5

LT1617/LT1617-1

Choosing an Inductor

Several recommended inductors that work well with the

LT1617 and LT1617-1 are listed in Table 1, although there

are many other manufacturers and devices that can be

used. Consult each manufacturer for more detailed infor-

mation and for their entire selection of related parts. Many

different sizes and shapes are available. Use the equations

and recommendations in the next few sections to find the

correct inductance value for your design.

Table 1. Recommended Inductors

PART

VALUE (

MAX DCR (

VENDOR

LQH3C4R7

4.7

0.26

Murata

LQH3C100

10

0.30

(714) 852-2001

LQH3C220

22

0.92

www.murata.com

CD43-4R7

4.7

0.11

Sumida

CD43-100

10

0.18

(847) 956-0666

CDRH4D18-4R7

4.7

0.16

www.sumida.com

CDRH4D18-100

10

0.20

DO1608-472

4.7

0.09

Coilcraft

DO1608-103

10

0.16

(847) 639-6400

D01608-223

22

www.coilcraft.com

Inductor Selection—Inverting Regulator

The formula below calculates the appropriate inductor

value to be used for an inverting regulator using the

LT1617 or LT1617-1 (or at least provides a good starting

point). This value provides a good tradeoff in inductor size

and system performance. Pick a standard inductor close

to this value (both inductors should be the same value). A

larger value can be used to slightly increase the available

output current, but limit it to around twice the value

calculated below, as too large of an inductance will in-

crease the output voltage ripple without providing much

additional output current. A smaller value can be used

(especially for systems with output voltages greater than

12V) to give a smaller physical size. Inductance can be

calculated as:

L

VV

I

t

OUT

D

LIM

OFF

=

+

















2

APPLICATIO S I FOR ATIO

For higher output voltages, the formula above will give

large inductance values. For a 2V to 20V converter (typical

LCD bias application), a 47

µH inductor is called for with

the above equation, but a 10

µH or 22µH inductor could be

used without excessive reduction in maximum output

current.

Inductor Selection—Inverting Charge Pump Regulator

For the inverting regulator, the voltage seen by the internal

power switch is equal to the sum of the absolute value of

the input and output voltages, so that generating high

output voltages from a high input voltage source will often

exceed the 36V maximum switch rating. For instance, a

12V to – 30V converter using the inverting topology would

generate 42V on the SW pin, exceeding its maximum

rating. For such a system, an inverting charge pump is the

best topology.

The formula below calculates the approximate inductor

value to be used for an inverting charge pump regulator

using the LT1617. As for the boost inductor selection, a

larger or smaller value can be used. For designs with

L

VV

V

I

t

OUT

IN MIN

D

LIM

OFF

=

−+

()

Current Limit Overshoot

For the constant off-time control scheme of the LT1617,

the power switch is turned off only after the 350mA (or

100mA) current limit is reached. There is a 100ns delay

between the time when the current limit is reached and

when the switch actually turns off. During this delay, the

inductor current exceeds the current limit by a small

amount. The peak inductor current can be calculated by:

II

VV

L

ns

PEAK

LIM

IN MAX

SAT

=+

−













()

100

current overshoot will be most evident for systems with

high input voltages and for systems where smaller induc-