Datasheet

Dt =
1
¾
2
C V - V
IN DET LOW
( )
2 2
TPS65250
SLVSAA3C –JUNE 2010–REVISED OCTOBER 2012 www.ti.com
Capacitance Reduction With TPS65250 Pump and Dump Circuit
Table 6 shows the capacitance reduction achieved compared to the typical application where the storage
capacitor is connected to the adapter supply the following formula applies.
(20)
Where:
P = Gasp power
∆t = Gasp time, 60 ms
V
DET
= Detection voltage (at this point is assumed input supply has disappeared), 11 V
V
LOW
= Minimum gasp voltage, 8.5 V
Size of Storage Capacitor
On choosing the capacitor for storage the following considerations should be taken into account:
• Capacitor type: Given the relatively high values required for gasp applications electrolytic are typically the
choice of capacitor due to their low cost.
• Capacitor voltage rating and derating: The electrolytic capacitors of interest for a 12-V application are rated at
25 V. Assuming an 80% derating factor, this will match the storage voltage. Bear in mind that the accuracy of
this voltage is 5%, therefore calculations should include the worst case number. For a 20-V setting the
storage voltage will be in the 19 V - 21 V range.
• Capacitor dimension: For this particular example a 25-mm height restriction is in place.
• Capacitor tolerance and life: As electrolytic capacitors degrade with time the choice of capacitor will dictate
the overall system life expectancy. It is recommended to add the following adjustment factors to the chosen
capacitor value.
• Capacitor package: Surface mount devices are considerably more expensive than through hole devices.
Based on these considerations the series M of Panasonic through hole capacitors is chosen.
http://industrial.panasonic.com/www-data/pdf/ABA0000/ABA0000CE12.pdf
Good savings and real estate reductions can be obtained if 25-V capacitors are used. As Table 5 shows only the
1000-µF part meets the height restriction of the design.
Table 5. 25-V Capacitors That
Can Be Used in TPS65250
Pump and Dump Circuit to
Support 2.85 W
C (µF) HEIGHT (mm)
1000 20
2200 25 (too tall)
3300 25 (too tall)
4700 31 (too tall)
Table 6 shows the tabulated results for the TPS65250 P&D circuit with a gasp time of 60 ms, power levels of 1
W to 4 W, showing the capacitance (C
STRG
) required for 20-V storage. The column C
IN
shows the capacitance
required using the typical approach of adding capacitance to the input supply. The data clearly shows that the
pump and dump circuit allows for a major reduction in the storage capacitor.
Table 6. Storage Capacitance required with TPS65250 Pump and Dump Circuit
PV
GASP
→ P = 1W P = 2 W P = 3 W P = 4 W
V
STRG
↓ C
IN
(µF) C
STRG
(µF) C
IN
(µF) C
STRG
(µF) C
IN
(µF) C
STRG
(µF) C
IN
(µF) C
STRG
(µF)
20 2,462 602 4,923 1,203 7,385 1,805 9,846 2,406
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