LT3470 Просмотр технического описания (PDF) - Linear Technology

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Компоненты Описание

производитель

LT3470

Micropower Buck Regulator with Integrated Boost and Catch Diodes

Linear Technology 

LT3470

Applications Information

Table 2. Inductor Vendors

VENDOR

URL

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Toko

Würth Elektronik

Coiltronics

Murata

www.tokoam.com

www.we-online.com

www.cooperet.com

www.murata.com

PART SERIES

DO1605

ME3220

DO3314

CR32

CDRH3D16/HP

CDRH3D28

CDRH2D18/HP

DB320C

D52LC

WE-PD2 Typ S

WE-TPC Typ S

SD10

LQH43C

LQH32C

INDUCTANCE RANGE (µH)

10 to 47

10 to 47

10 to 47

10 to 47

10 to 33

10 to 47

10 to 15

10 to 27

10 to 47

10 to 47

10 to 22

10 to 47

10 to 47

10 to 15

SIZE (mm)

1.8 × 5.4 × 4.2

2.0 × 3.2 × 2.5

1.4 × 3.3 × 3.3

3.0 × 3.8 × 4.1

1.8 × 4.0 × 4.0

3.0 × 4.0 × 4.0

2.0 × 3.2 × 3.2

2.0 × 3.8 × 3.8

2.0 × 5.0 × 5.0

3.2 × 4.0 × 4.5

1.6 × 3.8 × 3.8

1.0 × 5.0 × 5.0

2.6 × 3.2 × 4.5

1.6 × 2.5 × 3.2

be used. It is important to note that inductor saturation

current is reduced at high temperatures—see inductor

vendors for more information.

Input Capacitor

Step-down regulators draw current from the input sup-

ply in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the VIN pin of the LT3470 and to force this switching

current into a tight local loop, minimizing EMI. The input

capacitor must have low impedance at the switching

frequency to do this effectively. A 1µF to 2.2µF ceramic

capacitor satisfies these requirements.

If the input source impedance is high, a larger value ca-

pacitor may be required to keep input ripple low. In this

case, an electrolytic of 10µF or more in parallel with a 1µF

ceramic is a good combination. Be aware that the input

capacitor is subject to large surge currents if the LT3470

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be specified for such use.

Output Capacitor and Output Ripple

The output capacitor filters the inductor’s ripple current

and stores energy to satisfy the load current when the

LT3470 is quiescent. In order to keep output voltage ripple

low, the impedance of the capacitor must be low at the

LT3470’s switching frequency. The capacitor’s equivalent

series resistance (ESR) determines this impedance. Choose

one with low ESR intended for use in switching regulators.

The contribution to ripple voltage due to the ESR is ap-

proximately ILIM • ESR. ESR should be less than ~150mΩ.

The value of the output capacitor must be large enough to

accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal

to 1% of the output voltage, the output capacitor must be:

COUT

> 50

•L

•





ILIM

VOUT





2

where ILIM is the top current limit with VFB = 0V (see Elec-

trical Characteristics). For example, an LT3470 producing

3.3V with L = 33µH requires 22µF. The calculated value

can be relaxed if small circuit size is more important than

low output ripple.

Sanyo’s POSCAP series in B-case and provides very good

performance in a small package for the LT3470. Similar

performance in traditional tantalum capacitors requires

a larger package (C-case). With a high quality capacitor

filtering the ripple current from the inductor, the output

voltage ripple is determined by the delay in the LT3470’s

feedback comparator. This ripple can be reduced further

by adding a small (typically 22pF) phase lead capacitor

between the output and the feedback pin.

3470fd

10

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