LM3421 datasheet(12/72 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

SNVS574F – JULY 2008 – REVISED JULY 2015

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7.3 Feature Description

7.3.1 Current Regulators

Figure 12. Ideal CCM Regulator Inductor Current iL(t)

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 LM34xx-Q1 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 LM34xx-Q1 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 12 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)

7.3.2 Predictive Off-Time (PRO) Control

PRO control is used by the LM34xx-Q1 to control ILED. It is a combination of average peak current control and a

one-shot off-timer that varies with input voltage. The LM34xx-Q1 uses peak current control to regulate the

average LED current through an array of HBLEDs. This method of control uses a series resistor in the LED path

to sense LED current and can use either a series resistor in the MOSFET path or the MOSFET RDS-ON for both

cycle-by-cycle current limit and input voltage feed forward. D is indirectly controlled by changes in both tOFF and

tON, which vary depending on the operating point.

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LM3421 Datasheet(PDF) 12 Page - Texas Instruments

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