LM3429_14 Datasheet, PDF (9/50 Pages) Texas Instruments – LM3429Q1 N-Channel Controller for Constant Current LED Drivers

www.ti.com
V_IN
UVLO
HYSTERESIS
nDIM
1.24V
20 PA
RCT
COMP
CSH
HSP
HSN
IS
1.24V
LEB
Block Diagram
LM3429
SNVS616F – APRIL 2009 – REVISED JANUARY 2010
6.9V LDO
Regulator
VccUVLO
UVLO
Standby
1.24V
REFERENCE
TLIM _Thermal
Limit
Dimming
V_IN/25 _{Start new on time}
Reset
V_CC
Dominant
SQ
R QB
V_CC
GATE
PGND
PWM
800 mV
LOGIC
CURRENT
LIMIT
245 mV
OVP
HYSTERESIS
20 PA
OVP
1.24V
AGND
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 external 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 designing, 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 provide constant currents up to 5A.
This simple controller contains all the features necessary to implement a high efficiency versatile LED driver.
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 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 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_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 2 shows the inductor
current (i_L(t)) waveform for a regulator operating in CCM.
Copyright © 2009–2010, Texas Instruments Incorporated
Submit Documentation Feedback
9
Product Folder Links: LM3429