LT3032IDE datasheet(14/20 Pages) LINER

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LT3032

3032f

APPLICATIONS INFORMATION

at 25°C during normal operation where the BYPN pin

operates at approximately –60mV. DC leakages on the

order of 1μA into or out of these pins can throw off the

internal reference by 20% or more.

Output Capacitance and Transient Response

The LT3032 requires output capacitors for stability. It

is designed to be stable with most low ESR capacitors

(typically ceramic, tantalum or low ESR electrolytic). A

minimum output capacitor of 2.2μF with an ESR of 3Ω

or less is recommended to prevent oscillations on each

output. The LT3032 is a micropower device and output

transient response is a function of output capacitance.

Larger values of output capacitance decrease peak

deviations and provide improved transient response for

larger load current changes. Additional capacitors, used to

decouple individual components powered by the LT3032,

increase the effective output capacitor value. When using

bypass capacitors (for low noise operation), larger values

of output capacitors are needed. For 100pF of bypass

capacitance, 3.3μF of output capacitance is recommended.

With a 330pF bypass capacitor or larger, a 4.7μF output

capacitor is recommended. The shaded region of Figure 2

deﬁnes the range over which the LT3032 is stable. The

minimum ESR needed is deﬁned by the amount of bypass

capacitance used, while the maximum ESR is 3Ω. These

requirements are applicable to both the positive and

negative linear regulator.

Give extra consideration 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 dielectrics used

are speciﬁed with EIA temperature characteristic codes of

Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are

good for providing high capacitances in a small package,

but they tend to have strong voltage and temperature

coefﬁcients as shown in Figures 3 and 4. When used with

a 5V regulator, a 16V 10μF Y5V capacitor 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 temperature,

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 a given 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. In a

ceramic capacitor, the stress can be induced by vibrations

in the system or thermal transients. Tapping on the

ceramic bypass capacitor with a pencil generated the noise

shown in Figure 5. Similar vibration induced behavior can

masquerade as increased output voltage noise.

OUTPUT CAPACITANCE (μF)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

310

1762 F02

6 78 9

STABLE REGION

CBYP = 330pF

CBYP ≥ 3300pF

CBYP = 100pF

CBYP = 0

Figure 2. Stability

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