LT1767EMS8 Ver la hoja de datos (PDF) - Linear Technology

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componentes Descripción

Fabricante

LT1767EMS8

Monolithic 1.5A, 1.25MHz Step-Down Switching Regulators

Linear Technology 

LT1767/LT1767-1.8/

LT1767-2.5/LT1767-3.3/LT1767-5

APPLICATIONS INFORMATION

saturation of the inductor. In these applications, the

soft-start circuit shown in Figure 10 should be used.

2. Calculate peak inductor current at full load current to

ensure that the inductor will not saturate. Peak current

can be significantly higher than output current,

especially with smaller inductors and lighter loads, so

don’t omit this step. Powdered iron cores are forgiving

because they saturate softly, whereas ferrite cores

saturate abruptly. Other core materials fall somewhere

in between.

( ) VOUT VIN − VOUT

( )( )( ) IPEAK = IOUT +

2 L f VIN

VIN = Maximum input voltage

f = Switching frequency, 1.25MHz

3. Decide if the design can tolerate an “open” core geom-

etry like a rod or barrel, which have high magnetic field

radiation, or whether it needs a closed core like a toroid

to prevent EMI problems. This is a tough decision

because the rods or barrels are temptingly cheap and

small and there are no helpful guidelines to calculate

when the magnetic field radiation will be a problem.

Table 3

PART NUMBER VALUE (uH)

Coiltronics

TP1-2R2

2.2

TP2-2R2

2.2

TP3-4R7

4.7

TP4- 100

10

Murata

LQH1C1R0M04

1.0

LQH3C1R0M24

1.0

LQH3C2R2M24

2.2

LQH4C1R5M04

1.5

Sumida

CD73- 100

10

CDRH4D18-2R2 2.2

CDRH5D18-6R2 6.2

CDRH5D28-100

10

ISAT(Amps)

1.3

1.5

1.5

1.5

0.51

1.0

0.79

1.0

1.44

1.32

1.4

1.3

DCR (Ω) HEIGHT (mm)

0.188

1.8

0.111

2.2

0.181

2.2

0.146

3.0

0.28

1.8

0.06

2.0

0.1

2.0

0.09

2.6

0.080

3.5

0.058

1.8

0.071

1.8

0.048

2.8

4. After making an initial choice, consider the secondary

things like output voltage ripple, second sourcing, etc.

Use the experts in the Linear Technology’s applica-

tions department if you feel uncertain about the final

choice. They have experience with a wide range of

inductor types and can tell you about the latest devel-

opments in low profile, surface mounting, etc.

CATCH DIODE

The suggested catch diode (D1) is a UPS120 Schottky, or

its Motorola equivalent, MBRM120LTI/MBRM130LTI. It

is rated at 2A average forward current and 20V/30V

reverse voltage. Typical forward voltage is 0.5V at 1A. The

diode conducts current only during switch off time. Peak

reverse voltage is equal to regulator input voltage. Average

forward current in normal operation can be calculated

from:

( ) IOUT VIN − VOUT

ID(AVG) =

VIN

BOOST␣ PIN

For most applications, the boost components are a 0.1µF

capacitor and a CMDSH-3 diode. The anode is typically

connected to the regulated output voltage to generate a

voltage approximately VOUT above VIN to drive the output

stage. The output driver requires at least 2.7V of head-

room throughout the on period to keep the switch fully

saturated. However, the output stage discharges the boost

capacitor during the on time. If the output voltage is less

than 3.3V, it is recommended that an alternate boost

supply is used. The boost diode can be connected to the

input, although, care must be taken to prevent the 2x VIN

boost voltage from exceeding the BOOST pin absolute

maximum rating. The additional voltage across the switch

driver also increases power loss, reducing efficiency. If

available, an independent supply can be used with a local

bypass capacitor.

A 0.1µF boost capacitor is recommended for most appli-

cations. Almost any type of film or ceramic capacitor is

sn1767 1767fas

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