MAX868 Maxim, MAX868 Datasheet - Page 6

• Manufacturer Part Number: MAX868
• Description: Regulated / Adjustable -2x Inverting Charge Pump
• Manufacturer: Maxim

Regulated, Adjustable -2x
Inverting Charge Pump
Figure 2. a) In charge phase, the left-hand switches are
closed and the right-hand switches are open, charging the fly-
ing capacitors (C1 and C2) while the output capacitor (C
services the load. b) The equivalent circuit of the charge phase
of operation.
Set the output voltage using two external resistors, R1
and R2, as shown in Figure 4. Since the input bias cur-
rent at FB has a 50nA maximum, large resistor values in
the feedback loop do not significantly degrade accura-
cy. Begin by selecting R2 in the 100k
and calculate R1 using the following equation:
where V
any available regulated positive voltage. When the
MAX868 is powered by a regulated voltage, V
used as the reference for setting the output voltage.
6
__________________Design Procedure
_______________________________________________________________________________________
(a)
(b)
GND
IN
IN
OUT
is the desired output voltage, and V
R
1
C1+
C1-
Setting the Output Voltage
R
2
C2+
C2-
C1+
C1-
C2+
C2-
x
|
V
V
OUT
REF
|
to 500k
IN
C
V
C
V
OUT
OUT
OUT
OUT
can be
range,
OUT
REF
)
is
When the MAX868 is powered by an unregulated sup-
ply, such as when operating directly from a battery, use
any available positive reference voltage in the system.
Note that due to the MAX868’s doubling and inverting
charge-pump action, the output voltage is limited to
-2 x V
Alternatively, to configure the MAX868 as a simple,
unregulated doubler-inverter (V
nect FB to IN. In this configuration, the MAX868 runs at
its maximum oscillator frequency, operating as a con-
ventional, open-loop charge pump.
If multiple oscillator cycles are required to regulate the
output, reduce the values for R1 and R2, or parallel a
small capacitor (C
feedback loop and ensure stability. Choose the lowest
capacitor value that ensures stability; values up to 47pF
are adequate for most applications.
Figure 3. a) In discharge phase, the left-hand switches are
open and the right-hand switches are closed, transferring
energy from the flying capacitors (C1 and C2) to the output
capacitor (C
phase of operation.
(a)
IN
(b)
IN
.
OUT
). b) The equivalent circuit of the discharge
C
) across R1 to compensate the
C2+
C2-
C1+
C1-
C2+
C1+
C2-
C1-
OUT
C
OUT
V
OUT
= -2 x V
C
V
IN
OUT
OUT
), con-