SLOA198A September   2014  – December 2021 DRV2665 , DRV2667 , DRV2700 , DRV8662

 

  1.   Trademarks
  2. 1 Boost Converter Basics
  3. 2 DRV8662, DRV2700, DRV2665, and DRV2667 Boost Converter
    1. 2.1 DRV8662, DRV2700, DRV2665, and DRV2667 Boost Converter Efficiency
      1. 2.1.1 Boost Efficiency vs Boost Current
    2. 2.2 DRV8662, DRV2700, DRV2665, and DRV2667 Boost Converter Load Regulation
      1. 2.2.1 Boost Regulation vs Current
  4. 3 Configuring the Boost Converter
  5. 4 Boost Converter Output Voltage
  6. 5 Calculating the Load Current
  7. 6 Selecting an Inductor
    1. 6.1 Inductance Rating
    2. 6.2 Saturation Current Rating
    3. 6.3 Thermal Current Rating
    4. 6.4 Choosing REXT
    5. 6.5 What to Avoid: Using Incorrect Inductor Current Ratings
  8. 7 Calculate the Maximum Boost Current
  9. 8 Output Capacitor Selection
  10. 9 Input Capacitor Selection
  11. 10PCB Layout
    1. 10.1 What to Avoid: Incorrect Inductor Placement
  12. 11Examples
    1. 11.1 Example: Based on the DRV8662EVM
      1. 11.1.1 Configure the Boost Voltage
      2. 11.1.2 Configure the Inductor Current
      3. 11.1.3 Boost Performance Results
    2. 11.2 Example: Based on the DRV2667EVM-CT with 25-nF Piezo Module
      1. 11.2.1 Configure the Boost Voltage
      2. 11.2.2 Configure the Inductor Current
      3. 11.2.3 Boost Performance Results
  13. 12Revision History

9 Input Capacitor Selection

The input capacitor provides current to the DRVxxxx when there are large current transients during startup and heavy load periods.

GUID-460B928E-A5EF-4C2F-9D5B-8206488820D7-low.gif

When a bulk input capacitor is included, the following diagram shows that the battery actually sees a filtered version of ILIM.

GUID-907E1238-F0CF-47BF-8544-7660BD878543-low.gif

When the boost converter enters a heavy load condition or during a startup sequence, the switching frequency reaches a maximum value set by the slope of the charge/discharge curve and the ILIM value.

GUID-E0BC4E29-8889-4F95-8238-32F72BA1856A-low.gif

Calculating the exact IAVG value is rather difficult, but as long as the bulk decoupling capacitance in the system is sufficiently high, the average current drawn from the battery will be less than one-half of ILIM as a general guideline.

To estimate the bulk input capacitor value, use Equation 11.

Equation 11. GUID-0B479B08-2BE6-4F59-B707-B46AC5DEF516-low.gif
Symbol Description Value Unit
CBULK Minimum VDD bulk capacitance required – F
ITR Input transient current (maximum is ILIMIT) – A
LTrace Input trace inductance (estimate 50 nH, if unknown) – H
VDROOP Maximum boost output voltage droop
(Ex.: 3.6 V – 0.1 V = 3.5 V, 0.1 V is the allowable droop)		 – V

This equation was derived from the RLC circuit formed by the trace resistance and inductance combined with the bulk capacitance and capacitor ESR.

GUID-8C0F4477-B13F-4A52-9452-7861C261B9DC-low.gif

Keep in mind that it is difficult to calculate the exact input current required for the DRVxxxx, so a minimum capacitance between 22 µF and 47 µF is recommend.