LM3421 datasheet(15/24 Pages) NSC

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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.

30067362

FIGURE 6. Boost Topology with Inrush Diode

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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