MAX828, MAX829
http://onsemi.com
6
MAX828: C
1
= C
2
= C
3
= 10 F
MAX829: C
1
= C
2
= C
3
= 3.3 F
6
4
2
3
1
OSC
−V
out
C
1
C
2
R
L
+
+
C
3
V
in
+
Figure 20. Test Setup/Voltage Inverter
DETAILED OPERATING DESCRIPTION
The MAX828/829 charge pump converters inverts the
voltage applied to the V
in
pin. Conversion consists of a
two−phase operation (Figure 21). 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 21. 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 MAX828/829 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 nominal
at 25°C and 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 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 22
and 23).
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 22. Ideal Switched Capacitor Model
R
L
C
2
V
in
V
out
R
EQUIV
R
EQUIV
+
1
f C
1
Figure 23. Equivalent Output Resistance
