Rainbow Electronics MAX865 - Manual (Page 7)

A substantial voltage difference exists between (V+ -
V
IN ) and V IN for the positive pump, and between

V+

and

V-

if the impedances of the pump capacitors
(C1 and C2) are large with respect to their output
loads.
Larger values of reservoir capacitors (C3 and C4)
reduce output ripple. Larger values of both pump and
reservoir capacitors improve power efficiency.
Charge-Pump Capacitor Selection
To maintain the lowest output resistance, use capacitors
with low effective series resistance (ESR). The charge-
pump output resistance is a function of C1, C2, C3, and
C4’s ESR. Therefore, minimizing the charge-pump
capacitors’ ESR minimizes the total output resistance.
__________Applications Information
Positive and Negative Converter
The MAX865 is most commonly used as a dual charge-
pump voltage converter that provides positive and neg-
ative outputs of two times a positive input voltage. The
Typical Operating Circuit shows that only four external
components are needed: capacitors C1 and C3 for the
positive pump, C2 and C4 for the negative pump. In
most applications, all four capacitors are low-cost,
3.3µF polarized electrolytics. For applications where PC
board space is at a premium and very low currents are
being drawn from the MAX865, 1µF capacitors may be
used for the pump capacitors C1 and C2, with 1µF
reservoir capacitors C3 and C4. Capacitors C2 and C4
must be rated at 12V or greater.
Paralleling Devices
Paralleling multiple MAX865s (Figure 3) reduces the
output resistance of both the positive and negative con-
verters. The effective output resistance is the output
resistance of one device divided by the number of
devices. Separate C1 and C2 charge-pump capacitors
are required for each MAX865, but the reservoir capac-
itors C3 and C4 can be shared.
Heavy Output Current Loads
When under heavy loads, where V+ is sourcing current
into V- (i.e., load current flows from V+ to V-, rather than
from supply to ground), do not allow the V- supply to
pull above ground. In applications where large currents
flow from V+ to V-, use a Schottky diode (1N5817)
between GND and V-, with the anode connected to
GND (Figure 4).
Layout and Grounding
Good layout is important, primarily for good noise per-
formance. To ensure good layout:
•
Mount all components as close together as possible
•
Keep traces short to minimize parasitic inductance
and capacitance
•
Use a ground plane.
MAX865
Compact, Dual-Output Charge Pump
_______________________________________________________________________________________ 7
MAX865
GND
V-
Figure 4. A Schottky diode protects the MAX865 when large
currents flow from V+ to V-.