Datasheet

TC7662A
DS21468B-page 6 2001-2012 Microchip Technology Inc.
4.0 TYPICAL APPLICATIONS
4.1 Simple Negative Voltage
Converter
The majority of applications will undoubtedly utilize the
TC7662A for generation of negative supply voltages.
Figure 4-1 shows typical connections to provide a
negative supply where a positive supply of +3V to +18V
is available.
FIGURE 4-1: SIMPLE NEGATIVE
CONVERTER AND ITS
OUTPUT EQUIVALENT
The output characteristics of the circuit in Figure 4-1
are those of a nearly ideal voltage source in series with
a resistance as shown in Figure 4-1b. The voltage
source has a value of -(V
DD
). The output impedance
(R
O
) is a function of the ON resistance of the internal
MOS switches (shown in the Functional Block
Diagram), the switching frequency, the value of C
P
and
C
R
, and the ESR (equivalent series resistance) of C
P
and C
R
. A good first order approximation for R
O
is:
Combining the four R
SWX
terms as R
SW
, we see that:
R
SW
, the total switch resistance, is a function of supply
voltage and temperature (See Section 5.0, Typical
Characteristics “Output Source Resistance” graphs),
typically 23 at +25°C and 5V. Careful selection of C
P
and C
R
will reduce the remaining terms, minimizing the
output impedance. High value capacitors will
reduce the 1/(f
PUMP
x C
P
) component, and low ESR
capacitors will lower the ESR term. Increasing the
oscillator frequency will reduce the 1/(f
PUMP
x C
P
) term,
but may have the side effect of a net increase in output
impedance when C
P
> 10F and there is not enough
time to fully charge the capacitors every cycle. In a typ-
ical application when f
OSC
= 12kHz and C = C
P
= C
R
=
10F:
Since the ESRs of the capacitors are reflected in the
output impedance multiplied by a factor of 5, a high
value could potentially swamp out a low 1/(f
PUMP
x C
P
)
term, rendering an increase in switching frequency
or filter capacitance ineffective. Typical electrolytic
capacitors may have ESRs as high as 10.
1
2
3
4
8
7
6
5
TC7662A
10μF
+
V
DD
+
10μF
V
OUT
= -V+
V
OUT
R
O
V
DD
V
DD
V
DD
V
DD
+
AB
R
O
2(R
SW1
+ R
SW2
+ ESR
CP
) + 2(R
SW3
+ R
SW4
+
ESR
CP
) + + ESR
CR
1
f
PUMP
x C
P
(f
PUMP
= , R
SWX
= MOSFET switch resistance)
f
OSC
2
R
O
2 x R
SW
+ + 4 x ESR
CP
+ ESR
CR
1
f
PUMP
x C
P
R
O
2 x 23 + + 4 x ESR
CP
+ ESR
CR
1
(5 x 12
3
x 10 x 10
-6
)
R
O
(46 + 20 + 5 x ESR
C
)