6

LT1945

1945f

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 this application, 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 LT1945. 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 LT1945,

the power switch is turned off only after the 350mA 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 regulators with

high input voltages and smaller inductor values. This

overshoot can be beneficial as it helps increase the amount

of available output current for smaller inductor values.

This will be the peak current seen by the inductor (and the

diode) during normal operation. For designs using small

inductance values (especially at input voltages greater

than 5V), the current limit overshoot can be quite high.

Although it is internally current limited to 350mA, the

power switch of the LT1945 can handle larger currents

without problem, but the overall efficiency will suffer. Best

for the LT1945.

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors should

be used at the output to minimize the output ripple voltage.

X5R or X7R multilayer ceramic capacitors are the best

choice, as they have a very low ESR and are available in

very small packages. Y5V ceramics are not recommended.

Their small size makes them a good companion to the

LT1945’s MS10 package. Solid tantalum capacitors (like

the AVX TPS, Sprague 593D families) or OS-CON capaci-

tors can be used, but they will occupy more board area

than a ceramic and will have a higher ESR. Always use a

capacitor with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT1945. A 4.7

µF input capacitor is suffi-

cient for most applications. Table 2 shows a list of several

capacitor manufacturers. Consult the manufacturers for

more detailed information and for their entire selection of

related parts.

Table 2. Recommended Capacitors

CAPACITOR TYPE

VENDOR

Ceramic

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

Ceramic

AVX

(803) 448-9411

www.avxcorp.com

Ceramic

Murata

(714) 852-2001

www.murata.com