LM3402 datasheet(2/18 Pages) NSC

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FIGURE 2. LM3402/04 PSOP Thermal PAD and Via Layout

Connecting to LED Array

The LM3402 / 04 evaluation board includes two standard 94

mil turret connectors for the cathode and anode connections

to a LED array.

Low Power Shutdown

The LM3402/04 can be placed into a low power shutdown

state (I

Q typically 90 µA) by grounding the DIM terminal. Dur-

ing normal operation this terminal should be left open-circuit.

Constant On Time Overview

The LM3402 and LM3404 are buck regulators with a wide in-

put voltage range and a low voltage reference. The controlled

on-time (COT) architecture is a combination of hysteretic

mode control and a one-shot on-timer that varies inversely

with input voltage. With the addition of a PNP transistor, the

on-timer can be made to be inversely proportional to the input

voltage minus the output voltage. This is one of the application

improvements made to this demonstration board that will be

discussed later (improved average LED current circuit).

The LM3402 / 04 were designed with a focus of controlling

the current through the load, not the voltage across it. A con-

stant current regulator is free of load current transients, and

has no need for output capacitance to supply the load and

maintain output voltage. Therefore, in this demonstration

board in order to demonstrate the fast transient capabilities, I

have chosen to omit the output capacitor. With any Buck reg-

ulator, duty cycle (D) can be calculated with the following

equations.

The average inductor current equals the average LED current

whether an output capacitor is used or not.

30061604

FIGURE 3. Buck Converter Inductor Current Waveform

A voltage signal, V

SNS, is created as the LED current flows

through the current setting resistor, R

SNS, to ground. VSNS is

fed back to the CS pin, where it is compared against a 200

mV reference (V

REF). A comparator turns on the power MOS-

FET when V

SNS falls below VREF. The power MOSFET con-

ducts for a controlled on-time, t

ON, set by an external resistor,

ON.

30061605

FIGURE 4. V

SNS Circuit

SETTING THE AVERAGE LED CURRENT

Knowing the average LED current desired and the input and

output voltages, the slopes of the currents within the inductor

can be calculated. The first step is to calculate the minimum

inductor current (LED current) point. This minimum level

needs to be determined so that the average LED current can

be determined.

LM3402 Datasheet(PDF) 2 Page - National Semiconductor (TI)

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