Texas Instruments LM3445 Evaluation Board LM3445-230VFLBK/NOPB LM3445-230VFLBK/NOPB Datenbogen
Produktcode
LM3445-230VFLBK/NOPB
25.2V
115
(350 kHz x 0.1A)
L2
=
2
= 580
P
H
25.2V 1
0.8
1
u
C11 =
t
OFF
1.276
V
LED
R4
= 175 pF
V
LED
R4 =
= 360 k
:
I
COLL
25.2V
135
2
u
3.23
P
s = 638 ns
0.8
1
u
t
ON (MIN)
=
25.2V
135
2
1
0.8
1
u
25.2V
115
(250 kHz)
2
= 3.23
P
s
1
0.8
1
u
t
OFF
=
V
BUCK(MAX)
= 135 2 = 190V
V
BUCK(MIN)
=
2
90
2
x SIN(135
o
)
= 45V
SNVS570L – JANUARY 2009 – REVISED MAY 2013
DESIGN EXAMPLE
The following design example illustrates the process of calculating external component values.
Known:
1. Input voltage range (90V
AC
– 135V
AC
)
2. Number of LEDs in series = 7
3. Forward voltage drop of a single LED = 3.6V
4. LED stack voltage = (7 x 3.6V) = 25.2V
Choose:
1. Nominal switching frequency, f
SW-TARGET
= 250 kHz
2. I
LED(AVE)
= 400 mA
3.
Δ
i (usually 15% - 30% of
ILED(AVE)
) = (0.30 x 400 mA) = 120 mA
4. Valley fill stages (1,2, or 3) = 2
5. Assumed minimum efficiency = 80%
Calculate:
1. Calculate minimum voltage V
BUCK
equals:
(36)
2. Calculate maximum voltage V
BUCK
equals:
(37)
3. Calculate t
OFF
at V
BUCK
nominal line voltage:
(38)
4. Calculate t
ON(MIN)
at high line to ensure that t
ON(MIN)
> 200 ns:
(39)
5. Calculate C11 and R4:
6. Choose current through R4: (between 50 µA and 100 µA) 70 µA
(40)
7. Use a standard value of 365 k
Ω
8. Calculate C11:
9.
(41)
10. Use standard value of 120 pF
11. Calculate ripple current: 400 mA X 0.30 = 120 mA
12. Calculate inductor value at t
OFF
= 3 µs:
(42)
13. Choose C10: 1.0 µF 200V
14. Calculate valley-fill capacitor values: V
AC
low line = 90V
AC
, V
BUCK
minimum equals 60V (no triac dimming at
maximum LED current). Set droop for 20V maximum at full load and low line.
28
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