LTC3312SAAVPBF datasheet(19/28 Pages) AD

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LTC3312SA

Rev. 0

For more information www.analog.com

Input Capacitors

Bypass the input of the LTC3312SA 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 pins 9

and 10 with the exposed pad. 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, additional 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 LTC3312SA circuit is plugged into a live sup-

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

possibly exceeding the LTC3312SA’s voltage rating. This

situation is easily avoided (see Application Note AN88).

Table 3. Ceramic Capacitor Manufacturers

VENDOR

URL

AVX

www.avxcorp.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 LTC3312SA 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 LTC3312SA’s control loop.

The LTC3312SA is internally compensated and has been

designed to operate at a high bandwidth for fast transient

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 = 6A 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 instanta-

neously 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 compo-

nents 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 factors, 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

APPLICATIONS INFORMATION

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