SLUA887A August   2018  – September 2023 LM2005 , LM2101 , LM2103 , LM2104 , LM2105 , LM25101 , LM27222 , LM2724A , LM5100A , LM5100B , LM5100C , LM5101 , LM5101A , LM5101B , LM5101C , LM5102 , LM5104 , LM5105 , LM5106 , LM5107 , LM5108 , LM5109 , LM5109A , LM5109B , LM5109B-Q1 , LM5113-Q1 , LMG1205 , LMG1210 , SM72295 , SM74104 , TPS28225 , TPS28225-Q1 , TPS28226 , TPS2832 , TPS2836 , TPS2837 , UC2950 , UCC27200 , UCC27200-Q1 , UCC27200A , UCC27201 , UCC27201A , UCC27201A-DIE , UCC27201A-Q1 , UCC27211 , UCC27211A , UCC27211A-Q1 , UCC27212 , UCC27212A-Q1 , UCC27222 , UCC27282 , UCC27282-Q1 , UCC27284 , UCC27284-Q1 , UCC27288 , UCC27289 , UCC27301A , UCC27301A-Q1 , UCC27311A , UCC27311A-Q1 , UCC27710 , UCC27712 , UCC27712-Q1 , UCC27714

 

  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

This time constant occuring during the high-side off time explains the dependency on duty cycle. This duty cycle being constant, the bootstrap resistor and bootstrap capacitor should be tuned appropriately to achieve the desired start-up time. Increasing the bootstrap resistor values will increase the time constant leading to slower start-up time.

Additionally, the bootstrap resistor chosen must be able to withstand high power dissipation during the first charging sequence of the bootstrap capacitor. This energy can be estimated by Equation 7:

Equation 7. 12×Cboot×VCboot2

This energy is dissipated during the charging time of the bootstrap capacitor and can be estimated using Equation 8:

Equation 8. E≅3×Cboot×Rboot

This resistor is essential in limiting the peak currents through the bootstrap diode at start-up and limiting the dv/dt of HB-HS (high-side floating supply to the return high-side floating supply). The peak current through this resistor can be calculated using Equation 9:

Equation 9. Ipk=VDD-VBootDiodeRboot

Figure 3-4 shows the fast ramp up on VDD (CH4) and HB-HS (CH1) when using a 0-Ohm resistor which leads to undesired change in voltage on LO(CH3) and HO(CH2).

GUID-B6651563-BCAE-4358-A5E9-BE5D38F066DD-low.gifFigure 3-4 VDD/HB-HS Fast Ramp Up (Rboot = 0Ohms)

Figure 3-5 shows how using slightly higher resistor value (Rboot = 2.2Ohms) solve this issue. It is important to note that the bias rising rate observed in Figure 3-5 does not apply to all drivers.

GUID-7CD761F4-46F1-430B-9655-CFE4183EA3F2-low.gifFigure 3-5 VDD/HB-HS Fast Ramp Up (Rboot = 2.2Ohms)