LM3424 Datasheet, PDF (42/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
Typical Applications (continued)
www.ti.com
L1 = 33 2H
8.2.2.2.6 Output Capacitance
Solve for C_O:
C_O =
I_LED x D
r_D x 'i_LED-PP x f_SW
(119)
C_O
=
1.95:
1A x 0.467
x 12 mA x 5
04
kHz
=
39.6 PF
The closest capacitance totals 40 µF therefore Δi_LED-PP is:
'i_LED-
PP
=
r_D
I_LED x D
x C_O x f_SW
'i_LED-PP = _1.95:
1A x 0.467
x 40 PF x 504
kHz
=
12
mA
Determine minimum allowable RMS current rating:
I_CO-RMS = I_LED x
D_MAX
1- D_MAX
= 1A
x
0.677
1- 0.677
=
1.45A
The chosen components from step 6 are:
C_O = 4 x 10 2F
(120)
(121)
(122)
(123)
8.2.2.2.7 Peak Current Limit
Solve for R_LIM:
R_LIM
=
245 mV
I_LIM
=
245 mV
6A
=
0.041:
The closest standard resistor is 0.04 Ω therefore I_LIM is:
I_LIM
=
245 mV
R_LIM
=
245 mV
0.04:
=
6.13A
The chosen component from step 7 is:
R_LIM = 0.04:
(124)
(125)
(126)
8.2.2.2.8 Slope Compensation
Solve for R_SLP:
R
SLP
=
1.5 x 10¹³ x L1
V_O x R_T x R_LIM
R _SLP
=
21
V
1.5e¹³
x 14.3
x 33 μH
kΩ x 0.04
Ω
=
41.2
kΩ
The chosen component from step 8 is:
R_SLP =41.2 kΩ
(127)
(128)
8.2.2.2.9 Loop Compensation
ω_P1 is approximated:
Z_P1 =
1+D
r_D x C_O
=
1.467
1.95: x 40
PF
rad
= 19k _sec
(129)
42
Submit Documentation Feedback
Copyright © 2009–2015, Texas Instruments Incorporated
Product Folder Links: LM3424 LM3424-Q1