LM5117: NO OUTPUT CURRENT

Francesco Da Re

Prodigy 250 points

Part Number: LM5117
Other Parts Discussed in Thread: CSD18563Q5A

Our design has two LM5117 with same caracteristics:

Vin variable between 30Vdc and 42Vdc

Vout 12Vdc

Current Out Max 10A

With no load we have 12V output, but under any load we do not get any output current.

In the past we designed other power units with LM5117 without problems. So this situation is quite upsetting...

Any suggestion or help?

over 2 years ago

0 Timothy Hegarty over 2 years ago

TI__Mastermind 49597 points

Hello Renan,

These FETs are old technology devices with extremely high gate charge and capacitances. The Qg is 200nC at Vgs = 7.5V, thus resulting in very slow switching. Moreover, the current limit of the VCC regulator (30mA min, 42mA typ) will be exceeded during startup when VOUT does not overdrive VCC. Ivcc for each FET = Qg*Fsw = 200nC*220kHz = 44mA.

Please select lower gate charge FETs, e.g. CSD18563Q5A 60V NexFET or similar. Use our MOSFET selector tool for more suggestions:

https://www.ti.com/tool/SYNC-BUCK-FET-LOSS-CALC

https://training.ti.com/nexfet-synchronous-buck-converter-fet-selection-tool-training

Given the voltage step down from 30-42Vin to 12Vout, the high-side FET in particular should be optimized for switching losses (i.e. choose a FET with higher Rdson, lower Qg). No need for such low Rdson value for a 10A rated output current.

Regards,

Tim

Power management

Power management forum

LM5117: NO OUTPUT CURRENT