LM3421 datasheet(25/65 Pages) TI1

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LM3421, LM3421-Q1

LM3423, LM3423-Q1

www.ti.com

SNVS574D – JULY 2008 – REVISED SEPTEMBER 2011

Figure 34. Dynamic Resistance

Obtaining rLED is accomplished by refering to the manufacturer's LED I-V characteristic. It can be calculated as

the slope at the nominal operating point as shown in Figure 34. For any application with more than 2 series

LEDs, RSNS can be neglected allowing rD to be approximated as the number of LEDs multiplied by rLED.

OUTPUT CAPACITOR

For boost and buck-boost regulators, the output capacitor (CO) provides energy to the load when the recirculating

diode (D1) is reverse biased during the first switching subinterval. An output capacitor in a buck topology will

simply reduce the LED current ripple (

ΔiLED-PP) below the inductor current ripple (ΔiL-PP). In all cases, CO is sized

to provide a desired

ΔiLED-PP. As mentioned in the INDUCTOR section, ΔiLED-PP is recommended by

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

CO should be carefully chosen to account for derating due to temperature and operating voltage. It must also

have the necessary RMS current rating. Ceramic capacitors are the best choice due to their high ripple current

rating, long lifetime, and good temperature performance. An X7R dieletric rating is suggested.

INPUT CAPACITORS

The input capacitance (CIN) provides energy during the discontinuous portions of the switching period. For buck

and buck-boost regulators, CIN provides energy during tON and during tOFF, the input voltage source charges up

CIN with the average input current (IIN). For boost regulators, CIN only needs to provide the ripple current due to

the direct connection to the inductor. CIN is selected given the maximum input voltage ripple (ΔvIN-PP) which can

be tolerated.

ΔvIN-PP is suggested to be less than 10% of the input voltage (VIN).

An input capacitance at least 100% greater than the calculated CIN value is recommended to account for derating

due to temperature and operating voltage. When PWM dimming, even more capacitance can be helpful to

minimize the large current draw from the input voltage source during the rising transistion of the LED current

waveform.

The chosen input capacitors must also have the necessary RMS current rating. Ceramic capacitors are again the

best choice due to their high ripple current rating, long lifetime, and good temperature performance. An X7R

dieletric rating is suggested.

For most applications, it is recommended to bypass the VIN pin with an 0.1 µF ceramic capacitor placed as close

as possible to the pin. In situations where the bulk input capacitance may be far from the LM3421/23 device, a

Ω series resistor can be placed between the bulk input capacitance and the bypass capacitor, creating a

150 kHz filter to eliminate undesired high frequency noise.

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LM3421 Datasheet(PDF) 25 Page - Texas Instruments