LM3421 datasheet(24/65 Pages) TI1

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LM3421/23

VIN

FLT

TIMR

LRDY

VIN

VSW

High = LED in regulation

LM3421, LM3421-Q1

LM3423, LM3423-Q1

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

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3. LED current is above regulation by more than 30%.

4. Over-voltage protection has engaged

5. Thermal shutdown has engaged.

6. A fault has latched the device off.

Note that the LRDY pin is pulled low during startup of the device and remains low until the LED current is in

regulation.

Figure 33. Fault Detection and LED Status Circuit

Design Considerations

This section describes the application level considerations when designing with the LM3421/23. For

corresponding calculations, refer to the Design Guide section.

INDUCTOR

The inductor (L1) is the main energy storage device in a switching regulator. Depending on the topology, energy

is stored in the inductor and transfered to the load in different ways (as an example, buck-boost operation is

detailed in the CURRENT REGULATORS section). The size of the inductor, the voltage across it, and the length

of the switching subinterval (tON or tOFF) determines the inductor current ripple (ΔiL-PP ). In the design process, L1

is chosen to provide a desired

ΔiL-PP. For a buck regulator the inductor has a direct connection to the load, which

is good for a current regulator. This requires little to no output capacitance therefore

ΔiL-PP is basically equal to

the LED ripple current

ΔiLED-PP. However, for boost and buck-boost regulators, there is always an output

capacitor which reduces

ΔiLED-PP, therefore the inductor ripple can be larger than in the buck regulator case

where output capacitance is minimal or completely absent.

In general,

ΔiLED-PP is recommended by manufacturers to be less than 40% of the average LED current (ILED).

Therefore, for the buck regulator with no output capacitance,

ΔiL-PP should also be less than 40% of ILED. For the

boost and buck-boost topologies,

ΔiL-PP can be much higher depending on the output capacitance value.

However,

ΔiL-PP is suggested to be less than 100% of the average inductor current (IL) to limit the RMS inductor

current.

L1 is also suggested to have an RMS current rating at least 25% higher than the calculated minimum allowable

RMS inductor current (IL-RMS).

LED DYNAMIC RESISTANCE

When the load is a string of LEDs, the output load resistance is the LED string dynamic resistance plus RSNS.

LEDs are PN junction diodes, and their dynamic resistance shifts as their forward current changes. Dividing the

forward voltage of a single LED (VLED) by the forward current (ILED) leads to an incorrect calculation of the

dynamic resistance of a single LED (rLED). The result can be 5 to 10 times higher than the true rLED value.

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Product Folder Links: LM3421 LM3421-Q1 LM3423 LM3423-Q1

LM3421 Datasheet(PDF) 24 Page - Texas Instruments

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