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SNVS574E – JULY 2008 – REVISED MAY 2013
function with external dimming FET driver allows for fast PWM dimming of the LED load. When designing, the
maximum attainable LED current is not internally limited because the LM3421/23 is a controller. Instead it is a
function of the system operating point, component choices, and switching frequency allowing the LM3421/23 to
easily provide constant currents up to 5A. This controller contains all the features necessary to implement a high
efficiency versatile LED driver.
i L (t)
I L-MAX
I L
0
t ON = DT S
t OFF = (1-D)T S
? i L-PP
I L-MIN
t
T S
Figure 15. 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 LM3421/23 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 LM3421/23 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 15 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
D=
Boost
D=
V O
V IN
V O - V IN
V O
(1)
(2)
Buck-boost
D=
V O
V O + V IN
(3)
Copyright ? 2008–2013, Texas Instruments Incorporated
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