LT1964

9

1964f

Bypass Capacitance and Low Noise Performance

The LT1964 may be used with the addition of a bypass

voltage noise. A good quality low leakage capacitor is

recommended. This capacitor will bypass the reference of

the LT1964, providing a low frequency noise pole. The

noise pole provided by this bypass capacitor will lower the

output voltage noise to as low as 30

addition of a 0.01

µF bypass capacitor. Using a bypass

capacitor has the added benefit of improving transient

response. With no bypass capacitor and a 10

µF output

capacitor, a –10mA to –200mA load step will settle to

within 1% of its final value in less than 100

µs. With the

addition of a 0.01

µF bypass capacitor, the output will stay

within 1% for the same –10mA to –200mA load step (see

LT1964-5 Transient Response in the Typical Characteris-

tics section). However, regulator start-up time is inversely

proportional to the size of the bypass capacitor.

Higher values of output voltage noise may be measured

if care is not exercised with regard to circuit layout

and testing. Crosstalk from nearby traces can induce

unwanted noise onto the output of the LT1964-X.

APPLICATIO S I FOR ATIO

1964 F01

GND

ADJ

IN

OUT

LT1964

+

R1

R2

OUTPUT RANGE = –1.22V TO –20V

R2

R1

Output Capacitance and Transient Response

The LT1964 is designed to be stable with a wide range of

output capacitors. The ESR of the output capacitor affects

stability, most notably with small capacitors. A minimum

output capacitor of 1

µF with an ESR of 3Ω or less is

recommended to prevent oscillations. The LT1964 is a

micropower device and output transient response will be

a function of output capacitance. Larger values of output

capacitance decrease the peak deviations and provide

improved transient response for larger load current

changes. Bypass capacitors, used to decouple individual

components powered by the LT1964, will increase the

effective output capacitor value.

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common di-

electrics used are Z5U, Y5V, X5R, and X7R. The Z5U and

Y5V dielectrics are good for providing high capacitances

in a small package, but exhibit strong voltage and tem-

perature coefficients as shown in Figures 2 and 3. When

used with a –5V regulator, a 10

µF Y5V capacitor can

exhibit an effective value as low as 1

µF to 2µF 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 temperature, while the X5R is less expen-

sive and is available in higher values.

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 1. Adjustable Operation