LM2735XMFSLASHNOPB datasheet(16/59 Pages) TI

16 / 59 page

L =

VIN

2 x 'iL

x DTS

¸¸

¨¨

Âi

¨¨

VIN

2 L

DTS

¸¸

LM2735, LM2735-Q1

SNVS485G – JUNE 2007 – REVISED AUGUST 2015

www.ti.com

(11)

A good design practice is to design the inductor to produce 10% to 30% ripple of maximum load. From the

previous equations, the inductor value is then obtained.

where

•

1/TS = FSW = switching frequency

(12)

Ensure that the minimum current limit (2.1 A) is not exceeded, so the peak current in the inductor must be

calculated. The peak current (ILPK ) in the inductor is calculated by:

ILpk = IIN + ΔIL

(13)

ILpk = IOUT / D' + ΔIL

(14)

When selecting an inductor, make sure that it is capable of supporting the peak input current without saturating.

Inductor saturation will result in a sudden reduction in inductance and prevent the regulator from operating

correctly. Because of the speed of the internal current limit, the peak current of the inductor need only be

specified for the required maximum input current. For example, if the designed maximum input current is 1.5 A

and the peak current is 1.75 A, then the inductor should be specified with a saturation current limit of >1.75 A.

There is no need to specify the saturation or peak current of the inductor at the 3-A typical switch current-limit.

Because of the operating frequency of the LM2735, ferrite based inductors are preferred to minimize core losses.

This presents little restriction since the variety of ferrite-based inductors is huge. Lastly, inductors with lower

series resistance (DCR) will provide better operating efficiency. For recommended inductors, see the following

design examples.

8.2.1.2.2

Input Capacitor

An input capacitor is necessary to ensure that VIN does not drop excessively during switching transients. The

primary specifications of the input capacitor are capacitance, voltage, RMS current rating, and ESL (Equivalent

Series Inductance). The recommended input capacitance is 10 µF to 44 µF depending on the application. The

capacitor manufacturer specifically states the input voltage rating. Make sure to check any recommended

deratings and also verify if there is any significant change in capacitance at the operating input voltage and the

operating temperature. The ESL of an input capacitor is usually determined by the effective cross sectional area

of the current path. At the operating frequencies of the LM2735, certain capacitors may have an ESL so large

that the resulting impedance (2

πfL) will be higher than that required to provide stable operation. As a result,

surface mount capacitors are strongly recommended. Multilayer ceramic capacitors (MLCC) are good choices for

both input and output capacitors and have very low ESL. For MLCCs, TI recommends using X7R or X5R

dielectrics. Consult capacitor manufacturer datasheet to see how rated capacitance varies over operating

conditions.

8.2.1.2.3

Output Capacitor

The LM2735 operates at frequencies allowing the use of ceramic output capacitors without compromising

transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple.

The output capacitor is selected based upon the desired output ripple and transient response. The initial current

of a load transient is provided mainly by the output capacitor. The output impedance will therefore determine the

maximum voltage perturbation. The output ripple of the converter is a function of the reactance of the capacitor

and its equivalent series resistance (ESR):

Submit Documentation Feedback

Product Folder Links: LM2735 LM2735-Q1

LM2735XMF/NOPB 数据表(PDF) 16 Page - Texas Instruments

LM2735XMF/NOPB 数据表(HTML) 16 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

数据表下载