TPS7A33: Positive and negative LDO can't power on Synchronously

JimmyLee

Part Number: TPS7A33
Other Parts Discussed in Thread: TPS7A47,

hello，

I used TPS7a4700 and TPS7A3301 to supplied ±18V for my audio amplifier TME49724 。 At powering on, the amplifier outputed a pulse voltage (tens of milliseconds，up to 8V) ,which damaged my audio ADC。 I suspect that negative and positive power sources have different ramp time that leads to this problem.According to waveform captured,+18 voltage can power on in a few milliseconds, but -18V takes hundreds of milliseconds .So, is there any suggestions can solve this problem ?

Besides, the capacitors added the two LDO ss/nr pins are 1uF。

over 4 years ago

0 JCHK over 4 years ago

TI__Mastermind 44140 points

Hi Jimmy,

You may need to delay enabling the TPS7A47/33 until after the negative rail comes up. Indirectly you can use something like the TPS3840 to wait for 100's of milli-seconds before enabling the TPS7A47 and TPS7A33. One idea for directly monitoring the negative rail would be to use the TPS38600.

I hope this helps.

0 JimmyLee over 3 years ago in reply to JCHK

Prodigy 185 points

Hi John,

The negative and positive power rails come up almost at the same time. I've solved it by changing SS capacitor 1uF to 47nF connected to TPS7A33. But the fly in the ointment is their different ramp characteristic TPS7A47 has a slope ramp and the TPS7A33 has a exponential ramp.

0 JCHK over 3 years ago in reply to JimmyLee

TI__Mastermind 44140 points

Hi Jimmy,

I see the TPS7A47 filters the gained up reference and the TPS7A33 filters the reference directly. One idea to try is to increase the NR capacitor on the TPS7A33 to slow down its soft-start time.

Can you try this?

0 JimmyLee over 3 years ago in reply to JCHK

Prodigy 185 points

Hi John,

I've tried that before.Under the same NR capacitor value, the TPS7A33 has a much longer start time than TPS7A47. Increasing NR capacitor only leads the two LDO soft start time's mismatch.

0 JCHK over 3 years ago in reply to JimmyLee

TI__Mastermind 44140 points

Hi Jimmy,

Have you tried either slowing the TPS7A47 down or speeding the TPS7A33 up?

There is no way to get them to match exactly but you should be able to get them closer to each other.

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TPS7A33: Positive and negative LDO can't power on Synchronously