0

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

0402, 6.3V, X5R

0603, 10V, X5R

100

80

60

40

20

13

LP3991

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SNVS296J – DECEMBER 2006 – REVISED JUNE 2016

Product Folder Links: LP3991

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Copyright © 2006–2016, Texas Instruments Incorporated

Figure 13. Effect of DC Bias on Capacitance Value

As an example Figure 13 shows a typical graph showing a comparison of capacitor case sizes in a capacitance

vs. DC Bias plot. As shown in Figure 13, 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 Figure 13 shows the

capacitance out of spec for the 0402 case size capacitor at higher bias voltages. TI therefore recommends that

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

some capacitor sizes (for example, 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 4.7-µF

ceramic capacitor is in the range of 20 m

Ω to 40 mΩ, 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 (4.7 µF) 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 changes 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 1-µF to 4.7-µF 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. The ESR of a typical tantalum increases about 2:1 as the temperature goes from 25°C down to

–40°C, so some guard band must be allowed.

8.2.2.6 Power Dissipation

Knowing the device power dissipation and proper sizing of the thermal plane connected to the tab or pad is

critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and

load conditions and can be calculated with Equation 1.

(1)

Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available

voltage drops result in better dynamic (that is, PSRR and transient) performance.

On the LP3991 DSBGA (YZR) package, the primary conduction path for heat is through the four bumps to the

Power dissipation and junction temperature are most often related by the junction-to-ambient thermal resistance

Equation 2 or Equation 3:

(2)

(3)