© 2016 Fairchild Semiconductor Corporation www.fairchildsemi.com
FL77944 • Rev. 1.2 8
FL77944 — Analog/PWM/Phase-cut Dimmable High Power LED Direct AC Driver
Functional Description
The FL77944 can drive LED strings attached directly to
the rectified AC mains using only two external RC
components (RCS and CVDD). With 4 integrated high
voltage current sink, LED current in each string is
precisely controlled with system compactness. High PF
and low THD are obtained by the optimized current sink
levels. Phase-cut dimming is easily obtained with wide
dimming range and good dimmer compatibility.
Dedicated DIM pin can be used to implement analog or
digital dimming function. Flicker index in the direct AC
drive topology can be improved by adopting proprietary
self valley-fill solution.
Operation
When the rectified AC line voltage, VIN, is higher than
the forward voltage of the consecutive LED groups,
each LED group turns on automatically as the
corresponding current sink has enough voltage
headroom across it. Each current sink increases up to
the predefined current level and maintains that level
until the following channel’s current sink get enough
voltage headroom across it.
AC Line
Voltage (VIN)
LED Current
(IF)
ILED1
ILED2
ILED3
ILED4
tD1 tD2 tD3 tD4 tD3 tD2 tD1
VF1
VF1'+VF2
VF1''+VF2'+VF3
VF1'''+VF2''+VF3'+VF4
· tD1: Current is directed to LED1 pin through 1st LED group.
· tD2: Current is directed to LED2 pin through 1st and 2nd LED groups.
· tD3: Current is directed to LED3 pin through 1st, 2nd, and 3rd LED groups.
· tD4: Current is directed to LED4 pin through 1st, 2nd, 3rd, and 4th LED groups.
· VF1/VF1'/VF1''/VF1''': Forward voltage at forward current of ILED1/ILED2/ILED3/ILED4
in 1st LED group.
· VF2/VF2'/VF2'': Forward voltage at forward current of ILED2/ILED3/ILED4 in 2nd LED
group.
· VF3/VF3': Forward voltage at forward current of ILED3/ILED4 in 3rd LED group.
· VF4: Forward voltage at forward current of ILED4 in 4th LED group.
Figure 11. FL77944 Operation
When VIN reaches to the forward voltage across the
1st LED group (VF1) at forward current IF = ILED1, the
current drawn from the VIN is directed to the LED1
through the 1st LED group. In sequence, when VIN
reaches forward voltage across 1st and 2nd LED
groups (VF1'+VF2) at IF = ILED2, the current is directed to
LED2 across 1st and 2nd LED groups. Then, when VIN
reaches VF1''+VF2'+VF3 at IF=ILED3, the LED current
goes through 1st, 2nd, and 3rd LED groups and sinks
to the LED3. Finally, when VIN reaches
VF1'''+VF2''+VF3'+VF4 at IF=ILED4, the current goes
through all 4 LED groups and is directed to the LED4.
Whenever the active channel (one that is sinking LED
current) is changed from one channel to the adjacent
channel with respect to the change in the VIN, the new
active channel's current increases gradually while the
existing active channel's current decreases gradually.
This smooth current transition reduces frequency
harmonic contents and improves power factor as well as
Electromagnetic Interference (EMI) characteristics.
By fully utilizing available headroom, the FL77944 offers
maximum power, high efficiency, power factor and low
harmonic distortion. Typically, power factor is higher
than 0.98 and THD is lower than 20%. The efficiency
heavily depends on a LED configuration.
LED Current and Power Setting
The LED current is managed by an external current
sense resistor RCS. Regulation target of each channel's
current sink is calculated as follows.
,
37.0
,
18.0 21 CS
LED
CS
LED R
I
R
I
.
92.0
and ,
83.0 43 CS
LED
CS
LED R
I
R
I
Root-mean-square (RMS) value of the input current can
be calculated using the peak regulated current, ILED4,
and crest factor. Since the LED current waveform is
similar to the AC line voltage, the crest factor is close to
the crest factor of a sine wave, √2=1.414. But the actual
crest factor depends on the flattened time of the ILED4
and LED configuration. With FL77944, the typical crest
factor approximately is 1.4. Thus, based on estimated
input power, PIN, the RCS resistor value can be
calculated as follows.
IN
RMSAC
CS P
V
R
4.1
92.0 .
The actual RCS needs to be adjusted with respect to the
LED configuration.
LED Configuration
In the LED configuration, it is required to increase the
total LED forward voltage to improve efficiency. For
example, compared to using 4 LEDs with VF of 60 V
(total VF = 60 V x 4 channels = 240 V) for each LED
group, using 4 LEDs with VF equal to 65 V (total VF =
65 V x 4 channels = 260 V) will improve the efficiency
simply due to the higher total VF. Each LED channel can
have different VF. For example, if a design is
implemented with 144 pieces of 3-V LEDs for
replacement of 2-feet fluorescent lamp, designer can
assign flexible numbers of LEDs for LED channels such
as 25s2p-32s2p-6s2p-18s1p (“s” stands for LEDs in
series and “p” stands for LEDs in parallel) or 18s2p-
18s2p-18s2p-36s1p.
Which needs to be considered is that VF of first LED
group should be higher than VIN-pin turn-on voltage,
which is 20 V. If the VF of the first LED group is
configured to be lower than VIN-pin turn-on voltage,
ILED1 will not have the correct regulation level when input
voltage, VIN, is just exceeds the VF.