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TCM829ECT Datasheet(PDF) 4 Page - Microchip Technology |
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TCM829ECT Datasheet(HTML) 4 Page - Microchip Technology |
4 / 9 page 4 TCM828/829-4 5/22/00 Switched Capacitor Voltage Converters TCM828 TCM829 © 2001 Microchip Technology Inc. DS21488A 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 ESRC1 and 2Ω RSW. Table 2 shows the output voltage ripple for various values of C2. The VRIPPLE values assume 10mA output load current and 0.1 Ω ESRC2. Table 1. Output Resistance vs. C1 (ESR = 0.1 Ω) C1( µF) TCM828 ROUT ( Ω) TCM829 ROUT ( 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 = 0.1 Ω) I OUT 10mA C2( µF) TCM828 VRIPPLE (mV) TCM829 VRIPPLE (mV) 1 835 286 3.3 254 88 10 85 31 47 20 8 100 10 5 Input Supply Bypassing The VIN 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 (0.1 µF) 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. 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 VIN) but will droop at least by ROUT 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. Paralleling Devices To reduce the value of ROUT, 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. Figure 3. Test Circuit C1 C1 C2 5 5 4 3 4 1 2 2 1 3 C2 V IN V OUT V OUT = –nVIN + TCM828 TCM829 "1" TCM828 TCM829 "n" . . . . . . Figure 4. Cascading TCM828s or TCM829s to Increase Output Voltage 3 2 4 5 1 C3 3.3 µF* C1 3.3 µF* C2 3.3 µF* V IN V OUT R L TCM828 TCM829 C1– IN OUT C1+ GND Voltage Inverter *10 µF (TCM828) |
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