Datasheet

0 1.0 2.0 3.0 4.0 5.0

CAP VALUE (% of NOM. 1 uF)

DC BIAS (V)

0402, 6.3V, X5R

0603, 10V, X5R

100

LP3991

SNVS296H –DECEMBER 2006–REVISED MAY 2013

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Figure 17. Effect of DC bias on Capacitance Value.

As an example Figure 17 shows a typical graph showing a comparison of capacitor case sizes in a Capacitance

vs. DC Bias plot. As shown in the graph, as a result of the DC Bias condition, the capacitance value may drop

below the minimum capacitance value given in the recommended capacitor table. Note that the graph shows the

capacitance out of spec for the 0402 case size capacitor at higher bias voltages. It is therefore recommended

that the capacitor manufacturers' specifications for the nominal value capacitor are consulted for all conditions as

some capacitor sizes (e.g. 0402) may not be suitable in the actual application. Ceramic capacitors have the

lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a typical 4.7µF

ceramic capacitor is in the range of 20mΩ to 40mΩ, which easily meets the ESR requirement for stability for the

LP3991. The temperature performance of ceramic capacitors varies by type. Capacitor type X7R is specified with

a tolerance of ±15% over the temperature range -55°C to +125°C. The X5R has a similar tolerance over the

reduced temperature range of -55°C to +85°C. Some large value ceramic capacitors (4.7µF) are manufactured

with Z5U or Y5V temperature characteristics, which can result in the capacitance dropping by more than 50% as

the temperature varies from 25°C to 85°C. Therefore X7R or X5R types are recommended in applications where

the temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1µF to 4.7µF range. Another

important consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This

means that while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it

would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the

same ESR value. It should also be noted that the ESR of a typical tantalum will increase about 2:1 as the

temperature goes from 25°C down to -40°C, so some guard band must be allowed.

ENABLE CONTROL

The LP3991 features an active high Enable pin, V

, which turns the device on when pulled high. When not

enabled the regulator output is off and the device typically consumes 2nA.

If the application does not require the Enable switching feature, the V

pin should be tied to V

to keep the

regulator output permanently on.

To ensure proper operation, the signal source used to drive the V

input must be able to swing above and

below the specified turn-on/off voltage thresholds listed in the Electrical Characteristics section under V

and V

Product Folder Links: LP3991