LM5118-Q1: Functions in Buck mode, but not Buck Boost mode.

Kieran Lee

Part Number: LM5118-Q1

Hello,

After revising my layout design due to prior advice, I have built a prototype and have begun testing it.

I have found that it functions in buck mode, where the input supply is greater than the output, but does not function in buck-boost mode where the input falls below the desired output.
When the input voltage is below the set output voltage of 23.8V, the output voltage equals the input voltage.

Please see the attached layout layers:
VregRework2TopSilkX2.pdf
VregRework2TopCopperX2.pdf
VregRework2BottomCopperX2.pdf

The component values are:

CIN1	22uF
CIN2	22uF
CINX	100nF
C_BOOT	100nF
C_POLE	47nF
C_ZERO	2.2uF
C_OUT1	100uF
C_OUT2	100uF
C_OUTX1	470nF
C_OUTX2	470nF
C_RAMP	1nF
C_SS	220nF
C_VCC	1uF
C_R	220nF
C_CS	1nF

RUV1	82.5k
RUV2	33k
R_COMP	1.5k
R_EN	1M
R_FBB	180
R_FBT	3300
R_RAMP	100k
R_SENSE	0.013
R_T	30k
R_CS	100

33uH

I can provide wave forms on request as well, just let me know which ones would be most useful.

Thanks in advance for your help.

over 4 years ago

0 Youhao Xi over 4 years ago

TI__Guru 68705 points

Hi Kieran,

Thank you. Could you please share your schematic, and show the two switch node waveforms?

Thanks,

Youhao Xi, Applications Engineering

0 Kieran Lee over 4 years ago in reply to Youhao Xi

Prodigy 70 points

Here is the schematic:
VregRework2schem.pdf

Here are the waveforms using various inputs and either no load or a 30 Ohm load.

No Load, Vin = 19V, Vout = 19V

No Load, Vin = 27V, Vout = 23.8V

30 Ohm Load, Vin = 18V, Vout = 14.9V

30 Ohm Load, Vin = 29V, Vout = 23.5V:

0 Youhao Xi over 4 years ago in reply to Kieran Lee

TI__Guru 68705 points

Thank you for the info. The buck duty can reach beyond the 75% duty threshold to entering the buck-boost mode. Can you monitor the COMP pin under the condition of 30Ohm Load, Vin = 29V, (Vout = 23.5V)?

Thanks,

Youhao

0 Kieran Lee over 4 years ago in reply to Youhao Xi

Prodigy 70 points

Here is the waveform on the COMP pin.

0 Youhao Xi over 4 years ago in reply to Kieran Lee

TI__Guru 68705 points

Hi Kieran,

It shows your COMP is oscillating. Is the horizontal scale 40ms/div? We need to first stabilize the loop. Can you place a large capacitor (say 100nF to 1uF) across COMP and FB, and slowly raise your load from zero, and see if the circuit is stable?

Thanks,

Youhao

0 Kieran Lee over 4 years ago in reply to Youhao Xi

Prodigy 70 points

Sorry, I uploaded the wrong picture, this is the correct one:
The time per divider in this one is 80us, and the voltage 200mV

It is still oscillating, so I will test with the larger capacitor as well.

0 Youhao Xi over 4 years ago in reply to Kieran Lee

TI__Guru 68705 points

Thank you, and please let us know the findings.

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LM5118-Q1: Functions in Buck mode, but not Buck Boost mode.