Datasheet

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SGLS246A − JUNE 2004 − REVISED JUNE 2008
4
POST OFFICE BOX 655303 DALLAS, TEXAS 75265
detailed description (continued)
efficiency considerations
The power efficiency of a switched-capacitor voltage converter is affected by three factors: the internal losses
in the converter IC, the resistive losses of the capacitors, and the conversion losses during charge transfer
between the capacitors. The internal losses are associated with the IC’s internal functions, such as driving the
switches, oscillator, etc. These losses are affected by operating conditions such as input voltage, temperature,
and frequency. The next two losses are associated with the voltage converter circuit’s output resistance. Switch
losses occur because of the on-resistance of the MOSFET switches in the IC. Charge-pump capacitor losses
occur because of their ESR. The relationship between these losses and the output resistance is as follows:
P
CAPACITOR
LOSSES
+ P
CONVERSION
LOSSES
= I
O
2
× R
O
R
SWITCH
= resistance of a single MOSFET-switch inside the converter
f
OSC
= oscillator frequency
The first term is the effective resistance from an ideal switched-capacitor circuit. Conversion losses occur during
the charge transfer between C
(fly)
and C
O
when there is a voltage difference between them. The power loss is:
P
CONV.LOSS
+
ƪ
1
2
C
(fly)
ǒ
V
2
I
* V
2
O
Ǔ
)
1
2
C
O
ǒ
V
2
RIPPLE
* 2V
O
V
RIPPLE
Ǔ
ƫ
ƒ
osc
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. When the inverter
is loaded from IN to OUT, the circuit draws 2 × I
O
constantly, except for short switching spikes. A 0.1-µF bypass
capacitor is sufficient.
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.
(2)