Hi Masters,
My customer is using this device in their infotainment project, 9-12V VIN and MAX 30V@85mA/CH Vout. Schematic is attached as below, I heard from other customer that the R7 of FB should be controlled under 100Kohm, which is different from datasheet. Could you please give some comments on this?
Thanks
Eric
Hello Eric,
There are two options for feedback network:
1. For 2- resistor network, recommended R2 (bottom resistor) value is 130kOhm as mentioned in the datasheet.
For 30.4V max boost, R1 should be ~610kOhm not 750kOhm.
Vboost_max = ((1.2/130) +0.0387) x 610 + 1.2 = (0.04793) x 610 + 1.2 = 29.237 + 1.2 = 30.4V
2. T-divider network or 3 - resistor network is shown below. Most customers now prefer to use T-divider instead since it allows for lower resistor values which are preferred and sometimes required to pass humidity tests. Also feedback resistors will not only determine Vout_max but also Vout_min. When using T-divider network, the Vout range will be smaller, allowing for a better resolution of the boost adaptive voltage. When using T-divider, R2 = 6kOhm, R3 = 27kOhm are recommended and modifying R1 to meet max boost output spec. For this case, R1 = 69kOhm for Vout_max 30.7V. I have attached a calculation tool for comparison.
Hi Eric,
Equations are derived from simple node analysis as shown below:
Where:
I1 = I2 + I3
I2 = V2 / R2
I3 = ISEL (38.7uA for max; 5 uA for min)
V2 = VBG + (R3 x I3)
VOUT = V2 + (R1 x I1)
VBG = 1.2V
Solve these equations for your unknowns V2, I2, I1 and VOUT.
