5 Calculation of Feedback Resistor Values

The resistive divider (R_FB1, R_FB2, R_FB3) defines both the minimum and maximum adaptive boost voltage levels, as well as UVP and OVP levels. For feedback network using two-resistor method as shown in Figure 5-1, V_MINBOOST, V_MAXBOOST, V_{BOOST_UVP,MAX}, V_{BOOST_OVP_LOW} and V_STRING can be expressed in Equation 5, Equation 6, Equation 7, Equation 8, and Equation 9:

Equation 5.

Equation 6.

Equation 7.

Equation 8.

Equation 9.

where:

• V_BG is the band-gap voltage = 1.21 V
• R_FB1 is the upper feedback resistor
• R_FB2 is the lower feedback resistor
• I_{SEL_MAX} is the maximum internal 11-bit DAC current I_SEL = 38.7 μA
• V_UVP is the undervoltage threshold at feedback pin = 0.886 V
• V_OVPL is the overvoltage low level at feedback pin = 1.423 V
• 0.886 is the ratio of initial I_SEL DAC current with respect to I_{SEL_MAX} to set initial boot voltage during start-up
• V_LED is the voltage cross an LED
• n is the number of LEDs in a string

Figure 5-1 Two-Resistor Feedback Network

For feedback network using three-resistor method as shown in Figure 5-2, V_MINBOOST, V_MAXBOOST, V_{BOOST_UVP,MAX} and V_{BOOST_OVP_LOW} can be expressed in Equation 10, Equation 11, Equation 12, and Equation 13:

Equation 10.

Equation 11.

Equation 12.

Equation 13.

where:

• R_FB3 is the third feedback resistor can be used in applications where less than 200-kΩ resistors are required.

Figure 5-2 Three-Resistor Feedback Network