LT3029IDE datasheet(14/20 Pages) LINER

14 / 20 page

LT3029

3029f

APPLICATIONS INFORMATION

2μF for the applied DC bias voltage and over the operat-

ing temperature range. 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.

Exercise care even 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 (voltage coefﬁcient)

with X5R and X7R capacitors is better than with 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 case size increases.

Linear Technology recommends verifying expected versus

actual capacitance values at operating voltage in situ for

an application.

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,

similar to the way a piezoelectric accelerometer or micro-

phone works. For a ceramic capacitor, the stress can be

induced by vibrations in the system or thermal transients.

The resulting voltages produced can cause appreciable

amounts of noise, especially when a ceramic capacitor is

used for noise bypassing. A ceramic capacitor produced

Figure 4’s trace in response to light tapping from a pencil.

Similar vibration induced behavior can masquerade as

increased output voltage noise.

Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor

VOUT

500μV/DIV

3029 F04

100ms/DIV

COUT = 10μF

CBYP = 0.01μF

ILOAD = 500mA

LT3029IDE Datasheet(PDF) 14 Page - Linear Technology