LM10506 datasheet(33/43 Pages) TI2

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IRMSCIN = D x I

OUT +

RIPPLE

IOUT x D

CIN x FS

VPPIN =

+ IOUT x ESRCIN

PD_CIN = I

RMS_CIN x RESR_CIN

VOUT (VIN - VOUT)

IRMS_CIN = IOUT

VIN

LM10506

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SNVS729F – SEPTEMBER 2011 – REVISED AUGUST 2014

9.2.2.3.2.3.2

Input Capacitor Selection

There are 3 buck regulators in the LM10506 device. Each of these buck regulators has its own input capacitor

which should be located as close as possible to their corresponding SWx_VIN and SWx_GND pins, where x

designates Buck 1, 2, or 3. The 3 buck regulators operate at 120° out of phase, which means that they switch on

at equally spaced intervals, in order to reduce the input power rail ripple. It is recommended to connect all the

supply/ground pins of the buck regulators, SWx_VIN to two solid internal planes located under the device. In this

way, the 3 input capacitors work together and further reduce the input current ripple. A larger tantalum capacitor

can also be located in the proximity of the device.

The input capacitor supplies the AC switching current drawn from the switching action of the internal power

FETs. The input current of a buck converter is discontinuous, so the ripple current supplied by the input capacitor

is large. The input capacitor must be rated to handle both the RMS current and the dissipated power.

The input capacitor must be rated to handle this current:

(6)

The power dissipated in the input capacitor is given by:

(7)

The device is designed to be used with ceramic capacitors on the inputs of the buck regulators. The

recommended dielectric type of these capacitors is X5R, X7R, or of comparable material to maintain proper

tolerances over voltage and temperature. The minimum recommended value for the input capacitor is 10 µF with

an ESR of 10 m

Ω or less. The input capacitors need to be mounted as close as possible to the power/ground

input pins of the device.

The input power source supplies the average current continuously. During the PFET switch on-time, however,

the demanded di/dt is higher than can be typically supplied by the input power source. This delta is supplied by

the input capacitor.

A simplified “worst case” assumption is that all of the PFET current is supplied by the input capacitor. This will

result in conservative estimates of input ripple voltage and capacitor RMS current.

Input ripple voltage is estimated as follows:

where

•

VPPIN: estimated peak-to-peak input ripple voltage,

•

IOUT: Output Current

•

CIN: Input capacitor value

•

ESRCIN: input capacitor ESR.

(8)

This capacitor is exposed to significant RMS current, so it is important to select a capacitor with an adequate

RMS current rating. Capacitor RMS current estimated as follows:

•

IRMSCIN: estimated input capacitor RMS current.

(9)

9.2.2.4 Recommendations For Unused Functions And Pins

If any function is not used in the end application then the following recommendations for tying-off the associated

pins on the circuit boards should be used.

FUNCTION

IF UNUSED

BUCK1

VIN_B1

Connect to VIN

SW_B1

Connect to VIN

FB_B1

Connect to GND

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