LT3015 datasheet(13/20 Pages) LINER

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LT3015

3015f

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 microphone works. For

a ceramic capacitor, the stress can be induced by vibra-

tions in the system or thermal transients. The resulting

voltages produced can cause appreciable amounts of

noise. A ceramic capacitor produced the trace in Figure 5

in response to light tapping from a pencil. Similar vibration

induced behavior can masquerade as increased output

voltage noise.

APPLICATIONS INFORMATION

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 tem-

perature, 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 in situ for all applications.

VOUT

1mV/DIV

VOUT = –1.3V

COUT = 10μF

IL = 10μA

3015 F05

1ms/DIV

Figure 5. Noise Resulting from Tapping on a Ceramic Capacitor

DC BIAS VOLTAGE (V)

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

–100

–80

3015 F03

–60

–40

X5R

Y5V

–20

Figure 3. Ceramic Capacitor DC Bias Characteristics

TEMPERATURE (°C)

–50

–100

–80

–25

100

3015 F04

–60

–40

Y5V

–20

125

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

X5R

Figure 4. Ceramic Capacitor Temperature Characteristics

Overload Recovery

Like many IC power regulators, the LT3015 has safe oper-

ating area protection. The safe operating area protection

activates at IN-to-OUT differential voltages greater than

8V. The safe area protection decreases current limit as

the IN-to-OUT differential voltage increases and keeps

the power transistor inside a safe operating region for

all values of forward input-to-output voltage up to the

LT3015’s Absolute Maximum Ratings.

When power is first applied and input voltage rises, the

output follows the input and keeps the IN-to-OUT differential

voltage small, allowing the regulator to supply large output

currents and start-up into high current loads. With a high

input voltage, a problem can occur wherein removal of

an output short does not allow the output voltage to fully

recover. Other LTC negative linear regulators such as the

LT1175 and LT1964 also exhibit this phenomenon, so it

is not unique to the LT3015.

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