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SP21111ECT-L Datasheet(PDF) 7 Page - Exar Corporation |
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SP21111ECT-L Datasheet(HTML) 7 Page - Exar Corporation |
7 / 19 page 7 Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 50-668-707 • www.exar.com SP207E_00_072309 The Exar charge-pump generates V+ and V- independently from V CC. Hence in a no–load condition V+ and V- will be symmetrical. Older charge pump approaches generate V+ and then use part of that stored charge to generate V-. Because of inherent losses, the magnitude of V- will be smaller than V+ on these older designs. Underlightlyloadedconditionstheintelligent pump oscillator maximizes efficiency by running only as needed to maintain V+ and V-. Since interface transceivers often spend muchoftheirtimeatidle,thispower-efficient innovation can greatly reduce total power consumption. This improvement is made possiblebytheindependentphasesequence of the Exar charge-pump design. The clock rate for the charge pump typically operates at greater than 5kHz, allowing thepumptorunefficientlywithsmall0.1µF capacitors.Efficientoperationdependson rapidly charging and discharging C and C 2, therefore capacitors should be mounted close to the IC and have low ESR (equivalent series resistance). Low cost surface mount ceramic capacitors (such as are widely used for power-supply decoupling) are ideal for use on the charge pump. However the charge pumps are designed to be able to function properly with a wide range of capacitor styles and values. If polarized capacitors are used, the positive and negative terminals should be connected as shown. Figure 4. Charge Pump — Phase 4 VCC = +5V +10V VSS Storage Capacitor VDD Storage Capacitor C1 C2 C3 C4 + + + + – – – – Voltage potential across any of the capaci- tors will never exceed 2 x V CC. Therefore capacitors with working v oltages as low as 0V rating may be used with a nominal V CC supply. C will never see a potential greater than V CC , so a working voltage of 6.3V is adequate. The reference terminal of the V DD capacitor may be connected either to V CC or ground, but if connected to ground a minimum 6V working voltage is required. Higherworkingvoltagesand/orcapacitance values may be advised if operating at higher V CC or to provide greater stability as the capacitors age. +10V a) C 2 + gND gND b) C 2 – –10V Figure 5. Typical waveforms seen on ca- pacitor C2 when all drivers are at maximum load. |
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