LM3444MM Datasheet, PDF (17/30 Pages) Texas Instruments – AC-DC Offline LED Driver

LM3444
www.ti.com
SNVS682C – NOVEMBER 2010 – REVISED MAY 2013
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 will cause a voltage above 1.27V on the ISNS
pin which will trip the I-LIM comparator. The I-LIM comparator will reset the RS latch, turning off Q2. It will also
inhibit the Start Pulse Generator and the COFF comparator by holding the COFF pin low. A delay circuit will
prevent the start of another cycle for 180 µs.
VALLEY FILL CAPACITORS
Determining voltage rating and capacitance value of the valley-fill capacitors:
The maximum voltage seen by the valley-fill capacitors is:
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.
Determining the capacitance value of the valley-fill capacitors:
The valley fill capacitors should 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 should be
calculated for the maximum LED current.
V_BUCK
30°
t_X
150°
8.33 ms
t
0°
180°
Figure 20. Two Stage Valley-Ffill V_BUCK Voltage
From the above illustration and the equation for current in a capacitor, i = C x dV/dt, the amount of capacitance
needed at V_BUCK will be calculated as follows:
At 60Hz, and a valley-fill circuit of two stages, the hold up time (t_X) required at V_BUCK is calculated as follows. The
total angle of an AC half cycle is 180° and the total time of a half AC line cycle is 8.33 ms. When the angle of the
AC waveform is at 30° and 150°, the voltage of the AC line is exactly ½ of its peak. With a two stage valley-fill
circuit, this is the point where the LED string switches from power being derived from AC line to power being
derived from the hold up capacitors (C7 and C9). 60° out of 180° of the cycle or 1/3 of the cycle the power is
derived from the hold up capacitors (1/3 x 8.33 ms = 2.78 ms). This is equal to the hold up time (dt) from the
above equation, and dv is the amount of voltage the circuit is allowed to droop. From the next section
(“Determining Maximum Number of Series Connected LEDs Allowed”) we know the minimum V_BUCK voltage will
be about 45V for a 90V_AC to 135V_AC line. At 90V_AC low line operating condition input, ½ of the peak voltage is
64V. Therefore with some margin the voltage at V_BUCK can not droop more than about 15V (dv). (i) is equal to
(P_OUT/V_BUCK), where P_OUT is equal to (V_LED x I_LED). Total capacitance (C7 in parallel with C9) can now be
calculated. See “ Design Example" section for further calculations of the valley-fill capacitors.
Determining Maximum Number of Series Connected LEDs Allowed:
The LM3444 is an off-line buck topology LED driver. A buck converter topology requires that the input voltage
(V_BUCK) of the output circuit must be greater than the voltage of the LED stack (V_LED) for proper regulation. One
must determine what the minimum voltage observed by the buck converter will be before the maximum number
of LEDs allowed can be determined. Two variables will have to be determined in order to accomplish this.
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