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MAX870 Datasheet PDF : 8 Pages
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Switched-Capacitor Voltage Inverters
_______________Detailed Description
The MAX870/MAX871 capacitive charge pumps invert
the voltage applied to their input. For highest perfor-
mance, use low equivalent series resistance (ESR)
capacitors (e.g., ceramic).
During the first half-cycle, switches S2 and S4 open,
switches S1 and S3 close, and capacitor C1 charges to
the voltage at IN (Figure 2). During the second half-
cycle, S1 and S3 open, S2 and S4 close, and C1 is level
shifted downward by VIN volts. This connects C1 in par-
allel with the reservoir capacitor C2. If the voltage across
C2 is smaller than the voltage across C1, then charge
flows from C1 to C2 until the voltage across C2 reaches
-VIN. The actual voltage at the output is more positive
than -VIN, since switches S1–S4 have resistance and the
load drains charge from C2.
Charge-Pump Output
The MAX870/MAX871 are not voltage regulators: the
charge pump’s output source resistance is approxi-
mately 20Ω at room temperature (with VIN = +5V), and
VOUT approaches -5V when lightly loaded. VOUT will
droop toward GND as load current increases. The
droop of the negative supply (VDROOP-) equals the cur-
rent draw from OUT (IOUT) times the negative convert-
er’s source resistance (RS-):
VDROOP- = IOUT x RS-
The negative output voltage will be:
VOUT = -(VIN – VDROOP-)
Efficiency Considerations
The power efficiency of a switched-capacitor voltage
converter is affected by three factors: the internal loss-
es in the converter IC, the resistive losses of the pump
capacitors, and the conversion losses during charge
transfer between the capacitors. The total power loss is:
ΣPLOSS = PINTERNAL LOSSES + PSWITCH LOSSES
+ PPUMP CAPACITOR LOSSES
+ PCONVERSION LOSSES
f
V+
VOUT
C1
C2
RL
S1
S2
IN
C1
S3
S4
C2
VOUT = -(VIN)
Figure 2. Ideal Voltage Inverter
The internal losses are associated with the IC’s internal
functions, such as driving the switches, oscillator, etc.
These losses are affected by operating conditions such
as input voltage, temperature, and frequency.
The next two losses are associated with the voltage
converter circuit’s output resistance. Switch losses
occur because of the on-resistance of the MOSFET
switches in the IC. Charge-pump capacitor losses
occur because of their ESR. The relationship between
these losses and the output resistance is as follows:
PPUMP CAPACITOR LOSSES +PCONVERSION LOSSES
= IOUT2 x ROUT
( ) ROUT ≅
1
fOSC x
C1 + 2RSWITCHES + 4ESRC1 + ESRC2
where fOSC is the oscillator frequency. The first term is
the effective resistance from an ideal switched-
capacitor circuit. See Figures 3a and 3b.
REQUIV
V+
REQUIV =
1
f × C1
C2
VOUT
RL
Figure 3a. Switched-Capacitor Model
Figure 3b. Equivalent Circuit
_______________________________________________________________________________________ 5
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