Datasheet

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SLVS324A − JULY 2001 REVISED NOVEMBER 2004
4
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efficiency considerations (continued)
The efficiency of the TPS6040x devices is dominated by their quiescent supply current at low output current and
by their output impedance at higher current.
h ^
I
O
I
O
) I
Q
ǒ
1 *
I
O
R
O
V
I
Ǔ
Where, I
Q
= quiescent current.
capacitor selection
To maintain the lowest output resistance, use capacitors with low ESR (see Table 1). The charge-pump output
resistance is a function of C
(fly)
’s and C
O
’s ESR. Therefore, minimizing the charge-pump capacitor’s ESR
minimizes the total output resistance. The capacitor values are closely linked to the required output current and
the output noise and ripple requirements. It is possible to only use 1-µF capacitors of the same type.
input capacitor (C
I
)
Bypass the incoming supply to reduce its ac impedance and the impact of the TPS6040x switching noise. The
recommended bypassing depends on the circuit configuration and where the load is connected. When the
inverter is loaded from OUT to GND, current from the supply switches between 2 x I
O
and zero. Therefore, use
a large bypass capacitor (e.g., equal to the value of C
(fly)
) if the supply has high ac impedance.
flying capacitor (C
(fly)
)
Increasing the flying capacitor’s size reduces the output resistance. Small values increases the output
resistance. Above a certain point, increasing C
(fly)
’s capacitance has a negligible effect, because the output
resistance becomes dominated by the internal switch resistance and capacitor ESR.
output capacitor (C
O
)
Increasing the output capacitor’s size reduces the output ripple voltage. Decreasing its ESR reduces both output
resistance and ripple. Smaller capacitance values can be used with light loads if higher output ripple can be
tolerated. Use the following equation to calculate the peak-to-peak ripple.
V
O(ripple)
+
I
O
f
osc
C
o
) 2 I
O
ESR
CO
Table 1. Recommended Capacitor Values
DEVICE
V
I
[V]
I
O
[mA]
C
I
[µF]
C
(fly)
[µF]
C
O
[µF]
TPS60400 1.8…5.5 60 1 1 1
TPS60401 1.8…5.5 60 10 10 10
TPS60402 1.8…5.5 60 3.3 3.3 3.3
TPS60403 1.8…5.5 60 1 1 1