LM3424 Datasheet, PDF (30/50 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

1. OPERATING POINT
Solve for V_O and r_D:
Solve for D, D', D_MAX, and D_MIN:
The chosen components from step 3 are:
4. THERMAL FOLDBACK
Find the resistances corresponding to T_BK and T_END (R_NTC-
BK ^{= 24.3 kΩ and R}NTC-END ^{= 7.15 kΩ) from the manufacturer's}
datasheet. Assuming R_REF1 = R_REF2 = 49.9 kΩ, then R_BIAS =
R_NTC-BK= 24.3 kΩ.
Solve for R_GAIN:
2. SWITCHING FREQUENCY
Solve for R_T:
The chosen components from step 4 are:
The closest standard resistor is 14.3 kΩ therefore f_SW is:
5. INDUCTOR RIPPLE CURRENT
Solve for L1:
The chosen component from step 2 is:
3. AVERAGE LED CURRENT
Solve for R_SNS:
The closest standard inductor is 33 µH therefore Δi_L-PP is:
Determine minimum allowable RMS current rating:
Assume R_CSH = 12.4 kΩ and solve for R_HSP:
The closest standard resistor for R_SNS is actually 0.1Ω and for
R_HSP is actually 1 kΩ therefore I_LED is:
The chosen component from step 5 is:
www.national.com
30