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LT1945

Dual Micropower DC/DC Converter with Positive and Negative Outputs

Linear Technology 

LT1945

APPLICATIONS INFORMATION

Inductor Selection—Inverting Regulator

The formula below calculates the appropriate inductor value

to be used for an inverting regulator using the LT1945 (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 increase 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 = 2⎝⎜

VOUT + VD

ILIM

⎞

⎠⎟

tOFF

where VD = 0.4V (Schottky diode voltage), ILIM = 350mA

and tOFF = 400ns.

For higher output voltages, the formula above will give

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

LCD bias application), a 47μH inductor is called for with the

above equation, but a 10μH 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 ap-

plication, 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

varying VIN such as battery powered applications, use the

minimum VIN value in the equation below.

L=

VOUT

−

VIN(MIN)

ILIM

+

VD

tOFF

6

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:

IPEAK

=

ILIM

+

⎛

⎝⎜

VIN(MAX)

L

−

VSAT

⎞

⎠⎟

100ns

Where VSAT = 0.25V (switch saturation voltage). The 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 results will be obtained when IPEAK

is kept below 700mA 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 capacitors

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 4.7μF input capacitor is

sufficient for most applications. Table 2 shows a list of

several capacitor manufacturers. Consult the manufacturers

for more detailed information and for their entire selection

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