LM3429BKBSTEVAL National Semiconductor, LM3429BKBSTEVAL Datasheet - Page 7

• Manufacturer Part Number: LM3429BKBSTEVAL
• Description: BOARD, EVALUATION, FOR LM3429
• Manufacturer: National Semiconductor

Specifications of LM3429BKBSTEVAL

• Kit Contents: LM3429 Assembled Evaluation Board, Application Note, LM3429 Datasheet
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Drives 4 To 8 LED At Max Average LED Current 1A, Overvoltage Protection,

Block Diagram
Theory of Operation
The LM3429 is an N-channel MosFET (NFET) controller for
buck, boost and buck-boost current regulators which are ideal
for driving LED loads. The controller has wide input voltage
range allowing for regulation of a variety of LED loads. The
high-side differential current sense, with low adjustable
threshold voltage, provides an excellent method for regulating
output current while maintaining high system efficiency. The
LM3429 uses a Predictive Off-time (PRO) control architecture
that allows the regulator to be operated using minimal exter-
nal control loop compensation, while providing an inherent
cycle-by-cycle current limit. The adjustable current sense
threshold provides the capability to amplitude (analog) dim
the LED current and the output enable/disable function allows
for PWM dimming using no external components. When de-
signing, the maximum attainable LED current is not internally
limited because the LM3429 is a controller. Instead it is a
function of the system operating point, component choices,
and switching frequency allowing the LM3429 to easily pro-
vide constant currents up to 5A. This simple controller con-
tains all the features necessary to implement a high efficiency
versatile LED driver.
7
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 LM3429
is designed to drive a ground referenced NFET which is per-
fect 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 LM3429 can then
be used to drive all three basic topologies as shown in the
Basic Topology Schematics section.
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
energy in the inductor (L1) while the output capacitor (C
provides energy to the LED load. When Q1 is turned off
(t
and L1 provides energy to both C
1
operating in CCM.
OFF
shows the inductor current (i
), the re-circulating diode (D1) becomes forward biased
ON
), the input voltage source stores
L
(t)) waveform for a regulator
O
and the LED load.
30094403
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Figure
O
)