LP3971 datasheet(42/52 Pages) TI1

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ISAT > IOUTMAX + IRIPPLE

where IRIPPLE =

§ V

IN - VOUT

§ V

OUT

VIN

* L

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

100%

80%

60%

40%

20%

0402, 6.3V, X5R

0603, 10V, X5M

LP3971

SNVS432V – JANUARY 2006 – REVISED MAY 2013

www.ti.com

in the capacitance value falling below the minimum value given in the recommended capacitor specifications

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.

Figure 23. Graph Showing a Typical Variation in Capacitance vs. DC Bias

The ceramic capacitor’s capacitance can vary with temperature. The capacitor type X7R, which operates over a

temperature range of

−55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R

has a similar tolerance over a reduced temperature range of

−55°C to +85°C. Many large value ceramic

capacitors, larger than 1 µF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance

can drop by more than 50% as the temperature varies from 25°C to 85°C. Therefore X7R is recommended over

Z5U and Y5V in applications where the ambient 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 0.47 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. 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.

BUCK CONSIDERATIONS

Inductor Selection

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor

current ripple is small enough to achieve the desired output voltage ripple. Different saturation current rating

specs are followed by different manufacturers so attention must be given to details. Saturation current ratings are

typically specified at 25°C so ratings at max ambient temperature of application should be requested from

manufacturer.

There are two methods to choose the inductor saturation current rating.

Method 1

The saturation current is greater than the sum of the maximum load current and the worst case average to peak

inductor current. This can be written as

(2)

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