LM3409 Datasheet, PDF (22/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

3. AVERAGE LED CURRENT
Determine I_L-MAX:
Determine I_IN-RMS:
Assume V_ADJ = 1.24V and solve for R_SNS:
The chosen component from step 5 is:
The closest 1% tolerance resistor is 0.2 Ω therefore I_LED is:
6. PFET
Determine minimum Q1 voltage rating and current rating:
The chosen component from step 3 is:
4. OUTPUT CAPACITANCE
Assume r_D = 2 Ω and determine Z_C:
A 70V, 5.7A PFET is chosen with R_DS-ON = 190mΩ and Q_g =
20nC. Determine I_T-RMS and P_T:
Solve for C_O-MIN and :
Choose C_O:
The chosen component from step 5 is:
5. INPUT CAPACITANCE
Determine t_ON:
Solve for C_IN-MIN:
Choose C_IN:
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The chosen component from step 6 is:
7. DIODE
Determine minimum D1 voltage rating and current rating:
A 60V, 5A diode is chosen with V_D = 750mV. Determine P_D:
The chosen component from step 7 is:
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