LM3405AXMY datasheet(11/32 Pages) TI

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LM3405A

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SNVS508C – OCTOBER 2007 – REVISED MAY 2013

SETTING THE LED CURRENT

LM3405A is a constant current buck regulator. The LEDs are connected between VOUT and the FB pin as shown

in the Typical Application Circuit. The FB pin is at 0.205V in regulation and therefore the LED current IF is set by

VFB and resistor R1 from FB to ground by the following equation:

IF = VFB / R1

(9)

IF should not exceed the 1A current capability of LM3405A and therefore R1 minimum must be approximately

0.2

Ω. IF should also be kept above 200mA for stable operation, and therefore R1 maximum must be

approximately 1

Ω. If average LED currents less than 200mA are desired, the EN/DIM pin can be used for PWM

dimming. See LED PWM DIMMING section.

OUTPUT VOLTAGE

The output voltage is primarily determined by the number of LEDs (n) connected from VOUT to FB pin and

therefore VOUT can be written as :

VOUT = ((n x VF) + VFB)

(10)

where VF is the forward voltage of one LED at the set LED current level (see LED manufacturer datasheet for

forward characteristics curve).

ENABLE MODE / SHUTDOWN MODE

The LM3405A has both enable and shutdown modes that are controlled by the EN/DIM pin. Connecting a

voltage source greater than 1.8V to the EN/DIM pin enables the operation of LM3405A, while reducing this

voltage below 0.4V places the part in a low quiescent current (0.3µA typical) shutdown mode. There is no

internal pull-up on EN/DIM pin, therefore an external signal is required to initiate switching. Do not allow this pin

to float or rise to 0.3V above VIN. It should be noted that when the EN/DIM pin voltage rises above 1.8V while the

input voltage is greater than UVLO, there is a finite delay before switching starts. During this delay the LM3405A

will go through a power on reset state after which the internal soft-start process commences. The soft-start

process limits the inrush current and brings up the LED current (IF) in a smooth and controlled fashion. The total

combined duration of the power on reset delay, soft-start delay and the delay to fully establish the LED current is

in the order of 100µs (refer to Figure 22).

The simplest way to enable the operation of LM3405A is to connect the EN/DIM pin to VIN which allows self start-

up of LM3405A whenever the input voltage is applied. However, when an input voltage of slow rise time is used

to power the application and if both the input voltage and the output voltage are not fully established before the

soft-start time elapses, the control circuit will command maximum duty cycle operation of the internal power

switch to bring up the output voltage rapidly. When the feedback pin voltage exceeds 0.205V, the duty cycle will

have to reduce from the maximum value accordingly, to maintain regulation. It takes a finite amount of time for

this reduction of duty cycle and this will result in a spike in LED current for a short duration as shown in

Figure 18. In applications where this LED current overshoot is undesirable, EN/DIM pin voltage can be

separately applied and delayed such that VIN is fully established before the EN/DIM pin voltage reaches the

enable threshold. The effect of delaying EN/DIM with respect to VIN on the LED current is shown in Figure 19.

For a fast rising input voltage (200µs for example), there is no need to delay the EN/DIM signal since soft-start

can smoothly bring up the LED current as shown in Figure 20.

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