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CAT661 Datasheet(PDF) 9 Page - Catalyst Semiconductor |
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CAT661 Datasheet(HTML) 9 Page - Catalyst Semiconductor |
9 / 16 page CAT661 9 Doc. No. 5003, Rev. J VRIPPLE (mV) IOUT (mA) FOSC (kHz) C2 ( µF) C2 ESR ( Ω) 45 100 25 150 0.2 25 100 135 150 0.2 CAPACITOR SELECTION Low ESR capacitors are necessary to minimize voltage losses, especially at high load currents. The exact values of C1 and C2 are not critical but low ESR capacitors are necessary. The ESR of capacitor C1, the pump capacitor, can have a pronounced effect on the output. C1 currents are approximately twice the output current and losses occur on both the charge and discharge cycle. The ESR effects are thus multiplied by four. A 0.5 Ω ESR for C1 will have the same effect as a 2 Ω increase in CAT661 output impedance. Output voltage ripple is determined by the value of C2 and the load current. C2 is charged and discharged at a current roughly equal to the load current. The internal switching frequency is one-half the oscillator frequency. VRIPPLE = IOUT/(FOSC x C2) + IOUT x ESRC2 For example, with a 25kHz oscillator frequency (12.5kHz switching frequency), a 150 µF C2 capacitor with an ESR of 0.2 Ω and a 100mA load peak-to-peak the ripple voltage is 45mV. VRIPPLE vs. FOSC Figure 6. Oscillator Figure 7. External Clocking BOOST/FC (1) LV (6) OSC (7) ~18pF I 7.0 I 7.0 I I V+ + -V+ C2 1 2 3 4 8 7 6 5 REQUIRED FOR TTL LOGIC CAT661 V+ 100k OSC INPUT NC + C1 BOOST/FC CAP+ GND CAP- V+ OSC LV OUT |
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