Datasheet

LM3402, LM3402HV
SNVS450E –SEPTEMBER 2006–REVISED MAY 2013 www.ti.com
Though most power LEDs are designed to run at several hundred milliamps, some can be seen to glow with a
faint light at extremely low current levels, as low as a couple microamps in some instances. In lab testing, the
forward voltage was found to be approximately 2V for LEDs that exhibited visible light at these low current levels.
For LEDs that did not show light emission at very low current levels, the forward voltage was found to be around
900mV. It is important to remember that the forward voltage is also temperature dependent, decreasing at higher
temperatures. Consequently, with a maximum Vcc voltage of 7.4V, current will be observed in the LEDs if the
total stack voltage is less than about 6V at a forward current of several microamps. No current is observed if the
stack voltage is above 6V, as shown in Figure 23. The need for absolute darkness during DIM mode is also
application dependent. It will not affect regular PWM dimming operation.
The fix for this issue is extremely simple. Place a resistor from the SW pin to ground according to the chart
below.
Number of LEDs Resistor Value (kΩ)
1 20
2 50
3 90
4 150
5 200
>5 300
The luminaire designer should ensure that the suggested resistor is effective in eliminating the off-state light
output. A combination of calculations based on LED manufacturer data and lab measurements over temperature
will ensure the best design.
Transient Protection Considerations
Considerations need to be made when external sources, loads or connections are made to the switching
converter circuit due to the possibility of Electrostatic Discharge (ESD) or Electric Over Stress (EOS) events
occurring and damaging the integrated circuit (IC) device. All IC device pins contain zener based clamping
structures that are meant to clamp ESD. ESD events are very low energy events, typically less than 5µJ
(microjoules). Any event that transfers more energy than this may damage the ESD structure. Damage is
typically represented as a short from the pin to ground as the extreme localized heat of the ESD / EOS event
causes the aluminum metal on the chip to melt, causing the short. This situation is common to all integrated
circuits and not just unique to the LM340X device.
CS PIN PROTECTION
When hot swapping in a load (e.g. test points, load boards, LED stack), any residual charge on the load will be
immediately transferred through the output capacitor to the CS pin, which is then damaged as shown in
Figure 24 below. The EOS event due to the residual charge from the load is represented as V
TRANSIENT
.
From measurements, we know that the 8V ESD structure on the CS pin can typically withstand 25mA of direct
current (DC). Adding a 1kΩ resistor in series with the CS pin, shown in Figure 25, results in the majority of the
transient energy to pass through the discrete sense resistor rather than the device. The series resistor limits the
peak current that can flow during a transient event, thus protecting the CS pin. With the 1kΩ resistor shown, a
33V, 49A transient on the LED return connector terminal could be absorbed as calculated by:
V = 25mA * 1kΩ + 8V = 33V (16)
I = 33V / 0.67Ω = 49A (17)
This is an extremely high energy event, so the protection measures previously described should be adequate to
solve this issue.
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