Rainbow Electronics MAX1681 - Manual (Page 8)

MAX1680/MAX1681
Bypass Capacitor
Bypass the input voltage to reduce AC impedance and
to prevent internal switching noise. Bypassing depends
on the source impedance location. The AC ripple cur-
rent is 2 x I
OUT for the doubler and the inverter. Use a
large bypass capacitor (equal to C1) if the supply has
high AC impedance.
Cascading Devices
To produce larger negative voltages, cascade two
devices (Figure 1). For two devices, the unloaded out-
put voltage is approximately -2 x V
INPUT , but this value
is reduced slightly by the first device’s output resis-
tance multiplied by the second device’s quiescent cur-
rent. The effective output resistance for a cascaded
device is larger than that for an individual device (20
Ω
for two devices). Cascading several devices increases
output resistance and reduces efficiency. If a large
negative voltage is required for several stages, an
inductive inverting switching regulator such as the
MAX629 or MAX774 may offer more advantages.
Paralleling Devices
Parallel two or more MAX1680/MAX1681s to reduce
output resistance voltage drop under a given load. With
reduced output resistance, paralleled devices deliver
higher load currents. Figure 2 shows two
MAX1680/MAX1681s connected in parallel. Output
resistance is inversely proportional to the number of
devices.
125mA, Frequency-Selectable,
Switched-Capacitor Voltage Converters
8 _______________________________________________________________________________________
250 1.2
125 2.5
1.9
3.8
500 0.6 0.9
Table 4. Minimum Recommended Capacitances for Maximum Output Current
CAPACITANCE (µF) (C1 = C2)
2.5
5.1
3.2
6.4
1.2 1.6
1000 0.3 0.4 0.6 0.8
V IN = 3V
f OSC (kHz)
V IN = 2V V IN = 5V V IN = 4V
MAX1681
FSEL IN
CAP+ SHDN
GND 1 µ F
1 µ F
1 µ F
LV
CAP- OUT
MAX1681
FSEL IN
CAP+ SHDN
GND
+V
INPUT
V OUT = -V INPUT
1 µ F
NOTE: USE 4.7 µ F CAPACITORS FOR MAX1680.
1 µ F
LV
CAP- OUT V OUT = -2V INPUT
Figure 1. Cascading MAX1680/MAX1681s to Increase Output Voltage