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SP682EU Datasheet(PDF) 4 Page - Sipex Corporation |
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SP682EU Datasheet(HTML) 4 Page - Sipex Corporation |
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4 / 8 page  4 Date: 5/20/04 SP682 Micro Power Inverting Charge Pump © Copyright 2004 Sipex Corporation Table 1. R OUT .vs. C1, C2 Table 2. C3 .vs. V RIPPLE SHUTDOWN FEATURE The SP682 charge pump includes a shutdown feature (pin 8) which disables the charge pump when the V OUT is not needed. A logic "1" will activate the shutdown mode. If shutdown is not needed, it can be left open where an internal pull-down resistor will always keep the charge pump active. Typical input current for the shutdown pin is 3 µA. The shutdown feature is another option for conserving power in portable applications, reducing current to only 1 µA. PARALLEL DEVICES Multiple SP682 charge pumps can be connected in parallel. However, the effective output resis- tance now is the output resistance of a single device divided by the number of devices. Con- necting multiple pumps allows the user to save on the storage capacitor. The charge pump capacitors still must be separate for each device. APPLICATIONS INFORMATION The SP682 charge pump produces a doubled, inverted voltage from the V CC input. As such, it can serve in many applications where a negative –5V to –10V output is needed. Typical applica- tions include powering analog switches, and biasing LCD displays and panels. The ESR of the charge pump capacitors also determine the output resistance. Assuming that switch resistances are approximately equal, the output resistance can be derived as shown be- low: R OUT = 16*(RSW1-4) + 4*( ESRC1+ ESRC2) + ESRC3 + 1 / (f OSC * C1) + 1 / (fOSC * C2) R OUT is typically 140Ω at +25°C with VCC at +5V using 3.3 µF capacitors. The total internal switch resistance (16*R SW) is approximately 90 Ω. The table below shows the comparison of R OUT versus C1&C2. C1, C2 ( µF) ROUT ( Ω) 0.05 4085 0.10 2084 0.47 510 1.00 285 3.30 140 4.70 125 10.00 105 22.00 94 The output voltage ripple is also affected by the capacitors, specifically C OUT. Larger values will reduce the output ripple for a given output current load of current. The formula represen- tation is: V RIPPLE = {1 / [2 * (fOSC * C3)] + 2 * (ESRC3)} * IOUT To minimize the output ripple, the C OUT storage capacitor can be increased to over 10 µF whereas the pump capacitors can range from 1 µF to 5µF. Table 2 shows the typical V RIPPLE for given C OUT values. C OUT (µF) V RIPPLE (mV) 0.50 1020 1.00 520 3.30 172 4.70 120 10.00 70 22.00 43 THEORY OF OPERATION |
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