MAX828 ON, MAX828 Datasheet - Page 3

• Manufacturer Part Number: MAX828
• Description: Switched Capacitor Voltage Converters
• Manufacturer: ON

voltage applied to the V
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 V
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.
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 25
at +25 C and V
loads, and droops according to the equation below:
Charge Pump Efficiency
affected by four factors:
The MAX828/829 charge pump converters invert the
The MAX828/829 perform voltage conversion but do not
The overall power efficiency of the charge pump is
Figure 1. Ideal Switched Capacitor Charge Pump
(1) Losses from power consumed by the internal
(2) I
(3) Charge pump capacitor losses due to effective
IN
V
V
oscillator, switch drive, etc. (which vary with input
voltage, temperature and oscillator frequency).
switches on–board the charge pump.
series resistance (ESR).
2
R losses due to the on–resistance of the MOSFET
DROP
OUT
S1
S3
IN
= – (V
= +5V. V
= I
C1
OUT
IN
IN
x R
– V
OUT
S2
S4
pin. Conversion consists of a
OUT
DROP
is approximately - 5V at light
)
C2
DETAILED OPERATING DESCRIPTION
MAX828/829
V out = –(V in )
IN
W
and load
nominal
MAX828 MAX829
http://onsemi.com
3
and (4) above. These losses are given by Equation 1.
resistance of an ideal switched capacitor circuit (Figures 2a,
2b).
shown in Equation 2. The output voltage ripple is given by
Equation 3.
V
P
P
LOSS(2,3,4)
LOSS(4)
RIPPLE
Most of the conversion losses are due to factors (2), (3)
The 1/(f
The losses in the circuit due to factor (4) above are also
V +
V +
(4) Losses that occur during charge transfer (from the
(f
Figure 2a. Ideal Switched Capacitor Model
Figure 2b. Equivalent Output Resistance
OSC
R
EQUIV
commutation capacitor to the output capacitor)
when a voltage difference between the two
capacitors exists.
1
OSC
+
(V
)C1
+
RIPPLE
)(C1) term in Equation 1 is the effective output
(f
R EQUIV
OSC
+
(0.5)(C1)( V
)
+
I
f
OUT
I
8R
f
C1
OUT
)(C2)
1
SWITCH
Equation 2.
Equation 3.
Equation 1.
C1
2
*
2
)
2V
IN
R
OUT
2(I
2
OUT
*
)
OUT
V
4ESR
V
OUT
^
RIPPLE
)(ESR
C2
C2
I
OUT
C1
2
)
C2
)
)
)
R L
2
R L
(0.5)(C2)
)
ESR
V out
f
V out
OSC
C2