LP3906 datasheet(38/47 Pages) TI

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(Vin ± Vout) x Vout

L x f x Ioutmax x Vin

where

r =

Irms = Ioutmax

VOUT

VIN

1 -

VOUT

VIN

0402, 6.3V, X5R

0603, 10V, X5R

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

20%

40%

60%

80%

100%

LP3906

SNVS456M – AUGUST 2006 – REVISED MAY 2013

www.ti.com

Figure 39. Graph Showing a Typical Variation in Capacitance vs. DC Bias

As shown in the graph, increasing the DC Bias condition can result in the capacitance value that falls 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.

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.

Input Capacitor Selection for SW1 and SW2

A ceramic input capacitor of 10 µF, 6.3V is sufficient for the magnetic dc/dc converters. Place the input capacitor

as close as possible to the input of the device. A large value may be used for improved input voltage filtering.

The recommended capacitor types are X7R or X5R. Y5V type capacitors should not be used. DC bias

characteristics of ceramic capacitors must be considered when selecting case sizes like 0805 and 0603. The

input filter capacitor supplies current to the PFET switch of the dc/dc converter in the first half of each cycle and

reduces voltage ripple imposed on the input power source. A ceramic capacitor’s low ESR (Equivalent Series

Resistance) provides the best noise filtering of the input voltage spikes due to fast current transients. A capacitor

with sufficient ripple current rating should be selected. The Input current ripple can be calculated as:

(7)

The worse case is when VIN = 2VOUT

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