LT7101 datasheet(20/38 Pages) AD

20 / 38 page

LT7101

Rev. 0

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APPLICATIONS INFORMATION

Note that in applications with VOUT > 6V, additional con-

straints on the inductance value may also apply. See

Operating at VOUT > 6V section for more information.

A trade-off between component size, efficiency and oper-

ating frequency can be seen from this equation. Accepting

larger values of ΔIL allows the use of lower value induc-

tors, but results in greater core loss in the inductor, greater

ESR loss in the output capacitor, and larger output ripple.

Generally, highest efficiency operation is obtained at low

operating frequency with small ripple current.

A reasonable starting point for setting the ripple current

is approximately 0.35AP-P. Note that the largest ripple

current occurs at the highest VIN. To guarantee the ripple

current does not exceed a specified maximum, the induc-

tance should be chosen according to:

L =

VOUT

f • ΔIL(MAX)

⎛

⎝⎜

⎞

⎠⎟

1–

VOUT

VIN(MAX)

⎛

⎝⎜

⎞

⎠⎟

The LT7101 contains a fast, average current limit loop

that limits the DC output current to a value determined by

the voltage on the ICTRL pin. (See Average Output Current

Limit and Monitor section for details.) However, some

applications may experience inductor current transients

that are limited by the peak current limit comparator,

which tracks nominally 0.53A above the average current

limit set point. To avoid saturation, choose an inductor

with a saturation current ISAT such that:

ISAT >

VICTRL – 0.4

0.77

+ 0.68A

This enables the use of an inductor with a current rating

that fits the needs of a given application. If the average

output current limit is set to the default value of 1.11A,

then an inductor with ISAT > 1.9A is required. However, if

the average current limit is set to 0.6A (VICTRL = 0.89V),

then an inductor with ISAT > 1.4A may be used. Note that

if there is a varying voltage on the ICTRL pin, always use

the highest value present on ICTRL when calculating the

required inductor saturation current. See Average Output

Current Limit and Monitor section for details on setting

the average current limit.

If fixed VOUT operation is selected using the VPRG1 and

VPRG2 pins, the RIND pin can be left floating, but only if

the inductance value is chosen according to Table 3. Since

the RIND pin resistor indicates the inductance value being

used, the LT7101 will automatically assume an inductance

value as shown in Table 3 when this pin is left floating.

These inductance values will provide an inductor ripple

current that is approximately 30% to 40% of the full load

current. If the nominal value of the inductance used differs

by more than 10% from the values specified in Table 3,

a resistor must be placed on the RIND pin to indicate this

value.

Table 3. Required Inductor Values with RIND Pin Floating

FIXED VOUT

REQUIRED INDUCTANCE VALUE (RIND = FLOAT)

f = 300kHz

f = 1MHz

f = ADJ

1.2V

10μH

3.3µH

L = 3.1/f

1.8V

15µH

4.7µH

L = 4.6/f

2.5V

22µH

6.8µH

L = 6.7/f

3.3V

33µH

10µH

L = 9.9/f

3.6V

33µH

10µH

L = 9.9/f

47µH

15µH

L = 14.6/f

12V

100µH

33µH

L = 31.5/f

15V

100µH

33µH

L = 31.5/f

Inductor Core Selection

Once the value for L is known, the type of inductor must be

selected. Actual core loss is independent of core size for a

fixed inductor value but is very dependent on the induc-

tance selected. As the inductance increases, core loss

decreases. Unfortunately, increased inductance requires

more turns of wire leading to increased copper loss.

Ferrite designs exhibit very low core loss and are pre-

ferred at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core materials saturate hard, meaning the induc-

tance collapses abruptly when the peak design current is

exceeded. This collapse will result in an abrupt increase

in inductor ripple current, so it is important to ensure the

core will not saturate.

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