LM43601: output drop from -5.5V to -1.8V

Amelie Zheng

Part Number: LM43601

Hi Team,

My customer now use LM43601 in their system and found an issue.

They use webench to design the circuit to convert 24V to -5.5V.

When add a 20 ohm res as point of load (POL), which means the output current is nearly 200mA. It could work for a while, but after 4 hours, the output dropped from -5.5V to -1.8V. In this abnormal situation, customer do the following test. At that time， FB is -1.4V. But it is not very hot.

When remove the POL res, it could work again and output -5.5V.
Or When put tweezers in FB pin, it could work again and output -5.5V.
Or When remove the LM43601 from PCB and then solder again the same LM43601 in PCB, It could work again but after 4 hours, the same issue still happen

It is really strange. Could you kindly help to give some advice? I think it is the feedback loop issue...

over 3 years ago

0 Andy Chen over 3 years ago

TI__Mastermind 27045 points

Hi Amelie,

FB3 should be placed between C65 and C66. Pls ask customer to short FB3, or place another 4.7uF 50V cap between VIN and PGND pin of U3

pls change R43 to 10.2K and R42 to about 44.3K for 5.5V output.

B R

Andy

0 Amelie Zheng over 3 years ago in reply to Andy Chen

TI__Genius 10865 points

Hi Andy，

Thanks for your advice!

Customer shorted the FB and it now works normal.

May I ask why can't VIN add FB3?

0 Andy Chen over 3 years ago in reply to Amelie Zheng

TI__Mastermind 27045 points

Hi Amelie,

LM43601 VIN pin is power pin which sink pules current with high di/dt, the high-freq bypass capacitor should be placed close to VIN and PGND pin to reduce the loop parasitic inductor, however on customer's design, there is a bead FB3 placed between C66 and VIN pin, this will cause strong ringing at high-side switch turn-on/off which may cause unstable switching or limitation on output current capability.

B R

Andy

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LM43601: output drop from -5.5V to -1.8V