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TC1121CUA Datenblatt(PDF) 5 Page - Microchip Technology |
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TC1121CUA Datenblatt(HTML) 5 Page - Microchip Technology |
5 / 14 page 2002 Microchip Technology Inc. DS21358B-page 5 TC1121 3.0 APPLICATIONS 3.1 Negative Voltage Converter The TC1121 is typically used as a charge-pump voltage inverter. C1 and C2 are the only two external capacitors used in the operating circuit (Figure 3-1). FIGURE 3-1: CHARGE PUMP INVERTER The TC1121 is not sensitive to load current changes, although its output is not actively regulated. A typical output source resistance of 11.8 Ω means that an input of +5V results in -5V output voltage under light load, and only decreases to -3.8V typ with a 100mA load. The supplied output current is from capacitor C2 during one-half the charge-pump cycle. This results in a peak-to-peak ripple of: VRIPPLE = IOUT/2(fPUMP) (C2) + IOUT (ESRC2) Where fPUMP is 5kHz (one half the nominal 10kHz oscillator frequency), and C2 = 150 µF with an ESR of 0.2 Ω, ripple is about 90mV with a 100mA load current. If C2 is raised to 390 µF, the ripple drops to 45mV. 3.2 Changing Oscillator Frequency The TC1121’s clock frequency is controlled by four modes: TABLE 3-1: OSCILLATOR FREQUENCY MODES The oscillator runs at 10kHz (typical) when FC and OSC are not connected. The oscillator frequency is lowered by connecting a capacitor between OSC and GND, but FC can still multiply the frequency by 20 times in this mode. An external clock source that swings within 100mV of V+ and GND may overdrive OSC in the inverter mode. OSC can be driven by any CMOS logic output. When OSC is overdriven, FC has no effect. Note that the frequency of the signal appearing at CAP+ and CAP– is half that of the oscillator. In addition, by lowering the oscillator frequency, the effective output resistance of the charge-pump increases. To compensate for this, the value of the charge-pump capacitors may be increased. Because the 5kHz output ripple frequency may be low enough to interfere with other circuitry, the oscillator frequency can be increased with the use of the FC pin or an external oscillator. The output ripple frequency is half the selected oscillator frequency. Although the TC1121’s quiescent current will increase if the clock frequency is increased, it allows smaller capacitance values to be used for C1 and C2. 3.3 Capacitor Selection In addition to load current, the following factors affect the TC1121 output voltage drop from its ideal value 1) output resistance, 2) pump (C1) and reservoir (C2) capacitor ESRs and 3) C1 and C2 capacitance. The voltage drop is the load current times the output resistance. The loss in C2 is the load current times C2’s ESR; C1’s loss is larger because it handles currents greater than the load current during charge-pump operation. Therefore, the voltage drop due to C1 is about four times C1’s ESR multiplied by the load current, and a low (or high) ESR capacitor has a greater impact on performance for C1 than for C2. In general, as the TC1121’s pump frequency increases, capacitance values needed to maintain comparable ripple and output resistance diminish proportionately. 1 2 .4V to 5.5V UT C1121 ND SC AP AP C UT N HDN SHDN should be tied to V N if not used. SHDN* FC OSC Oscillator Frequency Open Open 10kHz FC = V+ Open 200kHz Open or FC = V+ External Capacitor See Typical Operating Characteristics Open External Clock External Clock Frequency |
Ähnliche Teilenummer - TC1121CUA |
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Ähnliche Beschreibung - TC1121CUA |
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