LTC3441 datasheet(13/20 Pages) AD

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LTC3246

3246fa

For more information www.linear.com/LTC3246

VIN Capacitor Selection

ThefinitechargetransferarchitectureusedbytheLTC3246

makes input noise filtering much less demanding than the

sharp current spikes of conventional regulated charge

pumps. Depending on the mode of operation, the input

current of the LTC3246 can step from about 1A to 0A on

a cycle-by-cycle basis. Low ESR will reduce the voltage

steps caused by changing input current, while the ab-

solute capacitor value will determine the level of ripple.

The total amount and type of capacitance necessary for

input bypassing is very dependent on the applied source

impedance as well as existing bypassing already on the

VIN node. For optimal input noise and ripple reduction, it

is recommended that a low ESR ceramic capacitor be used

for CIN bypassing. An electrolytic or tantalum capacitor

may be used in parallel with the ceramic capacitor on CIN

to increase the total capacitance, but, due to the higher

ESR, it is not recommended that an electrolytic or tan-

talum capacitor be used alone for input bypassing. The

LTC3246 will operate with capacitors less than 1µF, but,

depending on the source impedance, input noise can feed

through to the output causing degraded performance.

For best performance 1µF or greater total capacitance is

suggested for CIN.

Flying Capacitor Selection

Ceramic capacitors should always be used for the flying

capacitor. The flying capacitor controls the strength of

the charge pump. In order to achieve the rated output

current, it is necessary for the flying capacitor to have

at least 1µF of capacitance over operating temperature

with a bias voltage equal to the programmed VOUT (see

Ceramic Capacitor Selection Guidelines). If only 100mA

or less of output current is required for the application,

the flying capacitor minimum can be reduced to 0.2µF.

The voltage rating of the ceramic capacitor should be

VOUT +1V or greater.

Ceramic Capacitor Selection Guidelines

Capacitors of different materials lose their capacitance

with higher temperature and voltage at different rates.

For example, a ceramic capacitor made of X5R or X7R

material will retain most of its capacitance from –40°C

to 85°C, whereas a Z5U or Y5V style capacitor will lose

considerable capacitance over that range (60% to 80%

loss typical). Z5U and Y5V capacitors may also have a

very strong voltage coefficient, causing them to lose an

additional60%ormoreoftheircapacitancewhentherated

voltage is applied. Therefore, when comparing different

capacitors, it is often more appropriate to compare the

amount of achievable capacitance for a given case size

rather than discussing the specified capacitance value. For

example, over rated voltage and temperature conditions,

a 4.7µF, 10V, Y5V ceramic capacitor in an 0805 case may

not provide any more capacitance than a 1µF, 10V, X5R

or X7R available in the same 0805 case. In fact, over bias

and temperature range, the 1µF, 10V, X5R or X7R will

provide more capacitance than the 4.7µF, 10V, Y5V. The

capacitor manufacturer’s data sheet should be consulted

to determine what value of capacitor is needed to ensure

minimum capacitance values are met over operating

temperature and bias voltage. Below is a list of ceramic

capacitor manufacturers and how to contact them:

MANUFACTURER

WEBSITE

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata

Taiyo Yuden

www.t-yuden.com

TDK

www.tdk.com

Wurth Elektronik

www.we-online.com

BIAS Pin and Capacitor Selection

TheBIASpinoftheLTC3246isa5Voutputthatisgenerated

by an internal Low Drop-Out (LDO) regulator supplied by

VIN. The BIAS voltage is used as a supply for the internal

low voltage circuitry. A capacitor on the BIAS pin is neces-

sary to stabilize the LDO output and minimize ripple during

transient conditions. A low ESR ceramic capacitor with a

minimum capacitance of 2µF over temperature with 5V

bias should be used. Since the BIAS voltage comes from

an LDO, the BIAS voltage will drop with VIN as VIN goes

below 5V. This is normal and expected operation. The

BIAS pin voltage is for internal circuitry only and should

not be loaded externally.

APPLICATIONS INFORMATION

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