LT3060 datasheet(13/20 Pages) LINER

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LT3060

3060f

voltages set for greater than 24V, use a minimum output

capacitor of 4.7μF. The LT3060 is a micropower device

and output load transient response is a function of output

capacitance. Larger values of output capacitance decrease

the peak deviations and provide improved transient

responseforlargerloadcurrentchanges.Bypasscapacitors,

used to decouple individual components powered by the

LT3060, increase the effective output capacitor value. For

applications with large load current transients, a low ESR

ceramic capacitor in parallel with a bulk tantalum capacitor

often provides an optimally damped response.

Give extra consideration to the use of ceramic capacitors.

Manufacturers make ceramic capacitors with a variety of

dielectrics, each with different behavior across tempera-

ture and applied voltage. The most common dielectrics

are speciﬁed with EIA temperature characteristic codes

of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics

provide high C-V products in a small package at low cost,

but exhibit strong voltage and temperature coefﬁcients, as

shown in Figures 4 and 5. When used with a 5V regulator,

a 16V 10μF Y5V 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 yield much more stable characteristics and are

more suitable for use as the output capacitor.

The X7R type works over a wider temperature range and

has better temperature stability, 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 maximum 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 signiﬁcant enough to drop capacitor values below

appropriate levels. Capacitor DC bias characteristics tend

to improve as component case size increases, but expected

capacitance at operating voltage should be veriﬁed.

Voltageandtemperaturecoefﬁcientsarenottheonlysources

of problems. Some ceramic capacitors have a piezoelectric

response. A piezoelectric device generates voltage across

its terminals due to mechanical stress, similar to the way

a piezoelectric accelerometer or microphone works. For a

ceramic capacitor, the stress is induced by vibrations in

the system or thermal transients. The resulting voltages

produced cause appreciable amounts of noise. A ceramic

capacitor produced the trace in Figure 6 in response to light

tapping from a pencil. Similar vibration induced behavior

can masquerade as increased output voltage noise.

Figure 4. Ceramic Capacitor DC Bias Characteristics

APPLICATIONS INFORMATION

DC BIAS VOLTAGE (V)

3060 F04

–20

–40

–60

–80

–100

X5R

Y5V

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

TEMPERATURE (°C)

–50

–20

–40

–60

–80

–100

3060 F05

–25

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

Figure 5. Ceramic Capacitor Temperature Characteristics

4ms/DIV

3060 F06

VOUT

500μV/DIV

VOUT = 0.6V

COUT = 10μF

CREF/BYP = 10nF

ILOAD = 100mA

Figure 6. Noise Resulting from Tapping on a Ceramic Capacitor

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