SLVAFU0 Application note

SLVAFU0 April 2024 TPS61377

2.2 Improvement of Gap Between Negative Voltage Rail and Positive Voltage Rail

Figure 2-2 shows the improvement. To select appropriate R3 and forward voltage of D1 can improve the gap. The negative voltage rail formula can be edited as Equation 5. For best practice have the design meet Equation 6 to reduce the gap. To reduce the gap in full load current range, the commendation is to design to eliminate the forward voltage drop. And can simplify the Equation 7 to Equation 8.

Equation 5.

{- V}_{O U T 2} = (V_{O U T 1} + V_{D 1} + \frac{I_{O U T 1}}{1 - D} \times R_{3}) - (I_{C H G 1} + I_{C H G 2}) \times R_{C H G} - V_{D 2} - V_{D 3}

Equation 6.

V_{D 1} + \frac{I_{O U T 1}}{1 - D} \times R_{3} = \frac{1}{D (1 - D)} \times I_{O U T 2} \times R_{C H G} + V_{D 2} + V_{D 3}

Equation 7.

I_{O U T 1} \times R_{3} = \frac{1}{D} \times I_{O U T 2} \times R_{C H G}

Equation 8.

R_{3} = \frac{1}{D} \times R_{C H G} \times \frac{I_{O U T 2}}{I_{O U T 1}}

Figure 2-2 Improvement of Voltage Gap Between Negative Voltage Rail and Positive Voltage Rail