LT1961IMS8E-PBF データシートの表示（PDF） - Linear Technology

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LT1961IMS8E-PBF

1.5A, 1.25MHz Step-Up Switching Regulator

Linear Technology 

LT1961

APPLICATIONS INFORMATION

INDUCTOR CHOICE AND MAXIMUM OUTPUT

CURRENT

When choosing an inductor, there are 2 conditions that

limit the minimum inductance; required output current,

and avoidance of subharmonic oscillation. The maximum

output current for the LT1961 in a standard boost con-

verter configuration with an infinitely large inductor is:

IOUT(MAX)

=

1.5A

VIN • η

VOUT

Where η = converter efficiency (typically 0.87 at high

current).

As the value of inductance is reduced, ripple current

increases and IOUT(MAX) is reduced. The minimum induc-

tance for a required output current is given by:

LMIN

=

VIN(VOUT – VIN)

⎛

2VOUT (f)⎝⎜1.5

–

(VOUT )(IOUT )⎞

VIN • η ⎠⎟

The second condition, avoidance of subharmonic oscilla-

tion, must be met if the operating duty cycle is greater than

50%. The slope compensation circuit within the LT1961

prevents subharmonic oscillation for inductor ripple cur-

rents of up to 0.7AP-P, defining the minimum inductor

value to be:

LMIN

=

VIN(VOUT – VIN)

0.7VOUT (f)

These conditions define the absolute minimum induc-

tance. However, it is generally recommended that to

prevent excessive output noise, and difficulty in obtaining

stability, the ripple current is no more than 40% of the

average inductor current. Since inductor ripple is:

IP−P

RIPPLE

=

VIN(VOUT – VIN)

VOUT (L)(f)

The recommended minimum inductance is:

8

LMIN

=

(VIN)2 (VOUT – VIN)

0.4(VOUT )2 (IOUT )(f)

The inductor value may need further adjustment for other

factors such as output voltage ripple and filtering require-

ments. Remember also, inductance can drop significantly

with DC current and manufacturing tolerance.

The inductor must have a rating greater than its peak

operating current to prevent saturation resulting in effi-

ciency loss. Peak inductor current is given by:

ILPEAK

=

(VOUT )(IOUT

VIN • η

)

+

VIN(VOUT − VIN)

2VOUT (L)(f)

Also, consideration should be given to the DC resistance

of the inductor. Inductor resistance contributes directly to

the efficiency losses in the overall converter.

Suitable inductors are available from Coilcraft, Coiltronics,

Dale, Sumida, Toko, Murata, Panasonic and other manu-

factures.

Table 2

PART NUMBER VALUE (uH) ISAT(DC) (Amps) DCR (Ω) HEIGHT (mm)

Coiltronics

TP1-2R2

2.2

1.3

0.188

1.8

TP2-2R2

2.2

1.5

0.111

2.2

TP3-4R7

4.7

1.5

0.181

2.2

TP4- 100

10

1.5

0.146

3.0

Murata

LQH1C1R0M04

1.0

0.51

0.28

1.8

LQH3C1R0M24

1.0

1.0

0.06

2.0

LQH3C2R2M24

2.2

0.79

0.1

2.0

LQH4C1R5M04

1.5

1

0.09

2.6

Sumida

CD73- 100

10

1.44

0.080

3.5

CDRH4D18-2R2 2.2

1.32

0.058

1.8

CDRH5D18-6R2 6.2

1.4

0.071

1.8

CDRH5D28-100

10

1.3

0.048

2.8

Coilcraft

1008PS-272M

2.7

1.3

0.14

2.7

LPO1704-222M

2.2

1.6

0.12

1.0

LPO1704-332M

3.3

1.3

0.16

1.0

1961fa

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