SNVAA70 Application note

SNVAA70 june 2023 LM5176 , LM5176-Q1

2.3 Output Voltage Control of LM5176

Figure 2-2 shows the topology of the output voltage control using a microcontroller. Vout is regulated by using a resistor divider (R1 and R2) to the FB pin. The internal reference voltage of the LM5176 is 0.8 V. To control Vout through the DAC, the Resistor R3 is connected with the feedback divider and allows current to feed into the feedback node. For a detailed explanation, please refer to PMP9796 5 V Low-Power TEC Driver Reference Design.

Figure 2-2 Control of Vout Using MCU

According to the data sheet, the bottom resistor R2 in the resistor divider is in the 1 kΩ to 100 kΩ range. Once the value of R1 has been chosen, R2 and R3 can be calculated by using the following equations:

Equation 1.

R_{3} = \frac{V_{c t r l_m a x} - V_{c t r l_m i n}}{V_{o_m a x} - V_{o_m i n}} \times R_{1}

Equation 2.

R_{2} = \frac{V_{F B} \times R_{1} \times R_{3}}{V_{o_m a x} \times R_{3} - V_{F B} \times (R_{1} + R_{3}) + V_{c t r l_m i n} \times R_{1}}

where

R₃ is the resistor between control voltage and FB pin
V_{ctrl_max} is the maximum value of control voltage
V_{ctrl_min} is the minimum value of control voltage
V_{o_max} is the minimum value of output voltage
V_{o_min} is the minimum value of output voltage

For example, if R1 is selected as 280 kΩ, and Vout is from 5 V to 25 V, Vctrl needs to be varied from 0.5 V to 2 V. According to the previous equations, R2 and R3 are selected to be 11 kΩ and 21 kΩ respectively.