LM3424 Datasheet, PDF (32/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
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Typical Applications (continued)
Inductor current ripple: Δi_L-PP
LED current ripple: Δi_LED-PP
Peak current limit: I_LIM
Input voltage ripple: Δv_IN-PP
Output OVLO characteristics: V_TURN-OFF, V_HYSO
Input UVLO characteristics: V_TURN-ON, V_HYS
Thermal foldback characteristics: T_BK, T_END
Total start-up time: t_TSU
8.2.1.2 Detailed Design Procedure
8.2.1.2.1 Operating Point
Given the number of series LEDs (N), the forward voltage (V_LED) and dynamic resistance (r_LED) for a single LED,
solve for the nominal output voltage (V_O) and the nominal LED string dynamic resistance (r_D):
V_O = N x V_LED
(36)
r_D = N x r_LED
(37)
Solve for the ideal nominal duty cycle (D):
Buck
D
=
V_O
V_IN
(38)
Boost
D=
V_O - V_IN
V_O
(39)
Buck-Boost
D
=
V_O
V_O + V_IN
(40)
Using the same equations, find the minimum duty cycle (D_MIN) using maximum input voltage (V_IN-MAX) and the
maximum duty cycle (D_MAX) using the minimum input voltage (V_IN-MIN). Also, remember that D' = 1 - D.
8.2.1.2.2 Switching Frequency
Set the switching frequency (f_SW) by solving for R_T:
R_T
1 1.95x10^8 x f_SW
1.40x10^10 x f_SW
(41)
8.2.1.2.3 Average LED Current
For all topologies, set the average LED current (I_LED) knowing the desired current sense voltage (V_SNS) and
solving for R_SNS:
R_SNS =
V_SNS
I_LED
(42)
If the calculated R_SNS is too far from a desired standard value, then V_SNS will have to be adjusted to obtain a
standard value.
Setup the suggested signal current of 100 µA by assuming R_CSH = 12.4 kΩ and solving for R_HSP:
R_HSP =
I_LED
x R_CSH x R_SNS
1.24V
(43)
32
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