MAX828, MAX829
http://onsemi.com
7
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 C
1
will
lower the output resistance and larger values of C
2
will
reduce output voltage ripple. (See Equation 3).
Table 1 shows various values of C
1
, C
2
and C
3
with the
corresponding output resistance values at 25°C. Table 2
shows the output voltage ripple for various values of C
1
, C
2
and C
3
. The data in Tables 1 and 2 was measured not
calculated.
Table 1. Output Resistance vs. Capacitance
(C
1
= C
2
= C
3
), V
in
= 4.75 V and V
out
= −4.0 V
C
1
= C
2
= C
3
(mF)
MAX828 R
out
(W)
MAX829 R
out
(W)
0.7 127.2 55.7
1.4 67.7 36.8
3.3 36 26.0
7.3 26.7 24.9
10 25.9 25.1
24 24.3 25.2
50 24 24
Table 2. Output Voltage Ripple vs. Capacitance
(C
1
= C
2
= C
3
), V
in
= 4.75 V and V
out
= −4.0 V
C
1
= C
2
= C
3
(mF)
MAX828 Ripple
(mV)
MAX829 Ripple
(mV)
0.7 377.5 320
1.4 360.5 234
3.3 262 121
7.3 155 62.1
10 126 51.25
24 55.1 25.2
50 36.6 27.85
Input Supply Bypassing
The input voltage, V
in
should be capacitively bypassed to
reduce AC impedance and minimize noise effects due to the
switching internals in the device. If the device is loaded from
V
out
to GND, it is recommended that a large value capacitor
(at least equal to C
1
) be connected from V
in
to GND. If the
device is loaded from V
in
to V
out
a small (0.7 F) capacitor
between the pins is sufficient.
Voltage Inverter
The most common application for a charge pump is the
voltage inverter (Figure 20). This application uses two or
three external capacitors. The capacitors C
1
(pump
capacitor) and C
2
(output capacitor) are required. The input
bypass capacitor C
3
, may be necessary depending on the
application. The output is equal to −V
in
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 20.
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
MAX828 and MAX829 were 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
843−448−9411
www.avxcorp.com
TPS
Cornell Dubilier
508−996−8561
www.cornell−dubilier.com
ESRD
Sanyo/Os−con
619−661−6835
www.sanyovideo.com/oscon.htm
SN
SVP
Vishay
603−224−1961
www.vishay.com
593D
594
5
4
2
3
1
OSC
MAX828: Capacitors = 10 F
MAX829: Capacitors = 3.3 F
+
V
in
−V
out
Figure 24. Voltage Inverter
+
+
