LM3424 Datasheet, PDF (12/69 Pages) National Semiconductor (TI) – Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs

LM3424, LM3424-Q1
SNVS603C – AUGUST 2009 – REVISED AUGUST 2015
7.3 Feature Description
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7.3.1 Current Regulators
i_L (t)
I_L-MAX
I_L
Âi_L-PP
I_L-MIN
t_ON = DT_S t_OFF = (1-D)T_S
0
t
T_S
Figure 17. Ideal CCM Regulator Inductor Current i_L(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 (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 17 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
=
V_O
V_IN
(1)
Boost
D=
V_O - V_IN
V_O
(2)
Buck-Boost
D
=
V_O
V_O + V_IN
(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 R_DS-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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