LM3429: LM3429 power management issue - Buck boost LED dimming application

Rupert Gatterbauer

Part Number: LM3429

Dear TI Team,

I run into some issues with power management using a LM3429MHX/NOPB.

The PCB is designed to regulate two dimmable LEDs at a constant current of 500mA. So I tried to stay as close to the demonstration figure 34 of the LM3429MHX documentation "Buck Boost PWM dimming application - 6 LEDs at 500mA".

The PWM signal is provided by an ESP32 and shows correct behaviour.

When testing the PCB, i started to see some issues with VCC. The LM3429 is not really starting. Showing VCC to GND about 200mV only.

Some further measurements:

VCC ↔ PGND = O.L. ; VCC ↔ AGND = 5,1 Ω ; GATE ↔ PGND = 5,9 Ω ; GATE ↔ VCC = 0,9 Ω

I have tried to solve the problem, but with my second PCB try ending up in failure I thought it's better to talk with you.

Thank you very much.

19 天前

0 Taylor 19 天前

TI__Mastermind 45365 points

Hi,

We have received your post and the investigation will take some time. Thank you for your patience.

0 Daniel 18 天前

TI__Mastermind 44675 points

您好

May I know what is the purpose for C21? It shouldn't show in the circuit I think.

0 Rupert Gatterbauer 15 天前

Prodigy 10 points

Thank you for your reply.

I am not the original designer of this circuit, therefore I am not sure. I worked with a freelance PCB developer, which I forwarded your questino.

Do you think C21 is causing the problem?

0 Rupert Gatterbauer 15 天前回复 Daniel

Prodigy 10 points

Thank you for your reply.

I am not the original designer of this circuit, therefore I am not sure. I worked with a freelance PCB developer, which I forwarded your questino.

Do you think C21 is causing the problem?

0 Daniel 15 天前回复 Rupert Gatterbauer

TI__Mastermind 44675 points

You should remove it first and do the tests again. C21 shouldn't be there at the beginning.

0 Rupert Gatterbauer 4 天前

Prodigy 10 points

Thank you Daniel,

I removed C21 and bridged the gap. With C21 removed, I still have problems lighting up my LEDs.
Two findings:
1) Without C21 the Light does not light up at all - no change in behaviour to the previous version
2) If I add a resistor (100kΩ) parallel to the LED, it starts to light up just a bit but (not close to full brightness, but rather at minimal brightness to say it lights up). The dimming functions works.

A measurement of R400 showed, my output is not 500mA, but rather 25mA (5mV).

0 Daniel 3 天前回复 Rupert Gatterbauer

TI__Mastermind 44675 points

Do you have tested waveform? I'd like to see each of below in one figure.

1. Vin, IS, GATE, Inductor current

2. Vin, IS, GATE, VCC

3. Vin, IS, COMP, RCT

4. VIN, IS, VO, OVP

BTW, 1. what is LED string SPEC? 2. Pls provide the latest schematic with C21 bypassed.

0 Rupert Gatterbauer 1 天前回复 Daniel

Prodigy 10 points

Unfortunately I don't have a tested waveform.

Neue Schematic.pdf

The Spec sheet of the LED is attached as well. More detailed information about the LED is not available to my knowledge.

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LM3429: LM3429 power management issue - Buck boost LED dimming application