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TC962CPA Folha de dados(PDF) 3 Page - TelCom Semiconductor, Inc |
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TC962CPA Folha de dados(HTML) 3 Page - TelCom Semiconductor, Inc |
3 / 5 page 4-39 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 HIGH CURRENT CHARGE PUMP DC -TO-DC CONVERTER TC962 This applies to all types of capacitors, including film types (polyester, polycarbonate, etc.). Some applications information suggest that the capaci- tor is not critical and attribute the limiting factor of the capacitor to its reactive value. Let's examine this: where DS (duty cycle) = 50%. Thus, ZC ≈ 2.6Ω at f = 12kHz, where C = 10µF. For the TC962, f = 12,000 Hz, and a typical value of C would be 10 µF. This is a reactive impedance of 2.6 Ω. If the ESR is as great as 5 Ω, the reactive value is not as critical as it would first appear, as the ESR would predominate. The 5 Ω value is typical of a general-purpose electrolytic capacitor. TEST CIRCUIT Latch Up All CMOS structures contain a parasitic SCR. Care must be taken to prevent any input from going above or below the supply rail, or latch up will occur. The result of latch up is an effective short between VDD and VSS. Unless the power supply input has a current limit, this latch-up phenomena will result in damage to the device. (See Application Note 31 for additional information.) APPLICATIONS INFORMATION Theory of Operation The TC962 is a capacitive pump (sometimes called a switched capacitor circuit), where four MOSFET switches control the charge and discharge of a capacitor. The functional diagram (page 1) shows how the switch- ing action works. SW1 and SW2 are turned on simulta- neously, charging CP to the supply voltage, VIN. This as- sumes that the on resistance of the MOSFETs in series with the capacitor results in a charging time (3 time con- stants) that is less than the on time provided by the oscilla- tor frequency as shown: 3 (RDS(ON) CP) <CP/(0.5 fOSC) In the next cycle, SW1 and SW2 are turned off and after a very short interval of all switches being off (this prevents large currents from occurring due to cross con- duction), SW3 and SW4 are turned on. The charge in CP is then transferred to CR, BUT WITH THE POLARITY IN- VERTED. In this way, a negative voltage is now derived. Page 1 shows a functional diagram of the TC962. An oscillator supplies pulses to a flip-flop that is then fed to a set of level shifters. These level shifters then drive each set of switches at one-half the oscillator frequency. The oscillator has two pins that control the frequency of oscillation. Pin 7 can have a capacitor added that is re- turned to ground. This will lower the frequency of the oscillator by adding capacitance to the timing capacitor internal to the TC962. Grounding pin 6 will turn on a current source and double the frequency. This will double the charge current going into the internal capacitor, as well as any capacitor added to pin 7. A zener diode has been added to the TC962 for use as a reference in building external regulators. This zener runs from pin 1 to ground. Capacitors In early charge pump converters, the capacitors were not considered critical due to the high RDS(ON) of the MOS- FET switches. In order to understand this, let’s look at a model of a typical electrolytic capacitor (Figure 1). Note that one of its characteristics is ESR (equivalent series resistance). This parasitic resistance winds up in series with the load. Thus, both voltage conversion effi- ciency and power conversion efficiency are compromised if a low ESR capacitor is not used. In the test circuit, for example, just changing two capaci- tors, CP and CR, from capacitors with unspecified ESR to low ESR-type output, impedance changes from 36 Ω to 28Ω, an improvement of 23%! XC = and ZC = , 1 2 πf C XC DS ESL ESR C Figure 1. Typical Electrolytic Capacitor TC962 1 2 3 4 8 690 Ω 7 5 CP + 10 µF COSC R L V (–5V) OUT 10 µF CR I L IS V (+5V) + NC + |
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