1.34 x 10

LM3402/02HV

CS

+

-

VREF

+

-

One-shot

CS

Comparator

LED 1

LED n

LM3402, LM3402HV

SNVS450D – SEPTEMBER 2006 – REVISED FEBRUARY 2010

www.ti.com

APPLICATION INFORMATION

THEORY OF OPERATION

The LM3402 and LM3402HV 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 500 mA. 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 LM3402/02HV.

CONTROLLED ON-TIME OVERVIEW

Figure 1 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 LM3402/02HV regulators should be operated in continuous conduction mode (CCM), where inductor current

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

a constant switching frequency, which can be selected using the following equation:

(2)

(3)

10

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