LMZ13608 Output is not as what I want !

Hello Marshall

The regulated output voltage determined is determined by equation (4) on page 16 of the datasheet:  VOUT = 0.795V * (1 + RFBT / RFBB)

With your RFBT (R1) and RFBB (R2) combination you should expect VOUT = 3.26V which you're getting.

Your assumption of VFB = 1.2V it's incorrect; VFB = 0.795V.  So to get VOUT = 5V increase RFBB (R2) to 5.7 KOhms.

Best Regards,

-Juan

Dear Juan:

  Thanks a lot for your prompt response.

  I've checked the datasheet that the Rev.F Vfb=0.795 but Rev.e Vfb=0.12.

  OUTPUT VOLTAGE SELECTION
  Output voltage is determined by a divider of two resistors connected between VOUT and AGND. The midpoint of
the divider is connected to the FB input.
The regulated output voltage determined by the external divider resistors RFBT and RFBB is:
VOUT = 1.2V * (1 + RFBT / RFBB) (4)

I got the old datasheet. Rev.E  April 2013

Thank you again !

Marshall Wu