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LTC3114 instability when Iout > 150 mA

Question asked by peeter123 on Apr 10, 2018
Latest reply on Jun 22, 2018 by JNoon

We have a LTC3114 design on a 2 layer PCB which becomes unstable when the output current is higher then 150 mA. This is found using an electronic load.

 

The specs are as follows:

Vin: 18-38 V (Nominal 26 V)
Vout: 24 V

Iout-max: 0.8 A

 

The design is according to the datasheet with the following values:

L = 15 uH (Isat = 2 A) with 68 nF bootstrap capacitors

Rz = 390 k

Cp1 = 4.7 nF

Cp2 = 10 pF

Dsw1 = STPS160A

 

PROG pin is not used and pulled down using 22 k and 68 nF. MODE pin is pulled down using 22 k.

 

Feedback is 2 Meg and 86 k

 

Phase margin is about 60 deg at worst case (18 Vin). Changing  Rz to 100 k does not help with instability and the input voltage also does not matter much.

 

Cin = 10 uF (with 1000 uF bulk capacitance)

Cout = 10 uF (with 100 uF bulk capacitance)

 

The bottom plane is almost solid ground and stitched to top ground where possible. Attached are some waveforms, the unstable pictures maybe also the result of the power supply going into current limit. See also the schematic.

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