LTC3621 datasheet(11/20 Pages) AD

11 / 20 page

LTC3644/LTC3644-2

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

Output Voltage Programming

The output voltage is set by external resistive divider

according to the following equation:

VOUT = 0.6V • 1+

⎛

⎝⎜

⎞

⎠⎟

The resistive divider allows the FB pin to sense a fraction

of the output voltage as shown in Figure 3.

performance.FortheLTC3644/LTC3644-2,aminimumCOUT

of47μFisrecommendedtoensureloopstabilityforVOUTlower

than2V.Forgoodstartingvalues,seetheTypicalApplication

section.

Use X5R or X7R types. This choice will provide low

output ripple and good transient response. Transient

performance can be improved with a higher value output

capacitor and the addition of a feedforward capacitor

placed between VOUT and FB. Increasing the output

capacitance will also decrease the output voltage ripple.

A lower value of output capacitor can be used to save

space and cost but transient performance will suffer and

may cause loop instability. See the Typical Applications

in this data sheet for suggested capacitor values.

When choosing a capacitor, special attention should be

giventothedatasheettocalculatetheeffectivecapacitance

undertherelevantoperatingconditionsofvoltagebiasand

temperature. A physically larger capacitor or one with a

higher voltage rating may be required.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LTC3644 due to their piezoelectric

nature. When in Burst Mode operation, the LTC3644’s

switching frequency depends on the load current, and

at very light loads the LTC3644 can excite the ceramic

capacitor at audio frequencies, generating audible noise.

Since the LTC3644 operates at a lower current limit during

Burst Mode operation, the noise is typically very quiet to a

casual ear. If this is unacceptable, use a high performance

tantalum or electrolytic capacitor at the output. Low noise

ceramic capacitors are also available.

Output Power Good

When the LTC3644’s output voltages are within the ±7.5%

windowoftheregulationpoint,theoutputvoltagesaregood

andthePGOODpinsarepulledhighwithexternalresistors.

Otherwise, internal open-drain pull-down devices (275Ω)

willpullthePGOODpinslow.TopreventunwantedPGOOD

glitches during transients or dynamic VOUT changes, the

LTC3644’s PGOOD falling edge includes a blanking delay

of approximately 32 switching cycles.

Figure 3. Setting the Output Voltage

Input Capacitor (CIN) Selection

The LTC3644 has individual input supply pins for each

buck switching regulator and a separate SVIN pin that

supplies power to all top level control and logic. Each of

these pins must be decoupled with low ESR capacitors

to GND. These capacitors must be placed as close to

the pins as possible. Ceramic dielectric capacitors are a

good compromise between high dielectric constant and

stability versus temperature and DC bias. Note that the

capacitance of a capacitor deteriorates at higher DC bias.

It is important to consult manufacturer data sheets and

obtain the true capacitance of a capacitor at the DC bias

voltage it will be operated at. For this reason, avoid the

use of Y5V dielectric capacitors. The X5R/X7R dielectric

capacitors offer good overall performance.

Output Capacitor (COUT) Selection

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by the

LTC3644 to produce the DC output. In this role it determines

the output ripple, thus low impedance at the switching

frequency is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

LTC3644’s control loop. Ceramic capacitors have very low

equivalentseriesresistance(ESR)andprovidethebestripple

VOUT

CFF

3644 F03

GND

LTC3644

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