LM3401MMX National Semiconductor Corporation, LM3401MMX Datasheet - Page 6

• Manufacturer Part Number: LM3401MMX
• Description: Hysteretic Pfet Controller For High Power Led Drive
• Manufacturer: National Semiconductor Corporation

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Block Diagram
Operational Description
HYSTERETIC CONTROL
The LM3401 is a step-down DC-DC controller designed to
provide a constant current source for driving a high power
LED string.
The LM3401 uses comparator-based voltage mode hysteretic
control for a simple and stable design. Hysteretic control does
not utilize an internal oscillator, but relies on output conditions
to directly control switching. The LM3401 controls LED cur-
rent within the adjustable hysteresis window by monitoring
peak and valley voltage at the SNS pin. A dual sided hystere-
sis window is used to optimize accuracy.
Regulated LED current flows to ground through a sense re-
sistor at the SNS pin. The voltage generated at the SNS pin
is compared to the 200 mV internal reference. When the SNS
voltage falls below the reference voltage minus hysteresis,
the output of the hysteretic comparator goes low. This results
in the driver output, HG, pulling the gate of the PFET low and
turning on the PFET.
With the PFET on, LED current ramps up through the PFET
and the inductor. As the LED current increases, the SNS volt-
age reaches its upper threshold (reference voltage plus hys-
teresis). This forces the output of the comparator and HG to
go high, which turns the PFET off. When the PFET turns off,
current flows through the catch diode, and LED and inductor
current ramp down. When the SNS voltage falls to its lower
threshold, the cycle repeats. The resulting LED current, SNS,
and switch node waveforms are shown in Figure 2.
FIGURE 1. Block Diagram
6
OUTPUT CURRENT SETTING
The LED average current is programmed using a resistor be-
tween SNS and GND, shown as R1 in the typical application
schematic. The SNS resistor (R
lows:
Where V
LED current.
The sense resistor power rating must be higher than its power
dissipation. The required power rating can be calculated (in
Watts) as follows:
FIGURE 2. Hysteretic Switching Waveforms
SNS
is 200 mV typically, and I
SNS
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) can be calculated as fol-
LED
is the DC average
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