LM3405A Datasheet, PDF (8/20 Pages) National Semiconductor (TI) – 1.6MHz, 1A Constant Current Buck LED Driver with Internal Compensation in Tiny SOT23 Package

The fourth method can be used in an application which has
an external low voltage rail, V_EXT. C3 can be charged through
D2 from V_EXT, independent of V_IN and V_OUT voltage levels.
Again for best performance, ensure that the gate drive volt-
age, V_EXT - V_D2 + V_D1, falls in the range of 2.5V to 5.5V.
SETTING THE LED CURRENT
LM3405A is a constant current buck regulator. The LEDs are
connected between V_OUT and FB pin as shown in the Typical
Application Circuit. The FB pin is at 0.205V in regulation and
therefore the LED current I_F is set by V_FB and the resistor R1
from FB to ground by the following equation:
I_F = V_FB / R1
I_F should not exceed the 1A current capability of LM3405A
and therefore R1 minimum must be approximately 0.2Ω. I_F
should also be kept above 200mA for stable operation, and
therefore R1 maximum must be approximately 1Ω. 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 V_OUT to FB pin and therefore V_OUT
can be written as :
V_OUT = ((n x V_F) + V_FB)
where V_F 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 V_IN. 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 be-
fore 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 (I_F) 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 10).
The simplest way to enable the operation of LM3405A is to
connect the EN/DIM pin to V_IN 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 rapid-
ly. When the feedback pin voltage exceeds 0.205V, the duty
cycle will have to reduce from the maximum value according-
ly, 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 6. In applica-
tions where this LED current overshoot is undesirable, EN/
DIM pin voltage can be separately applied and delayed such
that V_IN is fully established before the EN/DIM pin voltage
reaches the enable threshold. The effect of delaying EN/DIM
with respect to V_IN on the LED current is shown in Figure 7.
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 8.
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FIGURE 6. Startup Response to V_IN with 5ms rise time
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FIGURE 7. Startup Response to V_IN with EN/DIM delayed
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FIGURE 8. Startup Response to V_IN with 200µs rise time
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