NCP1729
www.onsemi.com
5
6
4
2
3
1
OSC
−V
out
C
1
C
2
R
L
+
+
C
3
V
in
+
Figure 14. Test Setup/Voltage Inverter
5
C
1
= C
2
= C
3
= 3.3 mF
DETAILED OPERATING DESCRIPTION
The NCP1729 charge pump converter inverts the voltage
applied to the V
in
pin. Conversion consists of a two−phase
operation (Figure 15). During the first phase, switches S
2
and S
4
are open and S
1
and S
3
are closed. During this time,
C
1
charges to the voltage on V
in
and load current is supplied
from C
2
. During the second phase, S
2
and S
4
are closed, and
S
1
and S
3
are open. This action connects C
1
across C
2
,
restoring charge to C
2
.
Figure 15. Ideal Switched Capacitor Charge Pump
S3 S4
C
2
C
1
S1 S2
V
in
−V
out
From OSC
APPLICATIONS INFORMATION
Output Voltage Considerations
The NCP1729 performs voltage conversion but does not
provide regulation. The output voltage will drop in a linear
manner with respect to load current. The value of this
equivalent output resistance is approximately 26 W nominal
at 25°C with V
in
= 5.0 V. V
out
is approximately −5.0 V at
light loads, and drops according to the equation below:
V
DROP
+ I
out
R
out
V
out +
* (V
in
* V
DROP
)
Charge Pump Efficiency
The overall power conversion efficiency of the charge
pump is affected by four factors:
1. Losses from power consumed by the internal
oscillator, switch drive, etc. (which vary with input
voltage, temperature and oscillator frequency).
2. I
2
R losses due to the on−resistance of the MOSFET
switches on−board the charge pump.
3. Charge pump capacitor losses due to
Equivalent Series Resistance (ESR).
4. Losses that occur during charge transfer from the
commutation capacitor to the output capacitor when
a voltage difference between the two capacitors
exists.
Most of the conversion losses are due to factors 2, 3 and 4.
These losses are given by Equation 1.
P
LOSS(2,3,4)
+ I
out
2
R
out
^ I
out
2
ƪ
1
(f
OSC
)C
1
) 8R
SWITCH
) 4ESR
C
1
) ESR
C
2
ƫ
(eq. 1)
The 1/(f
OSC
)(C
1
) term in Equation 1 is the effective output
resistance of an ideal switched capacitor circuit (Figures 16
and 17).
The losses due to charge transfer above are also shown in
Equation 2. The output voltage ripple is given by Equation 3.
) 0.5C
2
(V
RIPPLE
2
* 2V
out
V
RIPPLE
)] f
OSC
LOSS
+ [0.5C
1
(V
in
2
* V
out
2
)
(eq.
V
RIPPLE
+
I
out
(f
OSC
)(C
2
)
) 2(I
out
)(ESR
C
2
)
(eq. 3)
R
L
C
2
C
1
V
in
V
out
f
Figure 16. Ideal Switched Capacitor Model
R
L
C
2
V
in
V
out
R
EQUIV
R
EQUIV
+
1
f C
1
Figure 17. Equivalent Output Resistance
