0.2

L

-

1.34 x 10

LM3404/04HV

CS

One-shot

CS

Comparator

LED 1

LED n

+

-

-

+

LM3404, LM3404HV

www.ti.com

SNVS465E – OCTOBER 2006 – REVISED FEBRUARY 2010

APPLICATION INFORMATION

THEORY OF OPERATION

The LM3404 and LM3404HV 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 1.2A. 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.

Hysteretic operation eliminates the need for small-signal control loop compensation. When the converter runs in

continuous conduction mode (CCM) the controlled on-time maintains a constant switching frequency over the

range of input voltage. Fast transient response, PWM dimming, a low power shutdown mode, and simple output

overvoltage protection round out the functions of the LM3404/04HV.

CONTROLLED ON-TIME OVERVIEW

Figure 19 shows the feedback system used to control the current through an array of LEDs. A voltage signal,

(1)

Figure 19. Comparator and One-Shot

The LM3404/04HV regulators should be operated in continuous conduction mode (CCM), where inductor current

stays positive throughout the switching cycle. During steady-state CCM operation, the converter maintains a

constant switching frequency that can be selected using the following equation:

(2)

AVERAGE LED CURRENT ACCURACY

The COT architecture regulates the valley of

current (which is also the average inductor current) the valley inductor current is calculated using the following

expression:

(3)

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