LM27761: LM27761 Start-Up Issue

Brian Ott

Part Number: LM27761

I have a new design utilizing the LM27761 to convert +5VDC to -5VDC. What I have encountered is if the -5Volt supply is not allowed to completely decay to 0V and power is reapplied to the DUT the -5V(LM27761) regulator will not start up and run.

over 8 years ago

0 Juha Karttunen over 8 years ago

TI__Intellectual 1120 points

Hi Brian,

Can you share your schematic, PCB layout and bill of materials? Do you have any oscilloscope waveforms which show the issue?

Regards,

Juha

0 Brian Ott over 8 years ago in reply to Juha Karttunen

Prodigy 180 points

Miller LM27761 Schematic.pdf Miller LM27761 Layout2.pdf Miller LM27761 Layout1.pdf

0 Brian Ott over 8 years ago in reply to Juha Karttunen

Prodigy 180 points

8764.Miller LM27761 Schematic.pdf

0 Brian Ott over 8 years ago in reply to Brian Ott

Prodigy 180 points

2475.Miller LM27761 Layout1.pdf

0 Brian Ott over 8 years ago in reply to Juha Karttunen

Prodigy 180 points

Juhu

Are there particular components of the BOM you are interested in?

I have isolated the output of LM27761(severed pcb trace) from rest of the circuit and issue persists.

2.2uF caps are type X5R, +/-10%.

1uF cap is also of type X5R, and +/- 10%.

I can provide oscilloscope traces if you define points of instrumentation.

0 Christophe Palandre over 8 years ago in reply to Brian Ott

Prodigy 10 points

Hi,

In order to start the charge pump, Enable pin (6) should be set at a High level (more than 1.2V) : tied to the Input pin (1).

Regards

0 Keith Keller over 8 years ago in reply to Christophe Palandre

TI__Expert 7230 points

The issue was found to be high impedance ground traces from the flying capacitors to ground pin on IC.

This caused excessive ground bounce and disturbs analog PFM mode comparator which halts switching.

Be sure to use a thick trace or copper pour to connect grounds. Also use several VIA to layer 2 ground plane.

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LM27761: LM27761 Start-Up Issue