LP78084 datasheet(18/20 Pages) POWER

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Preliminary Datasheet

LP78084

LP78084 – 02 Ver. 1.1 Datasheet

Nov.-2007

Page 18 of 22

ADP Bypass Capacitor

Many types of capacitors can be used for input bypassing;

however, caution must be exercised when using multi-layer

ceramic capacitors. Because of the self-resonant and high Q

characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up

conditions, such as connecting the battery charger input to a

live power source.

SWITCHING

REGULATOR

Inductor Selection

The output inductor is selected to limit the ripple current to

some predetermined value, typically 20%~40% of the full

load current at the maximum input voltage. Large value

inductors lower ripple currents. Higher VIN or VOU T also

increases the ripple current as shown in equation. A

reasonable starting point for setting ripple current is

IL =240mA (40% of 600mA).

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 720mA rated

inductor

should

enough

for

most

applications

(600mA+120mA). For better efficiency, choose a low

DC-resistance inductor.

CIN and COUT Selection

In continuous mode, the source current of the top MOSFET

is a square wave of duty cycle VOUT/VIN. The primary

function of the input capacitor is to provide a low

impedance loop for the edges of pulsed current drawn by the

LP78084. A low ESR input capacitor sized for the maximum

RMS current must be used. The size required will vary

depending on the load, output voltage and input voltage

source impedance characteristics. A typical value is around

4.7µF.

The input capacitor RMS current varies with the input

voltage and the output voltage. The equation for the

maximum RMS current in the input capacitor is:

The output capacitor COUT has a strong effect on loop

stability. The selection of COUT is driven by the required

effective series resistance (ESR).

ESR is a direct function of the volume of the capacitor;

that is, physically larger capacitors have lower ESR. Once

the ESR requirement for COUT has been met, the

RMS current rating generally far exceeds the IRI PPL E ( P- P )

requirement. The output ripple VOUT is determined by:

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics

have

the

best

temperature

and

voltage

characteristics of all the ceramics for a given value and

size.

Output Voltage Programming

The output voltage is set by a resistive divider according

to the following formula:

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure 5.

LINEAR

REGULATOR

External Capacitors

Like any low-dropout regulator, the regulator requires

external capacitors for regulator stability. The regulator is

specifically designed for portable applications requiring

minimum board space and smallest components. These

capacitors must be correctly selected for good performance.

Input Capacitor

A minimum input capacitance of 1 µF is required between

the regulator input pin and ground (the amount of the

capacitance may be increased without limit).This capacitor

must be located a distance of not more than 1cm from the

input pin and returned to a clean analog ground.

Any good quality ceramic, tantalum, or film capacitor may

be used at the input. If a tantalum capacitor is used at the

input, it must be guaranteed by the manufacturer to

have a surge current rating sufficient for the application.

No-Load Stability

The regulator will remain stable and in regulation with no

external load. This is specially important in CMOS RAM

keep-alive applications.

Output Capacitance

The regulator is specifically designed to employ ceramic

output capacitors as low as 2.2µF. Ceramic capacitors below

10µF offer significant cost and space savings, along with

high frequency noise filtering. Higher values and other

types and of capacitor may be used, but their equivalent

series resistance (ESR) should be maintained below 0 . 5 .

Ceramic capacitor of the value required by the regulator are

available in the following dielectric types: Z5U, Y5V, X5R,

and X7R. The Z5U and Y5V types exhibit a 50% or more

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