LP3997: Shootup voltage observed at the Vout of LP3997 during Vin switching

gautam jotkar

Expert 1165 points

Part Number: LP3997

Shootup voltage observed at the output of LP3997.

Applied Vin: 4.4V

Vout waveform attached.

Schematic diagram also attached that we have referred.

Capacitor on Vin and Vout are of order 10uF.

What will be the reason behind the shoot up voltage?

Under normal operating condition IC is working fine, generating 3,3Vout.

When switching done, means connecting, disconnecting 4.4V shoots observed.

over 6 years ago

0 kai klaas69 over 6 years ago

Guru 140200 points

Hi Gautam,

what load current is flowing? What is the purpose of R34? And what do you mean by "switching"? Connecting the input to 4.4V or toggling the /SD pin?

Kai

0 gautam jotkar over 6 years ago in reply to kai klaas69

Expert 1165 points

Hi,
Load Current is >50mA
We have put R34 (4.7kOhm) on the SD pin (The role of SD pin as in the datasheet)
We have also checked with putting R34 as 0 Ohm. (same problem of Vout overshoots observed)
SHUTDOWN
SD controls the turning on and off of the LP3997. VOUT is ensured to be on when the voltage on the SD pin is
greater than 0.95V. VOUT is ensured to be off when the voltage on the SD pin is less than 0.4V.

Switching Means:
5V0 point in my schematic is connecting on/off (fast)
on voltage: 4.4V
off voltage: 0V

0 kai klaas69 over 6 years ago in reply to gautam jotkar

Guru 140200 points

Hi Gautam,

when toggling Vin very quickly you might violate the output voltage to input voltage maximum rating of 0.3V. Or by other words, your output voltage becomes higher than your input voltage. This can make the regulator work improperly or even destroy it.

What happens if you install a protection diode from output to input?

Kai

0 Jason Song over 6 years ago in reply to kai klaas69

TI__Mastermind 23361 points

Hi Gautam,

Will you help us to understand what's the purpose of the fast switching Vin and Enable? What's the frequency of the switching? Can you also include the Vin in the scopeshot together with Vout?

Regards,
Jason Song

0 gautam jotkar over 6 years ago in reply to Jason Song

Expert 1165 points

Hi,

We have test case in which 200ms on-off is required. This is a part of Product generic standard.

As in the image switching freq. is of the order of 3Hz.

Vin (4.4V is stable)--- PINK

Vout --- Blue

Images attached.

BR,

0 gautam jotkar over 6 years ago in reply to kai klaas69

Expert 1165 points

Hi,
See the Vout/Vin images in the next thread.

BR,
Gautam

0 kai klaas69 over 6 years ago in reply to gautam jotkar

Guru 140200 points

Hi Gautam,

this looks like a damaged regulator! Take a fresh one and install a protection diode from output to input.

Kai

0 gautam jotkar over 6 years ago in reply to kai klaas69

Expert 1165 points

Hi, This is a fresh one. I have checked in 5 samples.

Thanks for your valuable time.

BR,
Gautam

0 Jason Song over 6 years ago in reply to gautam jotkar

TI__Mastermind 23361 points

Hi Gautam,

Can you try the same test but with slower Vin ramp? Right now, you ahve 200ms delay in two pulses, can you make it to 1-2 seconds and retake the wafeform?

Regards,
Jason Song

0 kai klaas69 over 6 years ago in reply to gautam jotkar

Guru 140200 points

Hi Gautam,

have you soldered a protection diode from output to input?

Kai

0 gautam jotkar over 6 years ago in reply to Jason Song

Expert 1165 points

Hi,

i already checked taht with 1-2sec, it is working fine.

What is the issue with 200ms.?? My use case is with 200ms..

Regards,

Gautam

0 Jason Song over 6 years ago in reply to gautam jotkar

TI__Mastermind 23361 points

Hi Gautam,

Thanks for the update. Since the issue is gone when you slowed down the cycles, we can confirm the overshot is due to the fact that between Vin cycles, the internal reference voltage is not being fully discharged. Essentially the overshot is due to the device startup when the internal reference is not zero. During normal startup, Vin is proportional to internal reference. But when internal reference is not zero, the unbalanced loop will try to pull vout high to catch up the non-zero internal reference.

Since there is no external connections for you to discharge the internal reference voltage, unfortunately you will need to slow down your Vin cycle to prevent excessive overshot due to startup with unbalanced loop.

Regards,
Jason Song

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LP3997: Shootup voltage observed at the Vout of LP3997 during Vin switching