LM3421 datasheet(11/65 Pages) TI1

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D =

V -

D =

iL (t)

ÂiL-PP

IL-MAX

IL-MIN

tON = DTS

tOFF = (1-D)TS

LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

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.

Figure 15. Ideal CCM Regulator Inductor Current iL(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 (tON), the input voltage source stores energy in the inductor (L1) while the output

capacitor (CO) provides energy to the LED load. When Q1 is turned off (tOFF), the re-circulating diode (D1)

becomes forward biased and L1 provides energy to both CO and the LED load. Figure 15 shows the inductor

current (iL(t)) waveform for a regulator operating in CCM.

The average output LED current (ILED) is proportional to the average inductor current (IL) , therefore if IL is tightly

controlled, ILED will be well regulated. As the system changes input voltage or output voltage, the ideal duty cycle

(D) is varied to regulate IL and ultimately ILED. For any current regulator, D is a function of the conversion ratio:

Buck

(1)

Boost

(2)

Buck-boost

(3)

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