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

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LTC3548EDD-2-TRPBF

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

Linear Technology 

LTC3548-2

APPLICATIONS INFORMATION

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.

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

COUT1

35482 F02

*MODE/SYNC = 0V: PULSE SKIPPING

MODE/SYNC = VIN: Burst Mode OPERATION

Figure 2. LTC3548-2 General Schematic

Inductor Selection

Although the inductor does not influence the operating fre-

quency, the inductor value has a direct effect on ripple cur-

rent. The inductor ripple current ΔIL decreases with higher

inductance and increases with higher VIN or VOUT:

ΔIL

=

VOUT

fO • L

•

⎛

⎝⎜1–

VOUT

VIN

⎞

⎠⎟

Accepting larger values of ΔIL 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

ΔIL = 0.3 • IOUT(MAX), where IOUT(MAX) is 800mA for channel 1

and 400mA for channel 2. The largest ripple current ΔIL

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 follow-

ing equation:

L

≥

VOUT

fO • ΔIL

⎛

• ⎜1–

⎝

VOUT

VIN(MAX)

⎞

⎟

⎠

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

materials are small and do not radiate much energy,

but generally cost more than powdered iron core induc-

tors with similar electrical characteristics. The choice of

which style inductor to use often depends more on the

price vs size requirements and any radiated field/EMI

requirements 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

0.075

CDRH3D16

3.3

0.110

4.7

0.162

1.20

3.8 × 3.8 × 1.8

1.10

0.90

Sumida

1.5

0.068

CDRH2D11

2.2

0.170

0.900

0.780

3.2 × 3.2 × 1.2

Sumida

CMD4D11

2.2

0.116

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 series

resistance (ESR) input capacitor sized for the maximum

35482fb

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