LM3409 Datasheet, PDF (8/30 Pages) National Semiconductor (TI) – PFET Buck Controller for High Power LED Drives

Block Diagram
Theory of Operation
The LM3409/09HV are P-channel MosFET (PFET) con-
trollers 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) architec-
ture that allows the converter to be operated in both continu-
ous 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, compo-
nent 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.
30085603
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 con-
nection to the load is excellent for current regulation. A buck
current regulator, using the LM3409/09HV, is shown in the
Typical Application section on the first page of this datasheet.
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 sup-
plied to the load keeping the LEDs forward biased. Figure 1
shows the inductor current (i_L(t)) waveform for a buck con-
verter operating in CCM.
The average inductor current (I_L) is equal to the average out-
put LED current (I_LED), therefore if I_L is tightly controlled, I_LED
will be well regulated. As the system changes input voltage
or output voltage, duty cycle (D) is varied to regulate I_L and
ultimately I_LED. For any buck regulator, D is simply the con-
version ratio divided by the efficiency (η):
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