Datasheet

TC7660S
DS20001467B-page 8  2001-2013 Microchip Technology Inc.
5.0 APPLICATIONS INFORMATION
5.1 Simple Negative Voltage
Converter
Figure 5-1 shows typical connections to provide a
negative supply where a positive supply is available. A
similar scheme may be employed for supply voltages
anywhere in the operating range of +1.5V to +12V,
keeping in mind that pin 6 (LV) is tied to the supply
negative (GND) only for supply voltages below 3.5V.
FIGURE 5-1: Simple Negative Converter.
The output characteristics of the circuit in Figure 5-1
are those of a nearly ideal voltage source in series with
a 70resistor. Thus, for a load current of -10 mA and
a supply voltage of +5V, the output voltage would be
-4.3V.
The dynamic output impedance of the TC7660S is due,
primarily, to capacitive reactance of the charge transfer
capacitor (C
1
). Since this capacitor is connected to the
output for only half of the cycle, the equation is:
EQUATION
5.2 Paralleling Devices
Any number of TC7660S voltage converters may be
paralleled to reduce output resistance (Figure 5-2). The
reservoir capacitor, C
2
, serves all devices, while each
device requires its own pump capacitor, C
1
. The resul-
tant output resistance would be approximately:
EQUATION
FIGURE 5-2: Paralleling Devices Lowers Output Impedance.
FIGURE 5-3: Increased Output Voltage By Cascading Devices.
+
V
+
+
1
2
3
4
8
7
6
5
TC7660S
V
OUT
*
C
1
10 µF
* V
OUT
= -V
+
for 1.5V  V+  12V
C
2
10 µF
X
C
2
2fC
1
----------- 3 . 1 8 ==
where:
f = 10 kHz and C1 = 10 µF.
R
OUT
R
OUT
of TC7660S
n number of devices
----------------------------------------------------=
“n”
“1”
R
L
+
V
+
+
1
2
3
4
8
7
6
5
TC7660S
C
1
C
2
+
1
2
3
4
8
7
6
5
TC7660S
C
1
V
OUT
*
“1”
+
V
+
+
1
2
3
4
8
7
6
5
TC7660S
10 µF
* V
OUT
= -n V
+
for 1.5V  V+  12V
“n”
+
1
2
3
4
8
7
6
5
TC7660S
10 µF
10 µF
+
10 µF