LM3424 datasheet(12/69 Pages) TI1

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D =

V -

D =

iL (t)

ÂiL-PP

IL-MAX

IL-MIN

tON = DTS

tOFF = (1-D)TS

LM3424, LM3424-Q1

SNVS603C – AUGUST 2009 – REVISED AUGUST 2015

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7.3 Feature Description

7.3.1 Current Regulators

Figure 17. 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 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 (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 17 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 Peak Current Mode Control

Peak current mode control is used by the LM3424 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. The controller has a fixed switching frequency set by an internal programmable oscillator

which means current mode instability can occur at duty cycles higher than 50%. To mitigate this standard

problem, an artificial ramp is added to the control signal internally. The slope of this ramp is programmable to

allow for a wider range of component choices for a given design. A detailed explanation of this control method is

presented in the following sections.

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