Back-up time of the BestCap™ may be calculated using the
following formula:
C = I * t /
V
V = Vo – Vt – I * RESR
At low current (as is the case in the back-up application):
V= Vo – Vt
t = C
V/I = C*(Vo – Vt)/I
Where
C is the Capacitance in F
Vo is the initial voltage
Vt is the end voltage
I is the back-up current drain in Amperes
RESR is the ESR in Ohms
t = back-up time in seconds
Example: System Clock Time Maintenance
Using a BZ015A503ZAB35 BestCap™ 5.5V, 50mF, 28 x 17 footprint, 3.5 mm (max) height
If Vo = 5.0V Vt = 2.5V I(Back-up) = 40µA (40*10 –6 Amp.)
The discharge current of the Cap shall be the sum of the back-up current drain plus leakage current.
Leakage Current is a function of the Capacitor voltage. In the specific case,
the Leakage Current is 5µA at 5.0V and it comes down to <1µA at 2.5V.
In the “worst” case the Leakage Current = 5µA. Practically, the Leakage Current is 3µA[(5+1)/2].
The total discharge current during the back-up period is
40µA (back-up) + 5µA (Leakage Current) = 45µA, in the worst case, or
40µA (back-up) + 3µA (Leakage Current) = 43µA, practically.
Thus the back-up time would be:
t= 50*10–3 * (5–2.5)/45*10–6 = 46 minutes
Engineering Guidance Notes
1. Operational and storage temperature
-20°C to +70°C. Storage at room temperature is
recommended.
2. Voltage
Rated voltage of the capacitor is per the data sheet and
the label on the product. A surge voltage of V rated
+2% may be applied to the capacitor for <5 sec without
damage or performance degradation.
3. Temperature/Voltage Recommendations
Temperature -20°C to +35°C +35°C to +70°C
Rated Voltage Recommended Maximum Applied Voltage
3.5V 3.5V 3.2V
4.5V 4.5V 4.2V
5.5V 5.5V 5.0V
4. Polarity
BestCap™capacitors are non-polar, so can handle both
positive and negative voltages. The polarity marking on
the label relates to internal test procedures only and may
be ignored in normal use.
5. Mounting
BestCap™may not be reflow or wave soldered. Hand
soldering of the device is acceptable. When hand
soldering, use a soldering iron of <30W rating and ensure
that the tip temperature does not exceed +350°C.
Duration of the soldering operation should be <3 sec.
Mounting chips which may be automatically placed, (i.e.
via Pick n’ Place machine) and reflow soldered, and into
which the BestCap™may subsequently be inserted, are
available for some versions of BestCap™. Please check
with your AVX Sales Office for further information.
6. Other Precautions
•Do not disassemble the capacitor
•Do not dispose of the capacitor by incineration.
•Should the internal material come in contact with the
skin or eyes, wash/rinse thoroughly with running
water.
BESTCAP™IN BATTERY BACK-UPAPPLICATIONS
BestCap™is normally utilized as a power-boosting device to assist the main battery during pulse power
demand periods. However, the capacitor may also be used as a back-up unit when, for example, the battery is
being replaced.