TC1240_13 Datasheet, PDF (8/18 Pages) Microchip Technology – Positive Doubling Charge Pumps with Shutdown in a SOT-23 Package

TC1240/TC1240A
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)(C₁) 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:
VRIPPLE = ₂----_--_F----^IS--^O--_W--^U---_--^T_---_C----₂----_ + 2IOUTESRC2
f
V+
C₁
V_OUT
C₂
R_L
FIGURE 5-1:
Model.
Ideal Switched Capacitor
R_EQUIV
V+
R_EQUIV
=
1
F_SW
x
C₁
C₂
V_OUT
R_L
FIGURE 5-2:
Resistance.
Equivalent Output
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 C₁ will
lower the output resistance and larger values of C₂ will
reduce output ripple (see Equation 5-1b).
Table 5-1 shows various values of C₁ 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
C₂. The V_RIPPLE values assume 5mA output load
current and 0.1 ESR_C2.
TABLE 5-1:
C₁ (µF)
0.47
1
2.2
3.3
4.7
10
47
100
OUTPUT RESISTANCE
VS. C₁ (ESR = 0.1)
TC1240
R_OUT()
TC1240A
R_OUT()
47
35
28.5
20.5
19.5
14
17
12
15.5
10.5
13.6
9.3
12.5
8.3
12.2
8.1
TABLE 5-2:
OUTPUT VOLTAGE RIPPLE
VS. C₂ (ESR = 0.1)
I_OUT 5 mA
C₁ (µF)
TC1240/TC1240A
V_RIPPLE (mV)
0.47
142
1
67
2.2
30
3.3
20
4.7
14
10
6.7
47
2.5
100
1.6
DS21516D-page 8
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