MAX1720 ON Semiconductor, MAX1720 Datasheet - Page 5

• Manufacturer Part Number: MAX1720
• Description: Switched Capacitor Voltage Inverter
• Manufacturer: ON Semiconductor

DETAILED OPERATING DESCRIPTION
applied to the V
operation (Figure 15). During the first phase, switches S
S
charges to the voltage on V
C
S
charge to C
APPLICATIONS INFORMATION
Output Voltage Considerations
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
loads, and drops according to the equation below:
4
3
2
V
The MAX1720 charge pump converter inverts the voltage
The MAX1720 performs voltage conversion but does not
. During the second phase, S
Figure 15. Ideal Switched Capacitor Charge Pump
are open and S
V
are open. This action connects C
From Osc
in
in
Figure 14. Test Setup/Voltage Inverter
+
C
2
.
3
S1
S3
in
V out + * (V in * V DROP )
C
1
2
3
V DROP + I out
in
= 5.0 V. V
1
= C
1
pin. Conversion consists of a two−phase
C
and S
1
OSC
2
= C
3
out
in
3
S2
S4
are closed. During this time, C
= 10 mF
and load current is supplied from
is approximately −5.0 V at light
2
and S
6
5
4
R out
4
1
are closed, and S
C
across C
2
+
C
1
+
C
2
−V
2
, restoring
out
−V
out
http://onsemi.com
2
1
R
and
and
L
MAX1720
1
5
Charge Pump Efficiency
pump is affected by four factors:
These losses are given by Equation 1.
resistance of an ideal switched capacitor circuit (Figures 16
and 17).
Equation 2. The output voltage ripple is given by Equation 3.
P LOSS + [ 0.5C 1 (V in
The overall power conversion efficiency of the charge
Most of the conversion losses are due to factors 2, 3 and 4.
The 1/(f
The losses due to charge transfer above are also shown in
1. Losses from power consumed by the internal
2. I
3. Charge pump capacitor losses due to Equivalent
4. Losses that occur during charge transfer from the
V
V
(f
in
in
oscillator, switch drive, etc. (which vary with input
voltage, temperature and oscillator frequency).
switches on−board the charge pump.
Series Resistance (ESR).
commutation capacitor to the output capacitor when
a voltage difference between the two capacitors
exists.
P
V
OSC
Figure 16. Ideal Switched Capacitor Model
2
LOSS(2,3,4)
Figure 17. Equivalent Output Resistance
R losses due to the on−resistance of the MOSFET
) 0.5C 2 (V RIPPLE
RIPPLE
R
1
OSC
EQUIV
)C 1
)(C
) 8R
R
1
+
+
) term in Equation 1 is the effective output
f
EQUIV
(f
f
C
+ I out
OSC
SWITCH
1
1
C 1
2
I out
* V out
)(C 2 )
2
2
* 2V out V RIPPLE )]
) 4ESR
) 2(I out )(ESR
R out ^ I out
2
)
C
C
C 1
2
2
) ESR
2
R
R
C 2
L
L
)
V
V
C 2
(eq. 3)
out
out
f OSC
(eq. 2)
(eq. 1)