LM3409MY-NOPB Datasheet, PDF (8/43 Pages) Texas Instruments – PFET Buck Controller for High Power LED Drivers

LM3409, LM3409HV, LM3409-Q1
SNVS602J – MARCH 2009 – REVISED MAY 2013
BLOCK DIAGRAM
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VIN
COFF
EN
UVLO
VCC
REGULATOR
OFF TIMER
COFF Complete
Start
THERMAL
SHUTDOWN
22
#A
+
1.24V ^-
LOGIC
VIN
VCC
UVLO
-
+
5R
VCC
R
R
5 #A
1.24V
VCC
PGATE
CSN
CSP
IADJ
GND
Theory of Operation
The LM3409/09HV are P-channel MosFET (PFET) controllers for step-down (buck) current regulators which are
ideal for driving LED loads. They have 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 LM3409/09HV uses a Controlled Off-Time
(COFT) architecture that allows the converter to be operated in both continuous conduction mode (CCM) and
discontinuous conduction mode (DCM) with no 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 over the full range and the fast output enable/disable function allows for high frequency
PWM dimming using no external components. When designing, the maximum attainable LED current is not
internally limited because the LM3409/09HV is a controller. Instead it is a function of the system operating point,
component choices, and switching frequency allowing the LM3409/09HV to easily provide constant currents up to
5A. This simple controller contains all the features necessary to implement a high efficiency versatile LED driver.
BUCK CURRENT REGULATORS
The buck regulator is unique among non-isolated topologies due to the direct connection of the inductor to the
load during the entire switching cycle. An inductor will control the rate of change of current that flows through it,
therefore a direct connection to the load is excellent for current regulation. A buck current regulator, using the
LM3409/09HV, is shown in the Typical Application section. During the time that the PFET (Q1) is turned on (t_ON),
the input voltage charges up the inductor (L1). When Q1 is turned off (t_OFF), the re-circulating diode (D1)
becomes forward biased and L1 discharges. During both intervals, the current is supplied to the load keeping the
LEDs forward biased. Figure 21 shows the inductor current (i_L(t)) waveform for a buck converter operating in
CCM.
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