LTC3314AACBZ-R7 datasheet(19/26 Pages) AD

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LTC3314A

Rev. 0

For more information www.analog.com

Input Capacitors

Bypass the input of the LTC3314A with at least two

ceramic capacitors close to the part, one near each PVIN

pin. Connect the ground of each capacitor to a wide PCB

trace on the top layer of the PCB that connects to the

PGND leads. These capacitors should be 0603 or 0805 in

size. Smaller 0201 capacitors can also be placed as close

as possible from PVIN1 to PGND, and from PVIN2 to PGND

to reduce input noise with minimal increase in application

footprint. See the PCB Layout Considerations section for

more detail. X7R or X5R capacitors are recommended for

best performance across temperature and input voltage

variations (see Table 3). Note that larger input capacitance

is required when a lower switching frequency is used. If

the input power source has high impedance, or there is

significant inductance due to long wires or cables, addi-

tional bulk capacitance may be necessary. This can be

provided with an electrolytic capacitor.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (underdamped) tank cir-

cuit. If the LTC3314A circuit is plugged into a live sup-

ply, the input voltage can ring to twice its nominal value,

possibly exceeding the LTC3314A’s voltage rating. This

situation is easily avoided (see Application Note AN88).

Table 3. Ceramic Capacitor Manufacturers

VENDOR

URL

Kyocera

www.kyocera-avx.com

Murata

www.murata.com

TDK

www.tdk.com

Taiyo Yuden

www.t-yuden.com

Samsung

www.samsungsem.com

Wurth Elektronik

www.we-online.com

Output Capacitor, Output Ripple and Transient

Response

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LTC3314A 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 LTC3314A’s control loop.

The LTC3314A is internally compensated and has been

designed to operate at a high bandwidth for fast tran-

sient response capability. The selection of COUT will affect

the bandwidth of the system, but the transient response

is also affected by VOUT, VIN, fSW, and other factors. A

good place to start is with the output capacitor value given

by Equation 8.

COUT = 20 •

IMAX

fSW

0.5

VOUT

(8)

where COUT is the recommended output capacitor value

in µF, fSW is the switching frequency in MHz, IMAX = 4A

per phase is the rated output current in Amps, and VOUT

is in Volts.

A lower value of output capacitor can be used to save

space and cost, but transient performance will suffer and

loop stability must be verified.

Ceramic capacitors have very low equivalent series

resistance (ESR) and provide the best output ripple and

transient performance. Use X5R or X7R ceramic capaci-

tors (see Table 3). Even better output ripple and transient

performance can be achieved by using low-ESL reverse

geometry or three-terminal ceramic capacitors.

During a load step, the output capacitor must instan-

taneously supply the current to support the load until

the feedback loop increases the switch current enough

to support the load. The time required for the feedback

loop to respond is dependent on the compensation com-

ponents and the output capacitor size. Typically, 3 to 4

cycles are required to respond to a load step, but only

in the first cycle does the output drop linearly. Although

affected by VOUT, VIN, fSW, tON(MIN), the equivalent series

inductance (ESL) of the output capacitor, and other fac-

tors, the output droop, VDROOP, is usually about 3 times

the linear drop of the first cycle (Equation 9).

VDROOP =

3 • ∆IOUT

COUT • fSW

(9)

Transient performance and control loop stability can

be improved with a higher COUT and/or the addition of

a feedforward capacitor CFF placed between VOUT and

FB. Capacitor CFF provides phase lead compensation by

APPLICATIONS INFORMATION

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