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Abnormal operation of LTM8073

I had a question from a customer about LTM8073.

 

My customers have confirmed the operation by changing the output voltage of the LTM8073 to 3.3V and the input voltage to 4.5V to 28V. When this operation is confirmed, when the voltage is between 4.5V and 5V and between 15V and 26V, 3.3V is output according to the set value.

However, there is a phenomenon that the output voltage drops by 50 mV while the input voltage is 5 V to 15 V.

Please tell me about this cause.

 

The solution was to short the connection between the BIAS and AUX pins to the BIAS and VIN pins to ensure that the output voltage did not fluctuate.

However, the data sheet states that if the output exceeds 3.2V, it will be connected to AUX. Is this a problem in this case?

Best Regards

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  • When Vin<~4.5V (or maybe 5V as you saw), Vin will be automatically used to supply the internal circuits (amplifiers and so on) no matter the value of Vbias. When Vin>~4.5V and Vbias >3.2V, Vbias will be used to supply internal circuits. So, the internal voltage reference (such as VFB reference) might change during the transit from Vin to Vbias. This change caused the output voltage changes. 

    When you connect Vbias to Vin instead of Vout, Vin always be the source for internal circuits and no such transit I mentioned above, so output voltage keeps more constant.

  • While connecting Vbias to Vin will have better performance in terms of Vout: input regulation, the tradeoff will be efficiency. Higher Vbias would hurt efficiency performance. 

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