TCM828ECT713 Datasheet, PDF (4/9 Pages) Microchip Technology – Switched Capacitor Voltage Converters

TCM828
TCM829
Switched Capacitor
Voltage Converters
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 2Ω R_SW. Table 2 shows the output voltage ripple
for various values of C2. The V_RIPPLE values assume 10mA
output load current and 0.1Ω ESR_C2.
Table 1. Output Resistance vs. C1 (ESR = 0.1Ω)
C1(µF)
TCM828 R_OUT (Ω) TCM829 R_OUT (Ω)
0.1
850
302
1
100
45
3.3
42
25
10
25
19
47
18
17
100
17
17
Table 2. Output Voltage Ripple vs. C2 (ESR = 0.1Ω) I_OUT 10mA
C2(µF)
TCM828 V_RIPPLE (mV) TCM829 V_RIPPLE (mV)
1
835
286
3.3
254
88
10
85
31
47
20
8
100
10
5
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
depends on the configuration of the TCM828/829.
If the device is loaded from OUT to GND it is recom-
mended that a large value capacitor (at least equal to C1) be
connected from the input to GND. If the device is loaded
from IN to OUT a small (0.1µF) capacitor from IN to OUT is
sufficient.
Voltage Inverter
The most common application for charge pump devices
is the inverter (Figure 3). This application uses two external
capacitors – C1 and C2 (plus a power supply bypass
capacitor, if necessary). The output is equal to V–_IN plus any
voltage drops due to loading. Refer to Table 1 and Table 2
for capacitor selection.
V_IN
C3
3.3µF*
1 _OUT
C1+ ⁵
2
IN
TCM828
TCM829
C1
3.3µF*
3 _C1–
4
GND
C2
3.3µF*
R_L
*10µF (TCM828)
Voltage Inverter
Figure 3. Test Circuit
V_OUT
Cascading Devices
Two or more TCM828/829’s can be cascaded to
increase output voltage (Figure 4). If the output is lightly
loaded, it will be close to (– 2 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 large negative voltage
requirements see the TC682 or TCM680 data sheets.
...
V_I⁺_N
2
2
3
3
C1
4 _TCM828
C1
4 _TCM828
TCM829
TCM829
5
"1"
1
5
"n"
1
V_OUT
...
C2
C2
V_OUT = –nV_IN
Figure 4. Cascading TCM828s or TCM829s to Increase Output Voltage
Paralleling Devices
To reduce the value of R_OUT, multiple TCM828/829s
can be connected in parallel (Figure 5). The output resis-
tance will be reduced by a factor of N where N is the number
of TCM828/829’s. Each device will require it’s own pump
capacitor (C1), but all devices may share one reservoir
capacitor (C2). However, to preserve ripple performance the
value of C2 should be scaled according to the number of
paralleled TCM828/829’s.
TCM828/829-4 5/22/00
4
© 2001 Microchip Technology Inc. DS21488A