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LM2731: Vout when SHDN=0V (shutdown functionality)

Part Number: LM2731


I've designed a printed circuit board with the typical application circuit of LM2731.

It is intended to use Vin=5V and Vout=9V. The values of the external components used are:

C1=2.2uF; L1=10uH; D=MBR020L; R1=106k; R2=16k; Cf=270pF; C2=4.7uF.

All good when the device is on (Vout=9V). However, when I try to shut down the component (SHDN=0V), I have Vout=4.8V, slightly below the Vin voltage. Is this the correct behaviour of the shutdown functionality? I was expecting 0V in the output.

  • Hi Matos,

    LM2731 doesn't support true shutdown function (Vout=0V) because it's non-synchronous boost converter. When SHDN=0V, there is no switching but the Vin will charge the output capacitor through inductor L, rectifying diode until Vout reaches close to Vin. That's why you have Vout=4.8V when Vin=5V.