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MAX828 Datasheet(PDF) 3 Page - ON Semiconductor

Part No. MAX828
Description  Switched Capacitor Voltage Converters
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Maker  ONSEMI [ON Semiconductor]
Homepage  http://www.onsemi.com
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MAX828 Datasheet(HTML) 3 Page - ON Semiconductor

 
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MAX828 MAX829
http://onsemi.com
3
DETAILED OPERATING DESCRIPTION
The MAX828/829 charge pump converters invert the
voltage applied to the VIN pin. Conversion consists of a
two–phase operation (Figure 1). During the first phase,
switches S2 and S4 are open and S1 and S3 are closed.
During this time, C1 charges to the voltage on VIN and load
current is supplied from C2. During the second phase, S2
and S4 are closed, and S1 and S3 are open. This action
connects C1 across C2, restoring charge to C2.
Figure 1. Ideal Switched Capacitor Charge Pump
S3
S4
C2
C1
S1
S2
IN
Vout = –(Vin)
MAX828/829
APPLICATIONS INFORMATION
Output Voltage Considerations
The MAX828/829 perform voltage conversion but do 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 25
W nominal
at +25
°C and V
IN = +5V. VOUT is approximately - 5V at light
loads, and droops according to the equation below:
VDROP = IOUT x ROUT
VOUT = – (VIN – VDROP)
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) I2R losses due to the on–resistance of the MOSFET
switches on–board the charge pump.
(3) Charge pump capacitor losses due to effective
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) above. These losses are given by Equation 1.
P
LOSS(2,3,4) +
I
OUT
2
R
OUT ^
I
OUT
2
Equation 1.
1
(f
OSC
)C1
) 8R
SWITCH )
4ESR
C1 )
ESR
C2
The 1/(fOSC)(C1) term in Equation 1 is the effective output
resistance of an ideal switched capacitor circuit (Figures 2a,
2b).
The losses in the circuit due to factor (4) above are also
shown in Equation 2. The output voltage ripple is given by
Equation 3.
P
LOSS(4) +
(0.5)(C1)( V
IN
2
* V
OUT
2)
) (0.5)(C2)
Equation 2.
(V
RIPPLE
2
* 2V
OUT
V
RIPPLE
)
f
OSC
V
RIPPLE +
I
OUT
(f
OSC
)(C2)
) 2(I
OUT
)(ESR
C2
)
Equation 3.
Figure 2a. Ideal Switched Capacitor Model
RL
C2
C1
V+
Vout
f
Figure 2b. Equivalent Output Resistance
RL
C2
V+
Vout
REQUIV
R
EQUIV +
1
f
C1


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