LT3483IDC データシートの表示（PDF） - Linear Technology

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LT3483IDC

Inverting Micropower DC/DC Converters with Schottky

Linear Technology 

LT3483/LT3483A

APPLICATIONS INFORMATION

value recommended, as too large of an inductance will

increase the output voltage ripple without providing much

additional output current.

Table 1. Recommended Inductors

PART

MAX

L IDC DCR HEIGHT

(µH) (mA) (Ω) (mm) MANUFACTURER

LQH2MCN4R7M02L 4.7 300 0.84 0.95 Murata

LQH2MCN6R8M02L 6.8 255 1.0

www.murata.com

LQH2MCN100M02L 10 225 1.2

SDQ12

Coupled

Inductor

4.7 1.45 0.40 1.2 Cooper Electronics

10 980 0.72

Tech

15 780 1.15

www.cooperet.com

LPD3015

Coupled

Inductor

4.7 860 0.52 1.4 Coilcraft

10 580 1.0

www.coilcraft.com

Capacitor Selection

The small size and low ESR of ceramic capacitors make

them ideal for LT3483/LT3483A applications. Use of X5R

and X7R types is recommended because they retain their

capacitance over wider voltage and temperature ranges

than other dielectric types. Always verify the proper volt-

age rating. Table 2 shows a list of several ceramic capaci-

tor manufacturers. Consult the manufacturers for more

detailed information on their entire selection of ceramic

capacitors.

A 4.7µF ceramic bypass capacitor on the VIN pin is rec-

ommended where the distance to the power supply or

battery could be more than a couple inches. Otherwise,

a 1µF is adequate.

A capacitor in parallel with feedback resistor R1 is recom-

mended to reduce the output voltage ripple. Use a 5pF

capacitor for the inverting charge pump, and a 22pF value

for the inverting flyback or other dual inductor configura-

tions. Output voltage ripple can be reduced to 20mV in

some cases using this capacitor in combination with an

appropriately selected output capacitor.

The output capacitor is selected based on desired output

voltage ripple. For low output voltage ripple in the inverting

flyback configuration, use a 4.7µF to 10µF capacitor. The

inverting charge pump utilizes values ranging from 0.22µF

to 4.7µF. The following formula is useful to estimate the

output capacitor value needed:

COUT

=

L •ISW 2

–VOUT • ∆VOUT

where ISW = 0.25A (LT3483) or ISW = 0.5A (LT3483A) and

∆VOUT = 30mV. The flying capacitor in the inverting charge

pump configuration ranges from 0.1µF to 0.47µF. Multiply

the value predicted by the above equation for COUT by 1/10

to determine the value needed for the flying capacitor.

Table 2. Recommended Ceramic Capacitor Manufacturers

MANUFACTURER

URL

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.tyuden.com

Setting the Output Voltage

The output voltage is programmed using one feedback

resistor according to the following formula:

R1=

–

VOUT

10µA

Inrush Current

When VIN is increased from ground to operating voltage,

an inrush current will flow through the input inductor and

integrated Schottky diode to charge the flying capacitor.

Conditions that increase inrush current include a larger,

more abrupt voltage step at VIN, a larger flying capacitor,

and an inductor with a low saturation current.

While the internal diode is designed to handle such events,

the inrush current should not be allowed to exceed 1.5A.

For circuits that use flying capacitors within the recom-

mended range and have input voltages less than 5V, inrush

current remains low, posing no hazard to the device. In

cases where there are large steps at VIN, inrush current

should be measured to ensure operation within the limits

of the device.

3483fc

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