0

1.0

2.0

3.0

4.0

5.0

DC BIAS (V)

100%

80%

60%

40%

20%

0402, 6.3V, X5R

0603, 10V, X5R

LM3687

www.ti.com

SNVS473A – DECEMBER 2007 – REVISED JULY 2008

In particular, the output capacitor selection should take account of all the capacitor parameters, to ensure that the

specification is met within the application. The capacitance can vary with DC bias conditions as well as

temperature and frequency of operation. Capacitor values will also show some decrease over time due to aging.

The capacitor parameters are also dependant on the particular case size, with smaller sizes giving poorer

performance figures in general. As an example, the graph below shows a comparison of different capacitor case

sizes in a Capacitance vs. DC Bias plot. As shown in the graph, increasing the DC Bias condition can result in

the capacitance value falling below the minimum recommended value. 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 10. 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 1µF 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 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. For the output

capacitor of the DC-DC converter, please note that the output voltage ripple is dependent on the ESR of the

output capacitor.

Table 2. Suggested Capacitors and their Suppliers

Capacitance / µF

Model

Voltage Rating

Vendor

Type

Case Size / Inch (mm)

10.0

C1608X5R0J106K

6.3V

TDK

Ceramic, X5R

0603 (1608)

4.7

C1608X5R1A475K

10V

TDK

Ceramic, X5R

0603 (1608)

2.2

C1608X5R1A225K

10V

TDK

Ceramic, X5R

0603 (1608)

1.0

C1005X5R1A105K

10V

TDK

Ceramic, X5R

0402 (1005)

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