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Zakładka z wyszukiwarką danych komponentów |
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TC7662AMJA Arkusz danych(PDF) 3 Page - TelCom Semiconductor, Inc |
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TC7662AMJA Arkusz danych(HTML) 3 Page - TelCom Semiconductor, Inc |
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3 / 5 page  4-79 TELCOM SEMICONDUCTOR, INC. 7 6 5 4 3 1 2 8 Note one of its characteristics is ESR (equivalent series resistance). This parasitic resistance winds up in series with the load. Thus, both voltage and power conversion effi- ciency are compromised if a low ESR capacitor is not used. For example, in the "Test Circuit", changing CP and CR capacitors from typical ESR to low ESR types, the effective converter output impedance changed from 45 Ω to 40Ω, an improvement of 12%. This applies to all types of capacitors, including film types (polyester, polycarbonate etc.). Some applications information suggests that the ca- pacitor is not critical and attributes the limiting factor to the capacitor's reactance value. Let's examine this: where DS (duty cycle) = 50%. Thus, ZC ≈ 1.33Ω at f = 12kHz, where C = 10 µF. For the TC7662A, f = 12,000Hz, and a typical value of C would be 10 µF. This is a reactive impedance of ≈1.33Ω. If the ESR is as great as 5 Ω, the reactive value is not as critical as it would first appear, since the ESR would dominate. The 5 Ω value is typical of a general-purpose electrolytic capacitor. Synchronizing The TC7662A may be synchronized by connecting pin 7 of the TC7662A through a 100k resistor in series with a diode to a negative-going pulse source. The negative pulse voltage can be +5V with a 5 microsecond duration going negative to 0V. TEST CIRCUIT Figure 2. Synchronization XC = and ZC = , XC DS APPLICATIONS INFORMATION Theory of Operation The TC7662A is a capacitive charge pump (some- times 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 C1 to the supply voltage, VDD. This assumes that the ON resistance of the MOSFETs in series with the capacitor produce a charging time (3 time con- stants) less than the ON time provided by the oscillator frequency, as shown: 3 (RDS(ON) C1) <C1/(0.5 fOSC). In the next cycle, SW1 and SW2 are turned OFF and, after a very short interval with all switches OFF (preventing large currents from occurring due to cross conduction), SW3 and SW4 are turned ON. The charge in C1 is then transferred to COUT, BUT WITH THE POLARITY IN- VERTED. In this way, a negative voltage is derived. An oscillator supplies pulses to a flip-flop that is 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 a pin that controls the frequency of oscillation. Pin 7 can have a capacitor added that is con- nected to ground. This will lower the frequency of the oscillator by adding capacitance to the internal timing ca- pacitor of the TC7662A. (See Oscillator Frequency vs. CEXT, page 5.) Capacitors In early charge pump converters, capacitors were not considered critical due to the high RDS(ON) of the MOSFET switches. In order to understand this, let’s look at a model of a typical electrolytic capacitor (Figure 1). 1 2 πf C TC7662A 1 2 3 4 8 7 5 CP + 10 µF COSC R L V (–5V) OUT 10 µF C R I L IS V (+5V) + NC NC 6 + TTL Q Q 100 k TO PIN 7 TC7662A Figure 1. Capacitor Equivalent Circuit EPR ESL ESR C CHARGE PUMP DC-TO-DC CONVERTER TC7662A |
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