LM3421_09 Datasheet, PDF (15/24 Pages) National Semiconductor (TI) – N-Channel Controllers for Constant Current LED Drivers

DETERMINING MAXIMUM NUMBER OF LEDS THAT CAN
BE DRIVEN
The LM3421/LM3423 devices can drive any string of LEDs
that will allow the current sense resistor to be below 75V. The
sense resistor may be embedded within the string of LEDs to
allow driving a stack of LEDs whose highest potential is above
75V. In this configuration, the IS pin must be tied to a source-
side resistor; R_DS(ON) sensing is not an option.
BOOST MODE INRUSH CURRENT
When configured as a boost converter, there is a “phantom”
power path comprised of the inductor, the output diode, and
the output capacitor. This path will cause two things to happen
when power is applied. First, there will be a very large inrush
of current to charge the output capacitor. Second, the energy
stored in the inductor during this inrush will end up in the out-
put capacitor, charging it to a higher potential than the input
voltage. This voltage could, depending on the impedance of
the source, reach a peak value determined by the following
equation:
Depending on the state of the EN pin, the output capacitor
would be discharged by:
1) EN < 1.3V, no discharge path (leakage only).
2) EN > 1.3V, the OVP divider resistor path, if present, and
10µA into each of the HSP & HSN pins. This output capacitor
voltage could be higher than the OVP voltage. In this situation,
the FLT pin (LM3423 only) is open and the PWM dimming
MOSFET is turned off. This condition (the system appearing
disabled) can persist for an undesirably long time; possible
solutions include:
• Add an inrush diode from V_IN to the output. See Figure 6
• Add an NTC thermistor to prevent the inrush from
overcharging the output capacitor so high.
• A current limited source supply.
• Raise the OVP threshold.
FIGURE 6. Boost Topology with Inrush Diode
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STARTUP INRUSH CURRENT
The LM3421/LM3423 devices implement a true current
source; they regulate current into a string of LEDs. When an
output capacitor is used to reduce the ripple current into the
LEDs, it is outside of the current control loop. During startup,
an inrush current associated with charging the output capac-
itor up to the LED string “on” voltage is observed. During this
inrush, there is little or no current flow in the LEDs so the error
amplifier pulls the COMP pin up as high as it is able to. The
input current rapidly reaches the current limit value set by the
current limit comparator, 245mV (typical) across the main
power switch or its source resistor depending on how the IS
pin is configured. The input current stays “regulated” at that
point until the voltage on the output capacitor rises high
enough to drive current into the LED string. When the LED
string exceeds the programmed current, the control loop
forces the voltage on the COMP pin down until the output
current into the LEDs is in regulation. This takes time and re-
sults in an overshoot in the LED current as the loop settles to
its programmed value.
Regardless, this overshoot in LED current can, in some con-
figurations, approach the 30% high over-current limit. As the
input voltage increases, the power intercepted from the input
source increases and therefore so does the associated over-
shoot. When the overshoot reaches 30%, the fault timer is
activated and a race starts between the control loop acting to
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