LT3083MPDF datasheet(13/28 Pages) AD

13 / 28 page

LT3083

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 5. Parallel Devices

capacitor can exhibit an effective value as low as 1μF to

2μF for the DC bias voltage applied and over the operating

temperature range. The X5R and X7R dielectrics result in

more stable characteristics and are more suitable for use

as the output capacitor. The X7R type has better stability

across temperature, while the X5R is less expensive and

is available in higher values. Care still must be exercised

when using X5R and X7R capacitors. The X5R and X7R

codes only specify operating temperature range and maxi-

mum capacitance change over temperature. Capacitance

change due to DC bias with X5R and X7R capacitors is

better than Y5V and Z5U capacitors, but can still be sig-

nificant enough to drop capacitor values below appro-

priate levels. Capacitor DC bias characteristics tend to

improve as component case size increases, but expected

capacitance at operating voltage should be verified.

Voltage and temperature coefficients are not the only

sources of problems. Some ceramic capacitors have a

piezoelectric response. A piezoelectric device generates

voltage across its terminals due to mechanical stress. In a

ceramic capacitor, the stress can be induced by vibrations

in the system or thermal transients.

Paralleling Devices

Higher output current is obtained by paralleling multiple

LT3083s together. Tie the individual SET pins together and

tie the individual IN pins together. Connect the outputs in

common using small pieces of PC trace as ballast resis-

tors to promote equal current sharing. PC trace resistance

in mΩ/inch is shown in Table 2. Ballasting requires only

a tiny area on the PCB.

Table 2. PC Board Trace Resistance

WEIGHT (oz)

10mil WIDTH

20mil WIDTH

54.3

27.1

13.6

Trace resistance is measured in mΩ/in

The worst-case room temperature offset, only ±4mV

(DD-PAK, T Packages) between the SET pin and the OUT

pin, allows the use of very small ballast resistors.

As shown in Figure 5, each LT3083 has a small 10mΩ

ballast resistor, which at full output current gives better

than 80% equalized sharing of the current. The external

Figure 3. Ceramic Capacitor DC Bias Characteristics

Figure 4. Ceramic Capacitor Temperature Characteristics

DC BIAS VOLTAGE (V)

3083 F03

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3083 F04

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10µF

–

LT3083

VIN

VCONTROL

OUT

SET

10m

–

LT3083

VIN

4.8V

TO 20V

VOUT

3.3V

VCONTROL

OUT

10µF

SET

33k

3083 F05

10m

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