Datasheet

D
=
INO
VV
+
O
V
D =
INO
VV -
O
V
D =
O
V
IN
V
t
i
L
(t)
Âi
L-PP
I
L-MAX
I
L-MIN
I
L
0
T
S
t
ON
= DT
S
t
OFF
= (1-D)T
S
LM3424
www.ti.com SNVS603B –AUGUST 2009–REVISED OCTOBER 2009
THEORY OF OPERATION
The LM3424 is an N-channel MosFET (NFET) controller for buck, boost and buck-boost current regulators which
are ideal for driving LED loads. The controller has wide input voltage range allowing for regulation of a variety of
LED loads. The high-side differential current sense, with low adjustable threshold voltage, provides an excellent
method for regulating output current while maintaining high system efficiency. The LM3424 uses peak current
mode control providing good noise immunity and an inherent cycle-by-cycle current limit. The adjustable current
sense threshold provides the capability to amplitude (analog) dim the LED current and the thermal foldback
circuitry allows for precise temperature management of the LEDs. Tthe output enable/disable function coupled
with an internal dimming drive circuit provides high speed PWM dimming through the use of an external MosFET
placed at the LED load. When designing, the maximum attainable LED current is not internally limited because
the LM3424 is a controller. Instead it is a function of the system operating point, component choices, and
switching frequency allowing the LM3424 to easily provide constant currents up to 5A. This simple controller
contains all the features necessary to implement a high efficiency versatile LED driver.
Figure 19. Ideal CCM Regulator Inductor Current i
L
(t)
CURRENT REGULATORS
Current regulators can be designed to accomplish three basic functions: buck, boost, and buck-boost. All three
topologies in their most basic form contain a main switching MosFET, a recirculating diode, an inductor and
capacitors. The LM3424 is designed to drive a ground referenced NFET which is perfect for a standard boost
regulator. Buck and buck-boost regulators, on the other hand, usually have a high-side switch. When driving an
LED load, a ground referenced load is often not necessary, therefore a ground referenced switch can be used to
drive a floating load instead. The LM3424 can then be used to drive all three basic topologies as shown in the
Basic Topology Schematics section. Other topologies such as the SEPIC and flyback converter (both derivatives
of the buck-boost) can be implemented as well.
Looking at the buck-boost design, the basic operation of a current regulator can be analyzed. During the time
that the NFET (Q1) is turned on (t
ON
), the input voltage source stores energy in the inductor (L1) while the output
capacitor (C
O
) provides energy to the LED load. When Q1 is turned off (t
OFF
), the re-circulating diode (D1)
becomes forward biased and L1 provides energy to both C
O
and the LED load. Figure 19 shows the inductor
current (i
L
(t)) waveform for a regulator operating in CCM.
The average output LED current (I
LED
) is proportional to the average inductor current (I
L
) , therefore if I
L
is tightly
controlled, I
LED
will be well regulated. As the system changes input voltage or output voltage, the ideal duty cycle
(D) is varied to regulate I
L
and ultimately I
LED
. For any current regulator, D is a function of the conversion ratio:
Buck
(1)
Boost
(2)
Buck-boost
(3)
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