LM3687 Datasheet, PDF (25/35 Pages) National Semiconductor (TI) – Step-Down DC-DC Converter with Integrated Low Dropout Regulator and Startup Mode

LM3687
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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.
100%
0603, 10V, X5R
80%
60%
40%
0402, 6.3V, X5R
20%
0
1.0 2.0 3.0 4.0 5.0
DC BIAS (V)
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.
Capacitance / µF
10.0
4.7
2.2
1.0
Table 2. Suggested Capacitors and their Suppliers
Model
C1608X5R0J106K
C1608X5R1A475K
C1608X5R1A225K
C1005X5R1A105K
Voltage Rating
6.3V
10V
10V
10V
Vendor
TDK
TDK
TDK
TDK
Type
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Ceramic, X5R
Case Size / Inch (mm)
0603 (1608)
0603 (1608)
0603 (1608)
0402 (1005)
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