LM3444_15 Datasheet, PDF (18/27 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
SNVS682D – NOVEMBER 2010 – REVISED DECEMBER 2015
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Application Information (continued)
Knowing the desired average LED current, I_AVE, and the nominal inductor current ripple, Δi_L, the peak current for
an application running in continuous conduction mode (CCM) is defined in Equation 23.
I_L2-PK
= I_AVE +
'i_L
2
(23)
The LED current would then be calculated using Equation 24.
I_AVE(UNDIM)
=
I_L2-PK(UNDIM)
-
'i_L
2
(24)
This is important to calculate because this peak current multiplied by the sense resistor R3 determines when the
internal comparator is tripped. The internal comparator turns the control MOSFET off once the peak sensed
voltage reaches 750 mV.
750 mV
I_L-PK(UNDIM) =
R3
(25)
Current Limit: The trip voltage on the PWM comparator is 750 mV. However, if there is a short circuit or an
excessive load on the output, higher than normal switch currents cause a voltage greater than 1.27 V on the
ISNS pin which trip the I-LIM comparator. The I-LIM comparator resets the RS latch, turning off Q2. It also
inhibits the Start Pulse Generator and the COFF comparator by holding the COFF pin low. A delay circuit
prevents the start of another cycle for 180 µs.
8.1.6 Valley Fill Capacitors
Determining voltage rating and capacitance value of the valley-fill capacitors:
The maximum voltage seen by the valley-fill capacitors is calculated by Equation 26.
V_VF-CAP
=
V_AC(MAX) 2
#stages
(26)
This is, of course, if the capacitors chosen have identical capacitance values and split the line voltage equally.
Often a 20% difference in capacitance could be observed between like capacitors. Therefore a voltage rating
margin of 25% to 50% should be considered.
8.1.7 Determining the Capacitance Value of the Valley-Fill Capacitors
The valley-fill capacitors must be sized to supply energy to the buck converter (V_BUCK) when the input line is less
than its peak divided by the number of stages used in the valley fill (t_X). The capacitance value must be
calculated for the maximum LED current.
V_BUCK
30°
t_X
150°
8.33 ms
t
0°
180°
Figure 17. Two Stage Valley-Fill V_BUCK Voltage
18
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