LMR23630: power up concerns

Richard Koski

Part Number: LMR23630
Other Parts Discussed in Thread: TLV62568,

I have the LMR23630 used into a Tlv62568. 24v to 5 v to 3.3 v. We had start up problems when hot plugging the 24 volt (instant 24 v to the input). The 5 volt output would not come up. If we slowed the ramp up of the 24 volt (R40 on attachment) we got the 5volt to come up. So we added a small resistor. Now we have times with the slower power up we will not get 5v reliably we have to drop power and quickly bring power back on. then we get 5 v .Can you weigh in on the issue? It seen the chip is touchy on how 24v is presented to the input.

over 5 years ago

0 Katelyn Wiggenhorn over 5 years ago

TI__Genius 11040 points

Hello,

I am unable to see your image. Would you be able to reshare this image by attaching the file as well as share this schematic?

Thank you,
Katelyn

0 Richard Koski over 5 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

lmr23630use.pdf

0 Katelyn Wiggenhorn over 5 years ago in reply to Richard Koski

TI__Genius 11040 points

Hello,

Thank you for resharing the schematic.

I do not expect the device to be sensitive to VIN=24V. The only sensitivity will be if the voltage goes above the maximum input voltage of this device during a hot plug event (due to the ringing).

Reviewing the schematic, I noticed that the Cboot (C1) is 680nF. This capacitor should be between 100nF to 470nF. Can you replace this capacitor and retest?

Best Regards,
Katelyn

0 Richard Koski over 5 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

I will try that. We also were having trouble going above 21 volts on the hot plug and we raised our output cap to 330 uF and that seemed to help but we are still having issues around 25 v hotplugging. We will try lowering the Cboot

0 Richard Koski over 5 years ago in reply to Katelyn Wiggenhorn

Prodigy 80 points

Katelyn,
The 680nf was given to us by the TI online design configurator, is it in error?

0 Richard Koski over 5 years ago in reply to Richard Koski

Prodigy 80 points

also what is the significance of higher Cboot? Is there a formula or such that determines cboot or trial and error?

0 Katelyn Wiggenhorn over 5 years ago in reply to Richard Koski

TI__Genius 11040 points

Hello,

The Cboot is typically determined based on the simulations results for a given device. Please see below for my explanation of the significance of a higher Cboot.

1. Too large means it may take a few switching cycles to charge CBOOT to the steady-state value. This may affect startup operation but should not damage anything. It will also cost more and may take more space for the larger cap.

2. Too small means the cap might not have enough energy to maintain the steady state voltage during switching cycles. This would reduce the effective gate voltage on the FET which would not turn on the FET as well which would lead to more power dissipation in the FET / worse efficiency overall.

Best Regards,
Katelyn

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LMR23630: power up concerns