LM3406_09 Datasheet, PDF(7/26 Page) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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LM3406_09 Datasheet, PDF (7/26 Pages) National Semiconductor (TI) – 1.5A Constant Current Buck Regulator for Driving High Power LEDs

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Block Diagram

Application Information

THEORY OF OPERATION

The LM3406 and LM3406HV are buck regulators with a wide

input voltage range, low voltage reference, and a fast output

enable/disable function. These features combine to make

them ideal for use as a constant current source for LEDs with

forward currents as high as 1.5A. The controlled on-time

(COT) architecture uses a comparator and a one-shot on-

timer that varies inversely with input and output voltage in-

stead of a fixed clock. The LM3406/06HV also employs an

integrator circuit that averages the output current. When the

converter runs in continuous conduction mode (CCM) the

controlled on-time maintains a constant switching frequency

over changes in both input and output voltage. These features

combine to give the LM3406/06HV an accurate output cur-

rent, fast transient response, and constant switching frequen-

cy over a wide range of conditions.

30020303

CONTROLLED ON-TIME OVERVIEW

Figure 1 shows a simplified version of the feedback system

used to control the current through an array of LEDs. A dif-

ferential voltage signal, VSNS, is created as the LED current

flows through the current setting resistor, RSNS. VSNS is fed

back by the CS pin, where it is integrated and compared

against an error amplifier-generated reference. The error am-

plifier is a transconductance (Gm) amplifier which adjusts the

voltage on COMP to maintain a 200 mV average at the CS

pin. The on-comparator turns on the power MOSFET when

VSNS falls below the reference created by the Gm amp. The

power MOSFET conducts for a controlled on-time, tON, set by

an external resistor, RON, the input voltage, VIN and the output

voltage, VO. On-time can be estimated by the following sim-

plified equation (for the most accurate version of this expres-

sion see the Appendix):

At the conclusion of tON the power MOSFET turns off and

must remain off for a minimum of 230 ns. Once this tOFF-MIN

is complete the CS comparator compares the integrated

VSNS and reference again, waiting to begin the next cycle.

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