Soft start, inrush and slope compensation
Soft-start charge, output-capacitor inrush, peak-current sense and sampled-current stability screening.
Development preview. Check the assumptions below before relying on a result. Calculations run in your browser.
How it calculates
Slope stability: ma > (m2−m1)/2; perturbation multiplier = −(m2−ma)/(m1+ma)
Assumptions
- Fixed-frequency peak-current-mode buck with linear inductor slopes and an additive comparator ramp starting at zero each cycle.
- Sense resistor sets the zero-ramp limit. Added slope reduces the actual duty-dependent peak limit unless the controller compensates internally.
- Soft-start model is a linear output ramp with a constant startup load; foldback, reference shaping and prebias are omitted.
Worked example
These are the calculator's default inputs. Change any input above to recalculate.
Inputs
- Buck input voltage
- 24 V
- Output voltage
- 12 V
- Load current during startup
- 1 A
- Effective output capacitance
- 0.001 F
- Linear soft-start ramp time
- 0.02 s
- Soft-start pin charge current
- 0.00001 A
- Soft-start threshold
- 1.2 V
- Buck inductance
- 0.000047 H
- Switching frequency
- 300000 Hz
- Desired sensed peak-current limit
- 3 A
- Current-sense comparator threshold
- 0.1 V
- Current-sense amplifier gain
- 1
- Added comparator-input ramp slope
- 0 V/s
Results
- Soft-start capacitor
- 1.66667e-7 F
- Sense resistor before slope correction
- 0.0333333 Ω
- Half-downslope design starting point
- 4255.32 V/s
- Output-capacitor charging current
- 0.6 A
- Output current during ramp
- 1.6 A
- Sense resistor continuous RMS loss
- 0.0858363 W
- Theoretical minimum added ramp
- 0 V/s
- Peak limit after selected ramp
- 3 A