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ZHCSQ40C January 2023 – September 2023 LM2105

PRODUCTION DATA

封装选项

机械数据 (封装 | 引脚)

D|8
- MSOI002K
DSG|8
- MPDS308C

散热焊盘机械数据 (封装 | 引脚)

DSG|8
- QFND141I

订购信息

8.2.2.1 Select Bootstrap and GVDD Capacitor

The bootstrap capacitor must maintain the V_BST-SH voltage above the UVLO threshold for normal operation. Calculate the maximum allowable drop across the bootstrap capacitor with Equation 11.

Equation 1.

∆ V_{B S T} = V_{G V D D} - V_{D H} - V_{B S T L} = 10 V - 2.1 V - 4.45 V = 3.45 V

where

V_GVDD = Supply voltage of the gate drive IC
V_DH = Bootstrap diode forward voltage drop
V_BSTL = BST falling threshold (V_BSTR(max) - V_BSTHYS)

Then, the total charge needed per switching cycle is estimated by Equation 12.

Equation 2.

Q_{T O T A L} = Q_{G} + I_{B S T S} \times \frac{D_{M A X}}{f_{S W}} + \frac{I_{B S T}}{f_{S W}} = 17 n C + 33.3 μ A \times \frac{0.95}{50 k H z} + \frac{130 μ A}{50 k H z} = 20 n C

where

Q_G = Total MOSFET gate charge
I_BSTS = BST to VSS leakage current
D_Max = Converter maximum duty cycle
I_BST = BST quiescent current

Next, use Equation 13 to estimate the minimum bootstrap capacitor value.

Equation 3.

C_{B O O T (M I N)} = \frac{Q_{T O T A L}}{{∆ V}_{B S T}} = \frac{20 n C}{3.45 V} = 5.8 n F

In practice, the value of the C_Boot 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.

Equation 4.

C_{B O O T} > \frac{Q_{T O T A L}}{{∆ V}_{B S T_R I P P L E}}

where

∆V_{BST_RIPPLE} = Maximum allowable voltage drop across the bypass capacitor based on system requirements

TI recommends having enough margins and to place the bootstrap capacitor as close to the BST and SH pins as possible.

Equation 5. C_BOOT = 100 nF

As a general rule, the local V_GVDD bypass capacitor must be 10 times greater than the value of C_BOOT, as shown in Equation 6.

Equation 6. C_GVDD = 1 µF

The bootstrap and bias capacitors must be ceramic types with X7R dielectric. The voltage rating must be twice that of the maximum V_GVDD considering capacitance tolerances once the devices have a DC bias voltage across them and to ensure long-term reliability.