TPS543820: Power problems

Robin Aebi

Part Number: TPS543820

I created a power supply that generates a 3.3V voltage from 12V to drive some LEDs. These need a larger current under full load. The power supply should be able to deliver a maximum current of 4A at 3.3V. With the Webench Designer I put together a circuit with the TPS543820RPYR. I have built the circuit in a test setup. This provides the desired voltage of 3.3V at the output. The problem, however, is that I can only put a minimal load on the feed. I have built the circuit exactly as calculated by WEBBENCH. The recommended components were perpetuated. Also the layout was taken as good as possible from the datasheet.

The circuit is a light, which switches a series of LEDs on and off via MOSFETs. The LEDs are to be dimmed via PWM.

Components according to Webbench designer:

- L1: MSS1048-472NLB

- Cout: 10uF

- Rfbt: 56k

-Rfbb: 10k

- Cff: 24pF

- Cboot: 100nF

-Rpg: 10k

- Rboot: 0

- Cbp5: 4.7uF

- RFsel: 24.3k

- Rmode: 60.4k

- Cin1,2: 10uF

- Cinx1,2: 47nF

I load the circuit with power resistors. (0.88Ohm) --> 3.3V/0.88Ohm = 3.75A

However, it then tears me the voltage together and the circuit pulses on the Output.

over 4 years ago

0 Robin Aebi over 4 years ago

Prodigy 10 points

At smaller loads, the circuit works. With a load of 6.6Ohm I get an expected output current of about 0.5A. At a load of 4.4Ohm, however, the limit seems to be reached. The voltage starts to pulse slightly and the losses across the coil become larger, so that it heats up strongly.

0 Robin Aebi over 4 years ago

Prodigy 10 points

When loaded with a short circuit, the circuit delivers a current of 4.3A. However, only at a voltage of approx 1V.

0 David_Daniels over 4 years ago in reply to Robin Aebi

TI__Mastermind 23975 points

do you have a schematic and pcb layout you can share

0 Robin Aebi over 4 years ago in reply to David_Daniels

Prodigy 10 points

This ist my schematic of the circuit.

This ist the WEBBENCH circuit

1602.WBDesign7.pdf

The layout was taken from the data sheet as good as possible. The PCB is cnc milled and double layered.

The 12V are on the bottom side and are led to the top side by means of vias. The other two open connections are made with small cables.

I hope this is helpful.

0 Robin Aebi over 4 years ago in reply to Robin Aebi

Prodigy 10 points

The enable resistors are not used. Just layouted for testing reasons.

0 Robin Aebi over 4 years ago in reply to Robin Aebi

Prodigy 10 points

This is how it looks like:

0 David_Daniels over 4 years ago in reply to Robin Aebi

TI__Mastermind 23975 points

The symptom of loading the converter and the voltage dropping out, implies the input voltage supply is noisy or a thermal limit is exceeded.

Can the voltage on VIN pins, BP5, SW (2us to 5us/div)and Vout be captured without and with the Load.

Some improvements I can see to the board.

Add Coutx2 near the blue box and the feedback sense trace. The webench shows 3x10uF

Move AGND pin 5 green wire to feedback resistor ground (blue line).

Move the small CIN ceramics closer to the device.

Add wire to input capacitor PGND (red line) to minimize impedance between PGND pins.

Add wire to input capacitor VIN (blue line) to minimize impedance between VIN pins

Move BP5 capacitor ground to PGND. Datasheet states AGND, but the AGND on the board is long and would .

Power management

Power management forum

TPS543820: Power problems