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second function is to store energy in order to satisfy tran-

sient loads and stabilize the LTC3307A’s control loop.

The LTC3307A is internally compensated and has been

designed to operate at a high bandwidth for fast tran-

sient response capability. The selection of COUT affects

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 capacitance value

of approximately:

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 = 3A

is the rated output current in Amps, and VOUT is in Volts.

A lower value output capacitor saves 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:

VDROOP =

3 • ΔIOUT

COUT • fSW

(9)

where ΔIOUT is the load step.

Input Capacitors

Bypass the input of the LTC3307A with at least two

ceramic capacitors close to the part, one on each side

from VIN to PGND, for best performance. These capaci-

tors should be 0603 or 0805 in size. Smaller, optional

0201 capacitors can also be placed as close as possible

to the LTC3307A directly on the traces leading from VIN

(Pin 3) and PGND (Pin 4) and on the traces leading from

VIN (Pin 8) and PGND (Pin 7) for better performance with

minimal (if at all) increase in application footprint. See the

layout 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 fre-

quency is used. If the input power source has high imped-

ance, 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 (under damped) tank

circuit. If the LTC3307A circuit is plugged into a live sup-

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

possibly exceeding the LTC3307A’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 LTC3307A at the SW pin to produce the DC output.

In this role, it determines the output ripple; thus, low

impedance at the switching frequency is important. The

LTC3307A

Rev. A

For more information www.analog.com

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