LTC3313EVPBF datasheet(15/28 Pages) AD

15 / 28 page

LTC3313

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

capacitors are recommended for best performance across

temperature and input voltage variations. Note that larger

input capacitance is required when a lower switching fre-

quency is used. For high frequency applications, adding two

small capacitors close to the part is also recommended. If

the input power source has high impedance, or if there is

significant inductance due to long wires or cables, additional

bulk capacitance may be necessary. This can be provided with

a low performance electrolytic capacitor.

A ceramic input capacitor combined with trace or cable

inductance forms a high quality (underdamped) tank cir-

cuit. If the LTC3313 circuit is plugged into a live supply,

the input voltage can ring to twice its nominal value, pos-

sibly exceeding the LTC3313’s voltage rating. This situ-

ation is easily avoided (see Analog Devices Application

Note 88). Table 3 provides a list of recommended capaci-

tor manufacturers.

Table 3. Ceramic Capacitor Manufacturers

VENDOR

URL

Kyocera AVX

www.kyocera-avx.com

Murata

www.murata.com

TDK

www.tdk.com

Taiyo Yuden

www.t-yuden.com

Samsung

www.samsungsem.com

Output Capacitor and Output Ripple

The output capacitor has two essential functions. Along

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

the LTC3313, to produce the DC output. In this role it deter-

mines the output ripple, thus, low impedance at the switch-

ing frequency is important. The second function is to store

energy in order to satisfy transient loads and stabilize the

LTC3313’s control loop. Ceramic capacitors have very low

equivalent series resistance (ESR) and provide the best

ripple performance.

X5R or X7R type capacitors will provide low output ripple

and good transient response. Transient performance is

improved with a higher value output capacitor and the addi-

tion 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

saves 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.

Multiphase Operation

The LTC3313 is easily configurable for multiphase opera-

tion. See Table 4.

Connecting the RT pin of the master phase to a resistor to

AGND programs the frequency and configures the MODE/

SYNC pin to become clock output used to drive the MODE/

SYNC pin of the slave phase(s).

Connecting the RT pin of the master phase to VIN con-

figures the MODE/SYNC pin to become an input capable

of accepting an external clock. The switching frequency

defaults to the nominal 2MHz internal frequency when

the external clock is unavailable, such as during start-up.

Connecting the FB pin to VIN configures a phase as a

slave. The MODE/SYNC becomes an input and the voltage

control loop is disabled. The slave phase current control

loop is still active and the peak current is controlled via the

shared ITH node. Careful consideration should be taken

when routing the ITH node between phases. Routing the

ITH and AGND nodes together is recommended to create

a low inductance path. See the multiphase demo board

PCB layout documentation as an example.

Connecting the PGOOD pins together and adding an

external pull-up resistor allows the master phase to com-

municate with the slave phases on when start-up has

been completed.

Table 4. LTC3313 Multiphase Configuration

MASTER/SLAVE

RT PIN

FB PIN

MODE/SYNC PIN

SWITCHING FREQUENCY (fSW)

Master

VIN

VOUT Divider

Clock Input

External Clock/2MHz Default

Master

Resistor to AGND

VOUT Divider

Clock Output

RT-Programmed

Slave

VIN Divider

VIN

Clock Input

External Clock

LTC3313EVPBF Datasheet(PDF) 15 Page - Analog Devices

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