LM46002PWPRG4 datasheet(27/54 Pages) TI2

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OUT

L MAX

) D

0.4 F

0.2 F

OUT

) D

L F

LM46002

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SNVSA13C – APRIL 2014 – REVISED APRIL 2019

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8.2.2.3 Switching Frequency

The default switching frequency of the LM46002 device is set at 500 kHz when RT pin is open circuit. The

switching frequency is selected to be 500 kHz in this application for one less passive components. If other

frequency is desired, use Equation 12 to calculate the required value for RT.

RT(kΩ) = 40200 / Freq (kHz) – 0.6

(12)

For 500 kHz, the calculated RT is 79.8 kΩ and standard value 80.6 kΩ can also be used to set the switching

frequency at 500 kHz.

8.2.2.4 Input Capacitors

The LM46002 device requires high frequency input decoupling capacitor(s) and a bulk input capacitor, depending

on the application. The typical recommended value for the high frequency decoupling capacitor is 4.7 µF to 10

µF. A high-quality ceramic type X5R or X7R with sufficiency voltage rating is recommended. The voltage rating

must be greater than the maximum input voltage. To compensate the derating of ceramic capactors, a voltage

rating of twice the maximum input voltage is recommended. Additionally, some bulk capacitance can be required,

especially if the LM46002 circuit is not located within approximately 5 cm from the input voltage source. This

capacitor is used to provide damping to the voltage spiking due to the lead inductance of the cable or trace. The

value for this capacitor is not critical but must be rated to handle the maximum input voltage including ripple.

For this design, a 10 µF, X7R dielectric capacitor rated for 100 V is used for the input decoupling capacitor. The

equivalent series resistance (ESR) is approximately 3 mΩ, and the current-rating is 3 A. Include a capacitor with

a value of 0.1 µF for high-frequency filtering and place it as close as possible to the device pins.

NOTE

DC bias effect: High-capacitance ceramic capacitors have a DC bias effect, which has a

strong influence on the final effective capacitance. Therefore, choose the right capacitor

value carefully. Package size and voltage rating in combination with dielectric material are

responsible for differences between the rated capacitor value and the effective

capacitance.

8.2.2.5 Inductor Selection

The first criterion for selecting an output inductor is the inductance itself. In most buck converters, this value is

based on the desired peak-to-peak ripple current, ΔiL, that flows in the inductor along with the DC load current.

As with switching frequency, the selection of the inductor is a tradeoff between size and cost. Higher inductance

gives lower ripple current and hence lower output voltage ripple with the same output capacitors. Lower

inductance could result in smaller, less expensive component. An inductance that gives a ripple current of 20% to

40% of the 2 A at the typical supply voltage is a good starting point. ΔiL = (1/5 to 2/5) × IOUT. The peak-to-peak

inductor current ripple can be found by Equation 13, and the range of inductance can be found by Equation 14

with the typical input voltage used as VIN.

(13)

(14)

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