LM3444_15 Datasheet, PDF (22/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
Typical Application (continued)
8.2.1 Design Requirements
Known:
1. Input voltage range (90 V_AC to 135 V_AC)
2. Number of LEDs in series = 7
3. Forward voltage drop of a single LED = 3.6 V
4. LED stack voltage = (7 × 3.6 V) = 25.2 V
Choose:
1. Nominal switching frequency, f_SW-TARGET = 350 kHz
2. I_LED(AVE) = 400 mA
3. Δi (usually 15% to 30% of _ILED(AVE)) = (0.30 × 400 mA) = 120 mA
4. Valley-fill stages (1, 2, or 3) = 2
5. Assumed minimum efficiency = 80%
8.2.2 Detailed Design Procedure
Calculate:
1. Calculate minimum voltage V_BUCK, as shown in Equation 35, which yields:
V_BUCK(MIN) = ⁹⁰
2 x SIN(135^o)
2
= 45V
2. Calculate maximum voltage V_BUCK, as shown in Equation 36, which yields:
V_BUCK(MAX) = 135 2 = 190V
3. Calculate t_OFF at V_BUCK nominal line voltage, as given by Equation 37.
1
t_OFF =
1 _u 25.2V
0.8 115 2
(250 kHz) ^{= 3.23 Ps}
4. Calculate t_ON(MIN) at high line to ensure that t_ON(MIN) > 200 ns, as given by Equation 38.
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
5. Calculate C11 and R4 in steps 6 through 9.
6. Choose current through R4 (from 50 µA to 100 µA): 70 µA as given by Equation 39.
V_LED
R4 = _ICOLL = 360 k:
7. Use a standard value of 365 kΩ.
8. Calculate C11 as given by Equation 40.
V_LED t_OFF
C11 =
R4
= 175 pF
1.276
9. Use standard value of 120 pF.
10. Calculate ripple current: 400 mA × 0.30 = 120 mA
11. Calculate inductor value at t_OFF = 3 µs as given by Equation 41.
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