveforms as well as COMP? In the meantime I will continue to check your components on your schematic.

Regards,

Henry

Hi Anshu,

Thanks for the signal waveforms. As mentioned earlier could you also send the switch node waveforms? (Node between inductor and FETs)

Hi Anshu,

It seems like the issue may be your FETs since your LDRV gate signal is switching, but SW node is not. This will take some more time to diagnose. Hopefully I'll finish reviewing your schematic by tomorrow. Thanks again for your patience.

Regards,

Henry

Hi Anshu,

I've reviewed the schematic and it seems the most obvious is that there is no damping on the input of your system. This will result in your system being unstable. The ferrite bead FB50 only has several mOhms of DCR resistance, so I recommend adding a large electrolytic, maybe 220uF, with a couple hundred mOhms of DCR parallel to C228. 

As another unrelated recommendation, I would advise not to include gate drive resistors on the LS FETs, R325, and R333. Finally I would recommend shorting out the ferrite beads on the output and measuring the VOUT directly.

Please let me know if there are further issues, otherwise I will leave this thread resolved.

Thanks for your patience.

Henry

Hi Anshu,

Thanks, marking resolved for now. Feel free to reopen if there are any issues.

Regards,

Henry

This is good news, feel free to send the layout, and I will review it.

HDRV1 and HDRV2 should swing at 5V since it has a 5V gate drive supply.

Regards,

Henry

Hi Anshu,

Could you do me a favor and measure three measurements on the same plot?

1. LDRV2 as you have currently

2. Voltage from TP33 and SW2 (pin 1 of L2)

3. Voltage from SW2 (pin 1 of L2) to PGND

Regards,

Henry

Hi Anshu,

The first diagram with the blue trace should be the correct VGS voltage driving the HS FET swinging from around 0V-5V. I'll check to confirm the shape of the waveform.

Regards,

Henry 

Hi Anshu,

Your HDRV2 is not the proper waveform. HDRV2 w.r.t. PGND should be swinging from 0V-29V (5V on top of your SW node.) This suggests that your HS gate driver within the controller is not functioning properly, probably due to stresses from the malfunctioning FETs from before.

With this in mind I'd advise the following:

1. Replacing the FETs (Q3, Q4, Q5, Q6), controller device (IC37), and boot diodes (D7, D6).

2. As well as implementing the recommendations from before: 

- I recommend adding a large electrolytic, maybe 220uF, with a couple hundred mOhms of DCR parallel to C228. 

- As another unrelated recommendation, I would advise not to include gate drive resistors on the LS FETs, R325, and R333, and instead shorting them out. Finally I would recommend shorting out the ferrite beads on the output and measuring the VOUT directly.

3. Remeasuring the HDRV1 and 2 w.r.t. PGND, LDRV1,2 w.r.t. PGND, SW1 w.r.t. PGND, SW2 w.r.t. PGND.

HDRV w.r.t. PGND should have square pulses ranging from 0V - 29V, LDRV w.r.t. PGND should have square pulses ranging from 0V - 5V, and SW w.r.t. PGND should have square pulses ranging from 0V - 24V. 

Let me know if you have further issues, thanks.

Regards,

Henry