LTC3370 datasheet(14/22 Pages) AD

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LTC3315B

Rev. 0

For more information www.analog.com

is significant inductance due to long wires or cables, ad-

ditional bulk capacitance may be necessary. This can be

provided with an electrolytic capacitor. A ceramic input

capacitor combined with trace or cable inductance forms

ahighquality(underdamped)tankcircuit.IftheLTC3315B

circuit is plugged in to a live supply, the input voltage can

ring to twice its nominal value, possibly exceeding the

voltage rating. This situation is easily avoided (see Linear

Technology Application Note 88).

Table 2. Ceramic Capacitor Manufacturers

MANUFACTURER

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 Loop Response

The output capacitor has two essential functions. Along

with the inductor, it filters the square wave generated by

the LTC3315B at the SW pin to produce the DC output. In

this role it determines the output ripple; thus, low imped-

ance at the switching frequency is important. The second

function is to store energy in order to satisfy transient

loads and to stabilize the LTC3315B’s control loop.

APPLICATIONS INFORMATION

The LTC3315B is internally compensated and designed to

operate at a high bandwidth for fast transient 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 an output capacitance value of approximately:

COUT =

80µF •MHz

fSW

500mV

VOUT

(7)

where COUT is the recommended output capacitor value

and fSWistheswitchingfrequency.Alowervalueofoutput

capacitorsavesspaceandcost,buttransientperformance

will suffer and loop stability must be verified. Ceramic ca-

pacitors have very low equivalent series resistance (ESR)

and provide the best output ripple and transient perfor-

mance. Use X5R or X7R ceramic capacitors. (see Table 2).

Even better output ripple and transient performance can

be achieved by using low-ESL reverse geometry or three

terminal ceramic capacitors.

Duringaloadstep,theoutputcapacitormustinstantaneously

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 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, fSW,

Table 1. Recommended Inductors

MANUFACTURER

FAMILY

L (nH)

MAX IDC (A)

MAX DCR (mΩ)

SIZE IN mm (L × W × H)

Murata

DFE18SAN_G0

240, 470

3.3 - 4.9

30 - 54

1.6 × 0.8 × 1.0

Murata

DFE201210S

110, 470

4.8 - 8.8

12 - 32

2.0 × 1.2 × 1.0

Murata

DFE201210U

240 - 680

3.6 - 6.5

25 - 60

2.0 × 1.2 × 1.0

Murata

DFE201610E

240 - 680

4.3 - 6.3

20 - 43

2.0 × 1.6 × 1.0

Murata

DFE252010F

220 - 820

4.5 - 7.2

16 - 40

2.5 × 2.0 × 1.0

Vishay

IHHP-0806AB-01

220 - 470

3.6-5.2

16 - 35

2.0 × 1.6 × 1.2

Vishay

IHHP-1008AB-01

220 - 470

4.5 - 6.5

10.5 - 26

2.5 × 2.0 × 1.2

NIC

NPMH0805

240 - 470

3.3 - 4.5

28 - 42

2.0 × 1.2 × 1.0

NIC

NPMH0806

240 - 470

3.5 - 4.7

23 - 42

2.0 × 1.6 × 1.0

NIC

NPMH1008

240 - 680

4.5 - 8.0

18 - 40

2.5 × 2.0 × 1.0

XFRMS

XFHCL43LT

220 - 470

3.8 - 7.0

13 - 25

2.5 × 2.0 × 1.2

WURTH ELECTRONIK

WE-PMCI-0806

240 - 680

3.4 - 5.4

19 -45

2.0 × 1.6 × 1.0

WURTH ELECTRONIK

WE-LQS-2512

160 - 680

3.5 - 6.4

16 - 35

2.5 × 2.0 × 1.2

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