TCM829ECT713 Datasheet, PDF (14/26 Pages) Microchip Technology – Switched Capacitor Voltage Converters

TCM828/TCM829
5.0.3 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 5-1).
Table 5-1 shows various values of C1 and the
corresponding output resistance values @ +25°C. It
assumes a 0.1Ω ESR_C1 and 2Ω R_SW. Table 5-2 shows
the output voltage ripple for various values of C2. The
V_RIPPLE values assume 10 mA output load current and
0.1Ω ESR_C2.
TABLE 5-1: OUTPUT RESISTANCE VS. C1
(ESR = 0.1Ω)
C1 (µF)
0.1
1
3.3
10
47
100
TCM828 R_OUT (Ω)
850
100
42
25
18
17
TCM829 R_OUT (Ω)
302
45
25
19
17
17
I
TABLE 5-2:
OUTPUT VOLTAGE RIPPLE
VS. C2 (ESR = 0.1Ω) IOUT
10MA
C2 (µF)
TCM828 V_RIPPLE
(mV)
TCM829 R_OUT (Ω)
1
835
286
3.3
254
88
10
85
31
47
20
8
100
10
5
5.0.4 INPUT SUPPLY BYPASSING
The V_IN input should be capacitively bypassed to
reduce AC impedance and minimize noise effects due
to the switching internal to the device. The
recommended capacitor depends on the configuration
of the TCM828/TCM829 devices.
If the device is loaded from OUT to GND, it is
recommended 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 (0.1 µF)
capacitor is sufficient.
5.0.5 VOLTAGE INVERTER
The most common application for charge pump
devices is the inverter (Figure 5-3). 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 5-1 and Table 5-1 for capacitor
selection.
C3
3.3 µF*
1 _{OUT C1}+ 5
2 _IN
3 _C1-
C1
3.3 µF*
GND ⁴
V_OUT
C2
3.3 µF*
R_L
V_OUT
Voltage Inverter
*10 µF (TCM828)
FIGURE 5-3:
Test Circuit.
5.0.6 CASCADING DEVICES
Two or more TCM828/829 devices can be cascaded to
increase output voltage (Table 5-4). If the output is
lightly loaded, it will be close to (– 2 x VIN) but will droop
at least by R_OUT of the first device multiplied by the IQ
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^+IN
2
2
3 _TCM828/
3 _TCM828/
C1
4 ^TCM829 _C1
4 ^TCM829
5 ^"1" 1
5 ^"n" 1 ^VOUT
...
C2
C2
V_OUT = -nV_IN
FIGURE 5-4:
Cascading TCM828 or
TCM829 Devices to Increase Output Voltage.
DS21488B-page 14
© 2010 Microchip Technology Inc.