LT1965

9

1965f

APPLICATIONS INFORMATION

capacitor values below appropriate levels. Capacitor DC

bias characteristics tend to improve as component case

size increases, but expected capacitance at operating

voltages 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,

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. A ceramic capacitor produced the trace

in Figure 4 in response to light tapping from a pencil.

Similar vibration induced behavior can masquerade as

increased output voltage noise.

Overload Recovery

Like many IC power regulators, the LT1965 has safe oper-

ating area protection. The safe area protection decreases

current limit as input-to-output voltage increases and keeps

the power transistor inside a safe operating region for all

values of input-to-output voltage. The protective design

provides some output current at all values of input-to-

output voltage up to the device breakdown.

When power is first applied, as input voltage rises, the

output follows the input, allowing the regulator to start up

into very heavy loads. During start-up, as the input voltage

is rising, the input-to-output voltage differential is small,

allowing the regulator to supply large output currents. With

a high input voltage, a problem can occur wherein removal

of an output short will not allow the output to recover.

Figure 2. Ceramic Capacitor DC Bias Characteristics

Figure 3. Ceramic Capacitor Temperature Characteristics

Figure 4. Noise Resulting from Tapping on a Ceramic Capacitor

DC BIAS VOLTAGE (V)

1965 F02

20

0

–20

–40

–60

–80

–100

0

4

8

10

26

12

14

X5R

Y5V

16

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

TEMPERATURE (°C)

–50

40

20

0

–20

–40

–60

–80

–100

25

75

1965 F03

–25

0

50

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10μF

1ms/DIV

1mV/DIV

1965 F04