11

LT1963 Series

1963fa

Figure 4. Ceramic Capacitor Temperature Characteristics

APPLICATIO S I FOR ATIO

TEMPERATURE (

°C)

–50

40

20

0

–20

–40

–60

–80

–100

25

75

1963 F04

–25

0

50

100

125

Y5V

X5R

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF

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.

Overload Recovery

Like many IC power regulators, the LT1963-X has safe

operating area protection. The safe area protection de-

creases the current limit as input-to-output voltage in-

creases and keeps the power transistor inside a safe

operating region for all values of input-to-output voltage.

The protection is designed to provide some output current

at all values of input-to-output voltage up to the device

breakdown.

When power is first turned on, as the input voltage rises,

the output follows the input, allowing the regulator to start

up into very heavy loads. During the 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 voltage to recover. Other regulators, such as the

LT1085, also exhibit this phenomenon, so it is not unique

to the LT1963-X.

The problem occurs with a heavy output load when the

input voltage is high and the output voltage is low. Com-

mon situations are immediately after the removal of a

short-circuit or when the shutdown pin is pulled high after

the input voltage has already been turned on. The load line

for such a load may intersect the output current curve at

two points. If this happens, there are two stable output

operating points for the regulator. With this double inter-

section, the input power supply may need to be cycled

down to zero and brought up again to make the output

recover.

Output Voltage Noise

The LT1963 regulators have been designed to provide low

output voltage noise over the 10Hz to 100kHz bandwidth

while operating at full load. Output voltage noise is typi-

cally 40nV/

√Hz over this frequency bandwidth for the

LT1963 (adjustable version). For higher output voltages

(generated by using a resistor divider), the output voltage

noise will be gained up accordingly. This results in RMS

noise over the 10Hz to 100kHz bandwidth of 14

the LT1963 increasing to 38

Higher values of output voltage noise may be measured

when care is not exercised with regards to circuit layout

and testing. Crosstalk from nearby traces can induce

unwanted noise onto the output of the LT1963-X. Power

supply ripple rejection must also be considered; the LT1963

regulators do not have unlimited power supply rejection

and will pass a small portion of the input noise through to

the output.

Figure 3. Ceramic Capacitor DC Bias Characteristics

DC BIAS VOLTAGE (V)

1963 F03

20

0

–20

–40

–60

–80

–100

0

4

8

10

26

12

14

X5R

Y5V

16

BOTH CAPACITORS ARE 16V,

1210 CASE SIZE, 10

µF