Comparison of capacitor DCL performance in a typical
battery circuit application As we have said, the typical energy source of a handheld
device is a lithium-ion rechargeable battery with a nominal voltage Va = 3.7V. To support device functionality, designers can choose from several different capacitor series. Figure 5 compares the maximum DCL of different capacitor series all with a nominal capacitance of 47µF.
Optimal voltage derating for minimal DCL To achieve the optimal leakage current ratings for the application (DCLa) at room temperature we need to consider two factors: the basic DCL defi ned at rated voltage Vr as in Equations 1; and the DCL ratio vs Voltage derating, see Figure 3.
Equations 2: TAJ series: DCLa = 0.01*C*Vr*Ri, TRJ series: DCLa = 0.0075*C*Vr*Ri, NOJ series: DCLa = 0.02*C*Vr*Ri, where Ri = ratio of DCLa/DCL (at Vr) – Figure 3. The maximum DCL multiplier vs Va/Vr for a fi xed application voltage Va is displayed in Figure 4.
In this example where the Va is 3.7V, the optimal rated
voltage (Vr) = 10V (Fig. 5), which means optimal operating conditions are at 37% of rated voltage (see Fig. 4). For different capacitances the optimal rated voltage will also be 10V.
The maximum actual DCL value varies with different
input conditions (chosen capacitor series, nominal capacitance, rated voltage), however shape of the graph (Fig. 4) will be the same. So we can identify the range of Va/Vr (derating) values with minimum actual DCL as the ‘optimal’ range. Therefore the minimum DCL is obtained when capacitor is used at 25 to 40 % of rated voltage - when the rated voltage of the capacitor is 2.5 to 4 times higher than actual application voltage.
January 14th 2012
Conclusion AVX’s TRJ, TAJ tantalum and NOJ niobium oxide capacitor series are suitable to support the battery in handheld and TPMS transmitter applications. These parts exhibit different basic leakage currents with TRJ series capacitors having the lowest DCL. However, voltage derating can be applied to reduce DCL to less than the catalogue value defi ned at rated voltage Vr. The optimum condition for minimum DCL is to use the capacitor at between 25 and 40% of its rated voltage. For 3.7V applications, the optimal rated voltage is 10V, therefore the best choice available from AVX is to use a TRJ capacitor with Vr = 10V, closely followed by a TAJ device with the same rating. Also special TAJ. * LE suffi xed tantalum capacitors exist with an even lower guaranteed maximum DCL.
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