Datasheet

STRG
V in
BUCK3
I_BUCK3
TPS65250
www.ti.com SLVSAA3C –JUNE 2010–REVISED OCTOBER 2012
Going through the iterative procedure, using only two 1000-µF, 25-V capacitors will allow for a gasp power in
excess of 2.85 W, instead of the 6 required in the application without the pump and dump feature.
Table 7. 25-V Capacitors Used in TPS65250 Pump and Dump Circuit to Support 2.85 W
ECA1CM222B C (µF) P_GASP (W) COMMENT
1 1000 1.63 Too little
2 2000 3.26 About right
3 3000 4.89 Too much
Note that the final capacitor value needs to be adjusted for the worst case of tolerance (typically -20%) and
reduction of capacitor value at the end of tis life expectancy (typically 20%).
Pump and Dump Plots
Figure 43 shows the pump stage plots. Green trace is the input supply (12 V), yellow trace is the storage
capacitor voltage, purple trace is the Buck 3 supply and blue trace is the PGOOD signal. The storage capacitor is
2000 µF and it takes ~200 ms to charge to the input supply. BUCK has a 470-nF capacitor connected to the EN
pin provides a delay of ~800 ms from the time when the input voltage is applied and the storage capacitor
charges to the 20-V target and the PGOOD signal has a 2-s delay before being asserted.
Figure 44 shows the dump stage plots. Yellow trace is the storage capacitor voltage, purple trace is the V
IN
supply and blue trace is the 7.5-V Buck 3 signal green trace is Buck 3 current. The total gasp time is ~70 ms
which is consistent with the calculations shown.
Figure 43. Pump Operation Figure 44. Dump Operation
Table 8 shows the loading on the rails plus rail efficiency to show the total power of ~3.18 W provided by the 12-
V adapter supply.
Table 8. Rail Loading for TPS65250 Pump and Dump Circuit Verification
RAIL V I (A) P
O
(W) EFFICIENCY PIN (W)
Buck 1 3.3 0.22 0.73 85% 0.85
Buck 2 2.5 0.21 0.53 87% 0.6
Buck 3 7.5 0.2 1.5 87% 1.72
12 V 12 0.02 0.24
Total 2.75 3.42
Copyright © 2010–2012, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Links: TPS65250