LM3409MY-NOPB Datasheet, PDF (29/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

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R_SNS
=
5
V_ADJ
x I_{L- MAX}
=
1.24V
5 x1.22A
=
0.203:
The closest 1% tolerance resistor is 0.2 Ω therefore I_LED is:
I_LED
=
5
V_ADJ
x R_SNS
-
'i_{L - PP}
2
=
1.24V _-
5 x 0.2:
445 mA
2
= 1.02A
The chosen component from step 3 is:
R_SNS = 0.2:
4. OUTPUT CAPACITANCE
Assume r_D = 2 Ω and determine Z_C:
Z_{C =}
r_D x 'i_LED-PP
'i_L-PP - 'i_LED-PP
=
2: x 50 mA
450 mA - 50 mA
=
250
m:
Solve for C_O-MIN and :
C_{O - MIN}
=
2
x
S
x
1
f_SW
x
Z_C
C_{O - MIN}
=
2
x
Sx
503
1
kHz
x
250
m:
=1.27 ^PF
Choose C_O:
C_O = C_{O- MIN} x1.75 = 2.2 PF
The chosen component from step 5 is:
C_O = 2.2 PF
5. INPUT CAPACITANCE
Determine t_ON:
t_{ON =}
1
f_SW
-
t _OFF
=
1
503 kHz
- 700 ns _=1.29 Ps
Solve for C_IN-MIN:
C_IN-MIN =
I_LED x t_ON
'v_{IN- PP}
=
1.02A x1.29 Ps
720 mV
= 1.82 PF
Choose C_IN:
C_IN = C_{IN- MIN} x 2 = 3.64 PF
Determine I_IN-RMS:
I_{IN- RMS} = I_LED x f_SW x t_ON x t_OFF
I_IN-RMS = 1.02A x 503 kHz x 1.29 Ps x 700 ns = 486 mA
The chosen component from step 5 is:
C_IN = 4.7PF
LM3409, LM3409HV, LM3409-Q1
SNVS602J – MARCH 2009 – REVISED MAY 2013
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