LTC3548EDD-2(RevA) Просмотр технического описания (PDF) - Linear Technology

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LTC3548EDD-2
(Rev.:RevA)

Dual Synchronous, Fixed/Adjustable Output, 2.25MHz Step-Down DC/DC Regulator

Linear Technology 

LTC3548-2

APPLICATIO S I FOR ATIO

A general LTC3548-2 application circuit is shown in

Figure 2. External component selection is driven by the

load requirement, and begins with the selection of the

inductor L. Once the inductor is chosen, CIN and COUT can

be selected.

Inductor Selection

Although the inductor does not influence the operating

frequency, the inductor value has a direct effect on ripple

current. The inductor ripple current ∆IL decreases with

higher inductance and increases with higher VIN or VOUT:

∆IL

=

VOUT

fO • L

•

⎛

⎝⎜1–

VOUT

VIN

⎞

⎠⎟

Accepting larger values of ∆IL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses and lower output current capability. A

reasonable starting point for setting ripple current is

∆IL = 0.3 • IOUT(MAX), where IOUT(MAX) is 800mA for

channel 1 and 400mA for channel 2. The largest ripple

current ∆IL occurs at the maximum input voltage. To

guarantee that the ripple current stays below a specified

maximum, the inductor value should be chosen according

to the following equation:

L

=

VOUT

fO • ∆IL

⎛

• ⎝⎜1–

VOUT ⎞

VIN(MAX) ⎠⎟

VIN

2.5V TO 5.5V

VOUT2

CIN

BM*

PS*

RUN2 VIN

MODE/SYNC

RUN1

POR

L2

C5

LTC3548-2

SW2

SW1

R5

POWER-ON

RESET

L1

VOUT1

R2

COUT2

VFB2

R1

VOUT1

GND

*MODE/SYNC = 0V: PULSE SKIP

MODE/SYNC = VIN: Burst Mode

Figure 2. LTC3548-2 General Schematic

COUT1

35482 F02

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation

begins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this to occur at lower

load currents. This causes a dip in efficiency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values will cause the burst frequency to

increase.

Inductor Core Selection

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy mate-

rials are small and don’t radiate much energy, but gener-

ally cost more than powdered iron core inductors with

similar electrical characterisitics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated field/EMI require-

ments than on what the LTC3548-2 requires to operate.

Table 1 shows some typical surface mount inductors that

work well in LTC3548-2 applications.

Table 1. Representative Surface Mount Inductors

PART

NUMBER

VALUE DCR

MAX DC

SIZE

(µH) (Ω MAX) CURRENT (A) W × L × H (mm3)

Sumida

2.2

CDRH3D16

3.3

4.7

0.075

0.110

0.162

1.20

3.8 × 3.8 × 1.8

1.10

0.90

Sumida

1.5

CDRH2D11

2.2

0.068

0.170

0.900

0.780

3.2 × 3.2 × 1.2

Sumida

2.2

0.116

CMD4D11

3.3

0.174

0.950

0.770

4.4 × 5.8 × 1.2

Murata

LQH32CN

1.0

0.060

2.2

0.097

1.00

2.5 × 3.2 × 2.0

0.79

Toko

D312F

2.2

0.060

3.3

0.260

1.08

2.5 × 3.2 × 2.0

0.92

Panasonic

3.3

0.17

ELT5KT

4.7

0.20

1.00

4.5 × 5.4 × 1.2

0.95

Input Capacitor (CIN) Selection

In continuous mode, the input current of the converter is

a square wave with a duty cycle of approximately VOUT/

VIN. To prevent large voltage transients, a low equivalent

35482fa

8

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