MAX1720
http://onsemi.com
6
Capacitor Selection
In order to maintain the lowest output resistance and
output ripple voltage, it is recommended that low ESR
capacitors be used. Additionally, larger values of C1 will
lower the output resistance and larger values of C2 will
reduce output voltage ripple. (See Equation 3).
Table 1 shows various values of C1, C2 and C3 with the
corresponding output resistance values at 25°C. Table 2
shows the output voltage ripple for various values of C1, C2
and C3. The data in Tables 1 and 2 was measured not
calculated.
Table 1. Output Resistance vs. Capacitance
(C1 = C2 = C3), Vin = 4.75 V and Vout = –4.0 V
C1 = C2 = C3
(F) Rout
()
0.7 129.1
1.4 69.5
3.3 37.0
7.3 26.5
10 25.9
24 24.1
50 24
Table 2. Output Voltage Ripple vs. Capacitance
(C1 = C2 = C3), Vin = 4.75 V and Vout = –4.0 V
C1 = C2 = C3
(F) Output Voltage Ripple
(mV)
0.7 382
1.4 342
3.3 255
7.3 164
10 132
24 59
50 38
Input Supply Bypassing
The input voltage, Vin should be capacitively bypassed to
reduce AC impedance and minimize noise ef fects due to the
switching internals in the device. If the device is loaded from
Vout to GND, it is recommended that a lar ge value capacitor
(at least equal to C1) be connected from Vin to GND. If the
device is loaded from V in to Vout, a small (0.7 µF) capacitor
between the pins is suf ficient.
Voltage Inverter
The most common application for a charge pump is the
voltage inverter (Figure 14). This application uses two or
three external capacitors. The C1 (pump capacitor) and C2
(output capacitor) are required. The input bypass capacitor,
C3, may be necessary depending on the application. The
output is equal t o –Vin plus any voltage drops due to loading.
Refer to Tables 1 and 2 for capacitor selection. The test setup
used for the majority of the characterization is shown in
Figure 14.
Layout Considerations
As with any switching power supply circuit, good layout
practice is recommended. Mount components as close
together as possible to minimize stray inductance and
capacitance. Also, use a large ground plane to minimize
noise leakage into other circuitry.
Capacitor Resources
Selecting the proper type of capacitor can reduce
switching loss. Low ESR capacitors are recommended. The
MAX1720 was characterized using the capacitors listed in
Table 3. This list identifies low ESR capacitors for the
voltage inverter application.
Table 3. Capacitor Types
Manufacturer/Contact Part Types/Series
AVX TPS
843–448–9411
www.avxcorp.com
Cornell Dubilier ESRD
u
508–996–8561
www.cornell–dubilier.com
San
o/Os–con SN
619–661–6835
SVP
www.sanyovideo.com/oscon.htm
Visha
593D
603–224–1961
594
www.vishay.com