LM3444_15 Datasheet, PDF (17/27 Pages) Texas Instruments – AC-DC Offline LED Driver

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LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
Application Information (continued)
The value of V_L(OFF-TIME) is relatively constant, because the LED stack voltage remains constant. If we rewrite the
equation for an inductor inserting what we know about the circuit during the off-time, Equation 18 shows that we
get:
V_L(OFF-TIME)
=
V_LED
=
L
x
'i
't
V_L(OFF-TIME)
=
V_LED
=
L
x
(I_(MAX) - I_(MIN))
't
(18)
Rearranging this gives Equation 19.
'i
#
t_OFF
x
V_LED
L2
(19)
From this, we can see that the ripple current (Δi) is proportional to off-time (t_OFF) multiplied by a voltage, which is
dominated by V_LED divided by a constant (L2).
These equations can be rearranged to calculate the desired value for inductor L2, as shown in Equation 20.
L2
#
t_OFF
x
V_LED
'i
(20)
The off time can be calculated using Equation 21:
t_OFF = ¹
1
K
u
V_LED
V_BUCK
f_SW
(21)
Substituting t_off in Equation 21 results in Equation 22:
V_LED 1
1
K
u
V_LED
V_BUCK
L2 =
f_SW x 'i
(22)
See Typical Application to better understand the design process.
8.1.5 Setting the LED Current
The LM3444 constant off-time control loop regulates the peak inductor current (I_L2). The average inductor current
equals the average LED current (I_AVE). Therefore the average LED current is regulated by regulating the peak
inductor current.
I_AVE
I_L2-PK
'i_L
I_L2 (t)
I_L2-MIN
t
t_ON
t_OFF
Figure 16. Inductor Current Waveform in CCM
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