VCS

10V/Div

VSNS

100 mV/Div

ILED

100 mA/Div

400 ns/DIV

VIN

SNS

HG

DIM

ILIM

GND

+

-

CS

on

20 PA

ILIM

VDD

regulator

+

-

+

-

-0.13V

UVLO

HYS

5.5 PA

X 0.2

VREF

200 mV

LM3401

www.ti.com

SNVS516C – AUGUST 2007 – REVISED MAY 2013

BLOCK DIAGRAM

Figure 14. 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 current within the adjustable hysteresis window by monitoring peak and valley voltage at

the SNS pin. A dual sided hysteresis window is used to optimize accuracy.

Regulated LED current flows to ground through a sense resistor 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 voltage reaches its upper threshold (reference voltage plus hysteresis). 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 15.

Figure 15. Hysteretic Switching Waveforms

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