TC1240 Datasheet, PDF (4/9 Pages) Microchip Technology – Positive Doubling Charge Pump with Shutdown in SOT Package

Positive Doubling Charge Pump with
Shutdown in SOT Package
TC1240
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 1(b)).
Table 1 shows various values of C1 and the correspond-
ing output resistance values @ +25°C. It assumes a 0.1Ω
ESR_C1 and 1.2Ω R_SW. Table 2 shows the output voltage
ripple for various values of C2. The V_RIPPLE values assume
5 mA output load current and 0.1Ω ESR_C2.
C3
5 _OUT
CC1+ ⁶
1
TC1240
VIN
C1
3 _{C1––}
4 _SHDN
2
GND
V_IN
C2
R_L
V_OUT
Table 1. Output Resistance vs. C1 (ESR = 0.1Ω)
C1 (µF)
0.47
1
2.2
3.3
4.7
10
47
100
TC1240
R_OUT(Ω)
47
28.5
19.5
17
15.5
13.6
12.5
12.2
Table 2. Output Voltage Ripple vs. C2 (ESR = 0.1Ω) I_OUT 5mA
C1 (µF)
TC1240
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
Device
TC1240
C1
3.3µF
C2
3.3µF
C3
3.3µF
Voltage Doubler
Figure 3. Test Circuit
VOLTAGE DOUBLER
The most common application for charge pump
devices is the doubler (Figure 3). This application uses
two external capacitors - C1 and C2 (plus a power supply
bypass capacitor, if necessary). The output is equal to 2 x
V_IN minus any voltage drops due to loading. Refer to Table
1 and Table 2 for capacitor selection.
V_IN
V_IN ¹
V_IN ¹
6 _C+
C1A
2
TC1240
GND
6 _C+
C1B
TC1240
2 _GND
3 _C–
"1"
5
OUT
4
SHDN
C2A
3 _{C– "n"}
4 _SHDN
OUT ⁵
V_OUT
C2B
V_OUT = (n + 1)V_IN
INPUT SUPPLY BYPASSING
The V_IN input should be capacitively bypassed to reduce
AC impedance and minimize noise effects due to the switch-
ing internal to the device. The recommended capacitor
should be a large value (at least equal to C1) connected from
the input to GND.
SHUTDOWN INPUT
TheTC1240 is disabled when SHDN is high, and
enabled when SHDN is low. This input cannot be allowed
to float.
Figure 4. Cascading Multiple Devies to Increase Output Voltage
CASCADING DEVICES
Two or more TC1240s can be cascaded to increase
output voltage (Figure 4). If the output is lightly loaded, it will
be close to ((n + 1) x V_IN) but will droop at least by R_OUT of
the first device multiplied by the I_Q of the second. It can be
seen that the output resistance rises rapidly for multiple
cascaded devices. For the case of the two-stage ‘tripler’output
resistance can be approximated as R_OUT = 2 x R_OUT1 +
R_OUT2, where R_OUT1 is the output resistance of the first
stage, and R_OUT2 is the output resistance of the second
stage.
TC1240-1 7/7/00
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