LT1767 查看數據表（PDF） - Linear Technology

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LT1767

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

Continuous Mode

IOUT(MAX) =

IP

−

(

VOUT ) (

2 (L)

VIN

(f)

− VOUT

( VIN )

)

Discontinuous operation occurs when

IOUT(DIS)

=

(VOUT )

2(L)(f)

For VIN = 8V, VOUT = 5V and L = 3.3µH,

( ) ( ) IOUT(MAX) =1.5− 2

(5)(8 −5)

3.3 • 10− 6 1.25 • 106

(8)

=1.5 − 0.23 =1.27 A

Note that the worst case (minimum output current avail-

able) condition is at the maximum input voltage. For the

same circuit at 15V, maximum output current would be

only 1.1A.

When choosing an inductor, consider maximum load cur-

rent, core and copper losses, allowable component height,

output voltage ripple, EMI, fault current in the inductor,

saturation, and of course, cost. The following procedure

is suggested as a way of handling these somewhat com-

plicated and conflicting requirements.

1. Choose a value in microhenries from the graphs of

maximum load current. Choosing a small inductor

with lighter loads may result in discontinuous mode

of operation, but the LT1767 is designed to work well

in either mode.

Assume that the average inductor current is equal to

load current and decide whether or not the inductor

must withstand continuous fault conditions. If maxi-

mum load current is 0.5A, for instance, a 0.5A inductor

may not survive a continuous 2A overload condition.

Also, the instantaneous application of input or release

from shutdown, at high input voltages, may cause

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 cur-

rent 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.

IPEAK

=

IOUT

+

( VOUT VIN− VOUT

2 (L) ( f) (VIN )

)

VIN = Maximum input voltage

f = Switching frequency, 1.25MHz

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

ometry 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.

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 applications

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 developments

in low profile, surface mounting, etc.

Table 1

PART NUMBER

Coiltronics

TP1-2R2

TP2-2R2

TP3-4R7

TP4- 100

Murata

LQH1C1R0M04

LQH3C1R0M24

LQH3C2R2M24

LQH4C1R5M04

Sumida

CD73- 100

CDRH4D18-2R2

CDRH5D18-6R2

CDRH5D28-100

VALUE (uH)

2.2

2.2

4.7

10

1.0

1.0

2.2

1.5

10

2.2

6.2

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)

0.188

0.111

0.181

0.146

0.28

0.06

0.1

0.09

0.080

0.058

0.071

0.048

HEIGHT (mm)

1.8

2.2

2.2

3.0

1.8

2.0

2.0

2.6

3.5

1.8

1.8

2.8

10

For more information www.linear.com/LT1767

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