LM46000-Q1: Problems with proper startup

Tom Lightning

Part Number: LM46000-Q1

Hi,

I got a problem with the simple switcher LM46000-Q1. When I apply the target input voltage on the Vin pin in one step (0V->24V) the LM46000-Q1 does not work properly. The output voltage reaches only 2V-3V instead of 12V. I'm able to fix that problem by increasing the input voltage in multiple steps (0V->1V->2V->...->24V) but the application doesn't allow this solution.

Do you have any idea to solve this problem?

Kind regards,

Tom

over 4 years ago

0 Arief Hernadi over 4 years ago

TI__Mastermind 23815 points

Hi,

Do you have schematic for your design? Also do you have a scope shot of the phenomena?

Maybe some probe on VIN, VOUT, VCC and IOUT if possible.

Thanks

-Arief

0 Tom Lightning over 4 years ago in reply to Arief Hernadi

Intellectual 305 points

Hi Arief,

thanks for your reply. I did some shots of Vin, Vout and the voltage at the switch node for the fault and normal case. The left side of every picture shows the normal operation and the right side the fault operation. The current in normal operation is about 28mA, in fault operation it is only 0.3mA.

Vin:

Vout:

Switch node:

Rise of Vin in one step:

Schematic:

Kind regards,

Tom

0 Marshall_Beck over 4 years ago in reply to Tom Lightning

TI__Mastermind 29590 points

Can you do VIN, VOUT, SW (or iL) on same plot? Sorry.

You are saying that slewing the input different causes issues with start up?

Besides the schematic you shared, is there a load or any additional capacitance on the output of the DCDC?

All the best.

0 Tom Lightning over 4 years ago in reply to Marshall_Beck

Intellectual 305 points

Yes, rising Vin step by step solves the problem. The problem is caused by turning on the power supply with 24V as starting voltage.

The biggest capacitors are the two 10µF which are placed directly at the output of the buck converter (schematic). The Load is a voltage regulator (12V to 5V) which supplies a small microcontroller. I also increased the output current by a resistor(47 Ohm ^= 255mA) but the same problem occured. I placed a 100nF capacitor from SS/TRK to AGND to increase the startup-time with no effect in relation to the mentioned problem.

Startup fault mode: