Datasheet

TC1240/TC1240A
DS21516D-page 8  2001-2012 Microchip Technology Inc.
The switching frequency in Equation 5-1b is defined as
one-half the oscillator frequency (i.e., F
SW
= F
OSC
/2).
The 1/(F
SW
)(C
1
) term in Equation 5-1b is the effective
output resistance of an ideal switched capacitor circuit
(Figure 5-1 and Figure 5-2).
The output voltage ripple is given by Equation 5-2.
EQUATION 5-2:
FIGURE 5-1: Ideal Switched Capacitor
Model.
FIGURE 5-2: Equivalent Output
Resistance.
5.3 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 ripple (see Equation 5-1b).
Table 5-1 shows various values of C
1
and the
corresponding output resistance values @ +25°C. It
assumes a 0.1 ESR
C1
and 0.9 R
SW
. Table 5-2
shows the output voltage ripple for various values of
C
2
. The V
RIPPLE
values assume 5mA output load
current and 0.1 ESR
C2
.
TABLE 5-1: OUTPUT RESISTANCE
VS. C
1
(ESR = 0.1)
TABLE 5-2: OUTPUT VOLTAGE RIPPLE
VS. C
2
(ESR = 0.1)
I
OUT
5mA
V
RIPPLE
I
OUT
2F
SW
C
2

--------------------------------
2I
OUT
ESR
C2
+=
V+ V
OUT
R
L
C
2
C
1
f
F
SW
x C
1
1
V+ V
OUT
R
EQUIV
R
EQUIV
=
R
L
C
2
C
1
(µF)
TC1240
R
OUT
()
TC1240A
R
OUT
()
0.47 47 35
128.520.5
2.2 19.5 14
3.3 17 12
4.7 15.5 10.5
10 13.6 9.3
47 12.5 8.3
100 12.2 8.1
C
1
(µF)
TC1240/TC1240A
V
RIPPLE
(mV)
0.47 142
167
2.2 30
3.3 20
4.7 14
10 6.7
47 2.5
100 1.6