6 Feedback Resistor Calculation Example

Using the 2-resistor network as an example, the feedback resistors calculation can be found from the product folder. The same calculator can also be used for 3-resistor network calculation by simply setting R_FB3 = 0 Ω.

In this example, there are 8 LEDs in a string, with forward voltage drop varies from 3.1 V to 3.4 V. The worst-case V_STRING,MIN and (V_STRING + V_HEADROOM,MAX) + Design Margin can be calculated as 24.8 V and 30.2 V, respectively, in Table 6-1.

Assuming 1% feedback resistors are used, select feedback resistor values to meet the worst-case conditions listed in Equation 4, Equation 2, Equation 4, and Equation 4. In this example, with R_FB1 = 620 kΩ and R_FB2 = 56 kΩ, the calculated V_MINBOOST = 14.61 V (< 24.8 V) and V_MAXBOOST = 38.60 V (> 30.2 V) are able to cover the voltage range of LED string, as shown in Table 6-2.

With V_UVP of 0.886 V, the calculated V_{BOOST_UVP,MAX} = 34.69 V as shown in Table 6-3, which is above 28.2 V so that LED open fault will not get false triggered during boost UVP event.

With V_OVPL of 1.423 V, the calculated V_{BOOST_OVP_LOW} = 38.44 V as shown in Table 6-4, which is below 48 V so that boost OVP low fault will always be reported before boost OVP High Fault.