MAX868 Maxim, MAX868 Datasheet - Page 8

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

Regulated, Adjustable -2x
Inverting Charge Pump
Since the difference between the voltage of the series-
connected flying capacitors and the output voltage
must be dissipated within the device, the MAX868’s
efficiency is very similar to that of a linear regulator.
Estimate efficiency using the following equation:
where k is a constant equal to 2 for the standard con-
figuration of Figure 5 and equal to 1 for the circuit of
Figure 6. This equation’s denominator is the voltage
resulting from the series connection of the flying capac-
itors (-2 x V
tor is simply the regulated output voltage.
For applications in which the output voltage will not be
more negative than -
using the circuit of Figure 6, as compared to the circuit
of Figure 5. In Figure 6, a single flying capacitor is con-
nected between C2+ and C1-, with C2- and C1+ left
Figure 5. Standard Configuration for Generating an Output
Voltage up to -2 x V
__________Applications Information
V
IN
= 5V
0.1 F
0.1 F
1 F
IN
, as shown in Figure 3b), while its numera-
Low-Output-Voltage Operation
SHDN
C2+
C2-
C1+
C1-
IN
|
V
MAX868
IN
PGND
IN
|
, the efficiency can be doubled
|
k x V
V
OUT
GND
OUT
IN
FB
|
R2
500k
R1
750k
V
10 F
OUT
= -7.5V
8
___________________________________
TRANSISTOR COUNT: 96
SUBSTRATE CONNECTED TO IN
unconnected. Furthermore, doubling the flying capaci-
tor to provide the same flying capacitance as the stan-
dard configuration (i.e., setting C
the same load-current capability as the standard con-
figuration and reduces the MAX868’s open-loop output
resistance by a factor of two, due to the reduction in the
number of switches in the current path.
Proper layout is important to obtain optimal perfor-
mance. Connect GND to PGND together using the
shortest trace possible, and similarly connect these
pins to the ground plane. Mount all capacitors as close
to the MAX868 as possible, keeping traces short to
minimize parasitics. Keep all connections to the FB pin
as short as possible. Specifically, locate R1 and R2
next to FB (Figures 7 and 8). Should it become neces-
sary in the final layout, leave room to parallel a feed-
forward capacitor across R1.
Figure 6. Alternative Configuration for
C
V
*C1+ AND C2- MUST BE LEFT UNCONNECTED.
F
IN
= 0.2 F
= 5V
1 F
*
*
SHDN
C2+
C2-
C1+
C1-
MAX868
PGND
IN
Layout and Grounding
GND
Chip Information
OUT
FB
F
|
V
= C1 + C2) provides
OUT
|
R2
500k
R1
330k
≤ V
IN
V
AT 20mA
10 F
OUT
= -3.3V