SNVA046E January   2002  – December 2021 LM2655

 

  1.   Trademarks
  2. 1Introduction
  3. 2Evaluation Board Design
  4. 3Soft Start
  5. 4LDelay
  6. 5Board Layout
  7. 6Revision History

3 Soft Start

Soft start in the LM2655 provides delay and current ramp-up rate control at startup. Both are a function of the value of Css. After a delay, pulses begin with the current limit set to zero then ramped up to full. The total time for soft start process is estimated as:

Equation 2. Tss = Css (.6 V/2 µA) + Css (2 V−.6 V)//10 µA

Which reduces to Tss = Css (440,000)

Table 3-1 Examples
CSS TSS
470 pF 225.5 µs
1 nF 451 µs
2.2 nF 992 µs
4.7 nF 2.07 ms

To begin with, use 4.7nF. This will assure the circuit will get up and running under full load.

If the output voltage does not attain 80% of its intended voltage when the under voltage lockout is enabled, (Css too small) the regulator will latch off. The actual time required for the output voltage to rise will depend upon the value of the inductor, output capacitor, the load on start up, and the supply voltage, as well as the value of Css. The calculated value of Tss approximates the time that the under voltage and over voltage lock out circuitry will be delayed from the application of power.