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  1.   1
  2.   Bootstrap Circuitry Selection for Half-Bridge Configurations
  3.   Trademarks
  4. 1Introduction
  5. 2Basic Operation of Bootstrap Circuit
  6. 3Bootstrap Components Selection
    1. 3.1 Bootstrap Capacitor
    2.     8
    3. 3.2 VDD Bypass Capacitor
    4. 3.3 External Bootstrap Diode
    5.     11
    6. 3.4 Bootstrap Resistor
    7.     13
  7. 4Layout Considerations for Bootstrap Components
  8. 5Summary
  9. 6References
  10. 7Revision History

Once the gate charge determined, the minimum value for the bootstrap capacitor can be estimated using Equation 2:

Equation 2. C boot ≥ 10 × C g

Alternatively, a more detailed calculation of the minimum bootstrap capacitor value can be done using Equation 3:

Equation 3. C boot ≥ Q total ∆ V HB Q total = Q G + I HBS × D max f sw + I HB f sw where : · Q G = Total   MOSFET   gate   charge   ( MOSFET ’ s   datasheet ) · I HBS = HB   to   VSS   leakage   current   ( gate   driver ' s   datasheet ) · D max = Maximum   duty   cycle · I HB = HB   Quiescent   current   ( Gate   driver ’ s   datasheet ) And ∆ V HB = V DD - V DH - V HBL where : · V DD = Supply   voltage   of   the   gate   driver   IC · V DH = Bootstrap   diode   forward   voltage   drop   ( Bootstrap   diode   datasheet ) · V HBL = HB   UVLO   falling   threshold   ( Gate   driver   datasheet )

It is important to note that values below the minimum required bootstrap capacitor value could lead to activation of the driver's UVLO therefore prematurely turning off the high-side FET. On the flip side, higher values of the bootstrap capacitor lead to lower ripple voltage and longer reverse recovery time in some conditions (when initially charging the bootstrap cap or with a narrow bootstrap charging period) as well as higher peak current through the bootstrap diode. Equation 4 relates the bootstrap cap and the peak currents through the bootstrap diode.

Equation 4. I peak = C boot × dv dt

It is generally recommended to use low ESR and ESL surface mount multi-layer ceramic capacitors (MLCC) with good voltage ratings (2xVDD), temperature coefficients and capacitance tolerances.