LM3409MY-NOPB Datasheet, PDF (21/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

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R_OFF
=
(C_OFF
+
20
- _{¨¨©§}1-
pF)
K
V_O
x V_IN
¸¸¹·
x f_SW x ln
¨¨©§^1-
1.24V
V_O
¸¸¹·
SNVS602J – MARCH 2009 – REVISED MAY 2013
(28)
2. INDUCTOR RIPPLE CURRENT
Set the inductor ripple current (Δi_L-PP) by solving for the appropriate inductor (L1):
L1 ₌
V_O x t_OFF
'i_L-PP
(29)
3. AVERAGE LED CURRENT
Set the average LED current (I_LED) by first solving for the peak inductor current (I_L-MAX):
I_{L- MAX}
=
I_LED
+
'i_{L- PP}
2
(30)
Peak inductor current is detected across the sense resistor (R_SNS). In most cases, assume the maximum value
(V_ADJ = 1.24V) at the IADJ pin and solve for R_SNS:
R_SNS
=
5
V_ADJ
x I_L-MAX
(31)
If the calculated R_SNS is far from a standard value, the beginning of the process can be iterated to choose a new
R_OFF, L1, and R_SNS value that is a closer fit. The easiest way to approach the iterative process is to change the
nominal f_SW target knowing that the switching frequency varies with operating conditions anyways.
Another method for finding a standard R_SNS value is to change the V_ADJ value. However, this would require an
external voltage source or a resistor from the IADJ pin to GND as explained in the Theory of Operation section of
this datasheet.
4. OUTPUT CAPACITANCE
A minimum output capacitance (C_O-MIN) may be necessary to reduce Δi_LED-PP below Δi_L-PP. With the specified
Δi_LED-PP and the known dynamic resistance (r_D) of the LED string, solve for the required impedance (Z_C) for C_O-
MIN^:
Z_C
=
r_D x 'i_LED-PP
'i_{L- PP} - 'i_{LED- PP}
(32)
Solve for C_O-MIN:
C_{O - MIN}
=
2
x
S
1
x f_SW
x
Z_C
(33)
5. INPUT CAPACITANCE
Set the input voltage ripple (Δv_IN-PP) by solving for the required minimum capacitance (C_IN-MIN):
C_{IN- MIN =}
I_LED x t_ON
'v_{IN - PP}
=
I_LED
x
¨¨©§
1
f_SW
-
t _{OFF¸¸¹·}
'v_{IN - PP}
(34)
The necessary RMS input current rating (I_IN-RMS) is:
I_IN-RMS = I_LED x f_SW x t_ON x t_OFF
(35)
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