I am looking for equations for the feedback network on the MP8004. The datasheet for the MP8004 and the EV8004-QW-00A do not provide any details on this and this device is not supported by MPS DCDC Designer. Is there any other design support; e.g. spice models, spreadsheets, app notes, etc for this device? Thanks!
Hello Matthew,
Referring to figure 9 in the data sheet, the complete schematic for a 12V output flyback, the Vout is set by the feedback resistor network to the TLV431, a reference/amplifier with a 1.24V reference. R17/R18 are set to divide the 12V output down to 1.24V. In this case adjusting R18 to a lower value would reduce Vout. The compensation may need to be adjusted as well as this will change the loop gain depending on the Vout needed.
The feedback voltage is 1.21V. Is this going to be for an isolated application, like the front page datasheet circuit?
The R17/R18/R15 I was able to scale to get the output voltage and maintain the opto LED current. I was thinking more about R1/R2/R7/R8/R16/C4/C12. Does the compensation also need to change with the different output voltage? The datasheet gives no details on this and says to reference the EV8004 docs for compensation parameters, but there is nothing in that documentation either.
Also, by my calculations on the existing design, the opto needs a CTR of 155% but the unranked PC357 can be as low as 50%.
In the old days we used to get demo boards and test it / tweak it ourselves. The fig 9 sch should work with a CTR of 35%. When everything is stable I will have 1.24V on R2 or 1.24mA if the CTR is 35% then in the LED I have a current of 3.6mA. Which is 10.8V on the R15, plus a volt for the LED well it is really close. Hey I used to do aps engineering so trust me datasheets are not at the same level as the 10 commandments. The primary side components look somewhat decorative to me, The loop comp is usually accomplished at the 431. The gain of that amp is about -6 at DC starts rolling off at 7khz and then by 15khz settles out at a gain of -3. Not too interesting.