ZHCSQ40C January 2023 – September 2023 LM2105
PRODUCTION DATA
The bootstrap capacitor must maintain the VBST-SH voltage above the UVLO threshold for normal operation. Calculate the maximum allowable drop across the bootstrap capacitor with Equation 11.
where
Then, the total charge needed per switching cycle is estimated by Equation 12.
where
Next, use Equation 13 to estimate the minimum bootstrap capacitor value.
In practice, the value of the CBoot capacitor must be greater than calculated to allow for situations where the power stage may skip pulse due to load transients. Equation 4 can be used to estimate the recommended bootstrap capacitance based on the maximum bootstrap voltage ripple desired for a specific application.
where
TI recommends having enough margins and to place the bootstrap capacitor as close to the BST and SH pins as possible.
As a general rule, the local VGVDD bypass capacitor must be 10 times greater than the value of CBOOT, as shown in Equation 6.
The bootstrap and bias capacitors must be ceramic types with X7R dielectric. The voltage rating must be twice that of the maximum VGVDD considering capacitance tolerances once the devices have a DC bias voltage across them and to ensure long-term reliability.