LTC3306CACBZ-R7 datasheet(11/16 Pages) AD

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LTC3306

Rev. 0

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3.15mm

2.92mm

VOUT

3306 F01

GND

VIN

GND

COUT

CIN

Figure 1. Recommended PCB Layout for the LTC3306

A lower value of output capacitor can be used to save

space and cost, but transient performance will suffer and

may cause loop instability.

Ceramic capacitors have very low equivalent series resis-

tance (ESR) and provide the best output ripple and tran-

sient performance. Use X5R or X7R ceramic capacitors.

During a load step, the output capacitor must instanta-

neously supply the current to support the load until the

feedback loop raises the switch current enough to sup-

port the load. The time required for the feedback loop to

respond is dependent on the compensation components

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, VSW, tON(MIN), and other factors, the output

droop, VDROOP, is usually about 2 to 4 times the linear

drop of the first cycle (Equation 5).

VDROOP =

3 • ∆IOUT

COUT • fSW

(5)

where ΔIOUT is the load step.

Transient performance and control loop stability can

be improved with a higher COUT. The values used in

the typical application circuits are good starting points.

LTpowerCAD® is a useful tool to help optimize COUT for a

desired transient performance.

Applying a load transient and monitoring the response of

the system or using a network analyzer to measure loop

response are two ways to experimentally verify transient

performance and control loop stability and optimize COUT.

APPLICATIONS INFORMATION

Input Capacitors

Bypass the input of the LTC3306 with a ceramic capacitor

close to the part from VIN to GND. See Output Capacitor,

Output Ripple, and Transient Response for more detail.

X7R or X5R capacitors are recommended for best perfor-

mance across temperature and input voltage variations.

Note that larger input capacitance is required when a lower

switching frequency is used. 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 a LTC3306 circuit is plugged into a live supply, the

input voltage can ring to twice its nominal value, possibly

exceeding the maximum input voltage rating. This situa-

tion is easily avoided (see Application Note AN88).

Table 2. 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

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 LTC3306 to produce a DC output. In this role,

it determines the output ripple, thus low impedance at

the switching frequency is important. The second func-

tion is to store energy in order to satisfy transient loads

and stabilize the LTC3306’s control loop. The LTC3306 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 sys-

tem, but the transient response is also affected by VOUT,

VIN, fSW, and other factors. A good place to start is with

the output capacitor shown in typical application circuits.

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