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LT8637 with soft-start function

Dears,

We started to use the LT8637 recently and have obtained most of the time the desired results. The output noise remains below the 10mVpkpk as stated in the datasheet while the output voltage is generally stable. However, we have encountered some problems as well with this chip.

The current settings of the chip are:

  • We are using the chip in spread spectrum mode with an RT = 86.6k (base frequency around 500k). The sync/mode pin is connected to INTvcc and bypassed to GND with a 1uF capacitor
  • The input voltage is stable between 33V and 34V, the output voltages are 3.3V, 5V, 7.5V and 28V. The output current is at highest 200mA per output channel. We are using higher value inductors of 22uH with a DCR of +-100mOhm. These inductors are less optimal (i.e. 20mOhm as stated in the datasheet) but we do not need the extreme efficiencies required for 5A.With the current setting we are able to obtain 80%-85% efficiency which correspond to the reported efficiencies reported in the datasheet for high Vin.
  • The combined output capacitors are around 100uF.
  • The Vin is bypassed with 4.7uF + 2x1uF.
  • In all cases, the feedback resistor network consists of a 1MOhm resistor + a 4.7pF capacitor with a calculated series resistor to obtain the desired voltages. The output voltages are within 50mV compared of the desired voltage.

We want to use the soft-start function to diminish the inrush-current, however the datasheet just states that a capacitor can be used to enable this function. There is not really a way to calculate the value of this cap. Everytime we want to use this function with a 100nF capacitor the chip stops working after several startup sequences and goes into short circuit. It can work well for a long amount of time, but at a random startup-sequence (EN-pin going 3.3V) the chip dies and causes a short circuit. When this capacitor is removed, no malfunctioning of the chip is observed. This chip is used to power analog circuitry, so when the chip is disabled (i.e. EN is low) the output voltage also goes to 0V.

We noticed several things from the datasheet:

  • This capacitor is only used when the spread-spectrum is not used.
  • The capacitor, in the datasheet when used, is always 10nF.

Are the conditioning limits that the spread-spectrum shall not be used, and, is the capacitor maximally 10nF? This is not clear at all from the datasheet. Other then that, the chip works well.

Best regards,
Laurent



Rephrased a sentence by removing "for"
[edited by: lasegers at 12:21 PM (GMT -4) on 25 Aug 2021]