MAX871 MAXIM [Maxim Integrated Products], MAX871 Datasheet - Page 5

• Manufacturer Part Number: MAX871
• Description: Switched-Capacitor Voltage Inverters
• Manufacturer: MAXIM [Maxim Integrated Products]

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 V
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
-V
than -V
load drains charge from C2.
The MAX870/MAX871 are not voltage regulators: the
charge pump’s output source resistance is approxi-
mately 20Ω at room temperature (with V
V
droop toward GND as load current increases. The
droop of the negative supply (V
rent draw from OUT (I
er’s source resistance (RS-):
The negative output voltage will be:
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:
Figure 3a. Switched-Capacitor Model
_______________Detailed Description
OUT
IN
P
. The actual voltage at the output is more positive
LOSS
approaches -5V when lightly loaded. V
IN
V+
, since switches S1–S4 have resistance and the
= P
C1
+ P
+ P
V
V
INTERNAL LOSSES
OUT
DROOP-
f
PUMP CAPACITOR LOSSES
CONVERSION LOSSES
_______________________________________________________________________________________
= -(V
OUT
Efficiency Considerations
IN
= I
Switched-Capacitor Voltage Inverters
IN
volts. This connects C1 in par-
) times the negative convert-
OUT
– V
Charge-Pump Output
C2
DROOP-
x RS-
DROOP-
+ P
SWITCH LOSSES
)
) equals the cur-
R
IN
L
V
OUT
= +5V), and
OUT
will
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:
where f
the effective resistance from an ideal switched-
capacitor circuit. See Figures 3a and 3b.
Figure 2. Ideal Voltage Inverter
Figure 3b. Equivalent Circuit
R
P
OUT
PUMP CAPACITOR LOSSES
= I
IN
OSC
OUT
V+
f
OSC
is the oscillator frequency. The first term is
2
R
S3
S1
EQUIV
x R
1
R
EQUIV
x C
OUT
=
C1
f
1
1
C1
2
R
SWITCHES
S4
S2
C2
+P
CONVERSION LOSSES
C2
4
R
L
ESR
V
OUT
C
V
OUT
1
= -(V
ESR
IN
)
C
2
5