MP103 does not run

Dear MPS, i have a standard application according to your reference design EV103-N-00A – OFFLINE INDUCTOR-LESS CONTROLLER EV BOARD. I changed the output voltage to get round abound 12-12.5V with 1-10mA output current. But if try to start this in my application, the VB Pin gets only 3.3V. What did i wrong? ![image|665x500]

Hi Robert:
Welcome to our community !!.
Does the change you do work on the MPS EVB but the same change does not work in your application? Do you have the same BOM as the EVB’s BOM? Please let us know. Thanks,

Dear ACDCguy,

the values of the parts are built in the uploaded picture. The difference between the EVB and my application are R1001 (changed from 2kOhm to 1kOhm), C1001 (changed from 470uF to 560uF) and R1003 + R1002 to get 12V-12.5V instead 5V. I need this LDO to start a PFC stage on the primary side without taking an additional winding on the main transformer. The controller of the PFC stage take a maximum of 1.38mA. With additional signal LED´s to sign “OK” the maximum load current for the LDO would be around 5-10mA.

Hi Robert:

I believe you have also changed the transistor Q1000 from the 3DD4802 to FJPF2145 which may be easier to procure in US. The change from 2K to 1K for R1001 will reduce the base current drive into the Q1001. This may increase the Vce voltage across the Q1000 which may reduce the voltage at pin 5 of MP103. What was the reason to reduce the R1001 from 2K to 1K?