Datasheet

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SLVS324A − JULY 2001 REVISED NOVEMBER 2004
3
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detailed description
operating principle
The TPS60400, TPS60401 charge pumps invert the voltage applied to their input. For the highest performance,
use low equivalent series resistance (ESR) capacitors (e.g., ceramic). During the first half-cycle, switches S2
and S4 open, switches S1 and S3 close, and capacitor (C
(fly)
) charges to the voltage at V
I
. During the second
half-cycle, S1 and S3 open, S2 and S4 close. This connects the positive terminal of C
(fly)
to GND and the
negative to V
O.
By connecting C
(fly)
in parallel, C
O
is charged negative. The actual voltage at the output is more
positive than −V
I
, since switches S1–S4 have resistance and the load drains charge from C
O
.
C
(fly)
1 µF
S2
S1
S3
S4
C
O
1 µF
V
O
(−V
I
)
GND
V
I
GND
Figure 1. Operating Principle
charge-pump output resistance
The TPS6040x devices are not voltage regulators. The charge pumps output source resistance is
approximately 15 Ω at room temperature (with V
I
= 5 V), and V
O
approaches –5 V when lightly loaded. V
O
will
droop toward GND as load current increases.
V
O
= −(V
I
– R
O
× I
O
)
R
O
[
1
ƒosc C
(fly)
) 4
ǒ
2R
SWITCH
) ESR
CFLY
Ǔ
) ESR
CO
R
O
= output resistance of the converter
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
(1)
(2)