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Boost converter calculator

Power stage values for a step-up converter in continuous conduction: duty cycle, inductance for a ripple target, peak and RMS inductor current, the output current the switch limit allows, and output ripple.

Presets
V
Switch current is worst at the lowest input.
V
A
kHz
%
From the datasheet curve, or 80 to 85 % as SLVA372 suggests.
%
Percent of I_OUT * V_OUT / V_IN, for the inductance result.
µH
µF
Effective value at the output voltage, after DC bias derating.
mΩ
A
Datasheet minimum. 0 to skip.
Minimum inductance for 30 % ripple
4.94µH
Vin3.3 VL4.7 µHQ1D1Vout5 VCout22 µFESR5 mΩLoad500 mAInductor currentPeak 1.045 AAvg 891.3 mAMin 737.1 mA0ΔI 308 mAT 1 µs D 43.9 %
Non-synchronous stage as drawn in SLVA372; a synchronous part replaces D1 with a second switch. Inductor current to scale from zero, with your L.
Duty cycle
43.9 %
Ripple current with chosen L
308 mA
41 % of I_OUT · V_OUT / V_IN (the target's basis), 35 % of the average inductor current
Average input (inductor) current
891.3 mA
Peak inductor and switch current
1.045 A
Inductor RMS current
895.7 mA
Maximum output current at switch limit
1.04 A
Output ripple from capacitance
9.98 mV
Output ripple from ESR
5.23 mV
Output ripple, sum (worst case)
15.2 mV
Method

How this is calculated

D = 1 − V_IN(min) · η / V_OUT
ΔI_L = V_IN(min) · D / (f_S · L)
L_min = V_IN · (V_OUT − V_IN) / (ΔI_L · f_S · V_OUT)
ΔI_L target = ripple % · I_OUT · V_OUT / V_IN
I_SW(max) = ΔI_L / 2 + I_OUT / (1 − D)
I_MAXOUT = (I_LIM(min) − ΔI_L / 2) · (1 − D)
ΔV_C = I_OUT · D / (f_S · C_OUT)
ΔV_ESR = ESR · ( I_OUT / (1 − D) + ΔI_L / 2 )
I_L(rms) = √( I_IN² + ΔI_L² / 12 )
Mode
Continuous conduction only. When the average inductor current I_OUT / (1 − D) falls below ΔI_L / 2 (SLVA061 section 2.3), the result is marked.
Worst case
Evaluated at the minimum input voltage, where duty cycle and switch current are highest.
Inductance and ripple
Minimum inductance follows SLVA372 Equation 5, which has no efficiency term, while the ripple uses the efficiency-adjusted duty cycle from Equation 1. Choosing exactly the minimum inductance therefore gives more ripple than the target: 39 % of I_OUT · V_OUT / V_IN for a 30 % target at the default inputs. Pick the next standard value up.
Limits not modeled
Maximum duty cycle, minimum on time and slope compensation are IC specific. Check the duty cycle against the datasheet.
Sources
FAQ

Questions

About 0.44 at 85 % efficiency: 1 − 3.3 × 0.85 / 5. The lossless figure is 0.34; losses push the duty cycle up because the converter must also supply them.

Power in equals power out over efficiency, so at 3.3 V in, 5 V and 0.5 A out, 85 % efficient, the inductor carries about 0.89 A on average. The switch and inductor must be rated for that plus half the ripple.

(I_LIM(min) − ΔI_L / 2) · (1 − D). A 2 A minimum switch limit at D = 0.44 with 0.31 A of ripple gives about 1.04 A, about half the switch limit.

SLVA372 suggests 20 to 40 % of I_OUT · V_OUT / V_IN, roughly the input current. Lower ripple needs a larger inductor but raises the current the IC can deliver.

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