LM3444MM Datasheet, PDF (20/30 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682C – NOVEMBER 2010 – REVISED MAY 2013
www.ti.com
V_BUCK(MAX) = 135 2 = 190V
(36)
3. Calculate t_OFF at V_BUCK nominal line voltage:
1 ¹ u ^25.2V
0.8 115 2
t_OFF =
(250 kHz)
= 3.23 Ps
(37)
4. Calculate t_ON(MIN) at high line to ensure that t_ON(MIN) > 200 ns:
1 _u 25.2V
0.8 135 2
t_{ON (MIN)} =
u 3.23 Ps = 638 ns
1 ¹ u ^25.2V
0.8 135 2
(38)
5. Calculate C11 and R4:
6. Choose current through R4: (between 50 µA and 100 µA) 70 µA
V_LED
R4 = _ICOLL = 360 k:
(39)
7. Use a standard value of 365 kΩ
8. Calculate C11:
V_LED t_OFF
C11 =
R4
= 175 pF
1.276
(40)
9. Use standard value of 120 pF
10. Calculate ripple current: 400 mA X 0.30 = 120 mA
11. Calculate inductor value at t_OFF = 3 µs:
25.2V ¹
1 _u 25.2V
0.8 115 2
L2 =
= 580 PH
(350 kHz x 0.1A)
(41)
12. Choose C10: 1.0 µF 200V
13. Calculate valley-fill capacitor values:
V_AC low line = 90V_AC, V_BUCK minimum equals 60V. Set droop for 20V maximum at full load and low line.
i
=
C
dv
dt
(42)
i equals P_OUT/V_BUCK (270 mA), dV equals 20V,
dt equals 2.77 ms, and
then C_TOTAL equals 37 µF.
Therefore, C7 = C9 = 22 µF
20
Submit Documentation Feedback
Product Folder Links: LM3444
Copyright © 2010–2013, Texas Instruments Incorporated