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LTC1755 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1755 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LTC1755/LTC1756 10kV ESD Protection All Smart Card pins (CLK, RST, I/O, AUX1, AUX2, VCC and GND) can withstand over 10kV of human body model ESD in situ. In order to ensure proper ESD protection, careful board layout is required. The GND pin should be tied directly to a ground plane. The VCC capacitor should be located very close to the VCC pin and tied immediately to the ground plane. Capacitor Selection The style and value of capacitors used with the LTC1755/ LTC1756 determine several parameters such as output ripple voltage, charge pump strength, Smart Card switch debounce time and VCC discharge rate. Due to the switching nature of a capacitive charge pump, low equivalent series resistance (ESR) capacitors are recommended for the capacitors at VIN and VCC. When- ever the flying capacitor is switched to the VCC charge storage capacitor, considerable current flows. The prod- uct of this high current and the ESR of the output capacitor can generate substantial voltage spikes on the VCC output. These spikes may cause problems with the Smart Card or may interfere with the regulation loop of the LTC1755/ LTC1756. Therefore, ceramic or tantalum capacitors are recommended rather than higher ESR aluminum capaci- tors. Between ceramic and tantalum, ceramic capacitors generally have the lowest ESR. Some manufacturers have developed low ESR tantalum capacitors but they can be expensive and may still have higher ESR than ceramic types. Thus, while they cannot be avoided, ESR spikes will typically be lowest when using ceramic capacitors. For ceramic capacitors there are several different materi- als available to choose from. The choice of ceramic material is generally based on factors such as available capacitance, case size, voltage rating, electrical perfor- mance and cost. For example, capacitors made of Y5V material have high packing density, which provides high capacitance for a given case size. However, Y5V capaci- tors tend to lose considerable capacitance over the – 40 °C to 85 °C temperature range. X7R ceramic capacitors are more stable over temperature but don’t provide the high packing density. Therefore, large capacitance values are generally not available in X7R ceramic. The value and style of the flying capacitor are important not only for the charge pump but also because they provide the large debounce time for the Smart Card detection channel. A 0.68 µF X7R capacitor is a good choice for the flying capacitor because it provides fairly constant capacitance over temperature and its value is not prohibitively large. The charge storage capacitor on the VCC pin determines the ripple voltage magnitude and the discharge time of the Smart Card voltage. To minimize ripple, generally, a large value is needed. However, to meet the VCC discharge rate specification, the value should not exceed 20 µF. A 10µF capacitor can be used but the ripple magnitude will be higher leading to worse apparent DC load regulation. Typically a 15 µF to 18µF Y5V ceramic capacitor is the best choice for the VCC charge storage capacitor. For best performance, this capacitor should be connected as close as possible to the VCC and GND pins. Note that most of the electrostatic discharge (ESD) current on the Smart Card pins is absorbed by this capacitor. The bypass capacitor at VIN is also important. Large dips on the input supply due to ESR may cause problems with the internal circuitry of the LTC1755/LTC1756. A good choice for the input bypass capacitor is a 10 µF Y5V style ceramic Dynamic Pull-Up Current Sources The current sources on the bidirectional pins (DATA, AUX2IN, AUX1IN, I/O, AUX2 and AUX1) are dynamically activated to achieve a fast rise time with a relatively small static current (Figure 1). Once a bidirectional pin is relin- quished, a small start-up current begins to charge the node. An edge rate detector determines if the pin is Figure 1. Dynamic Pull-Up Current Sources – + δV δt ISTART 17556 F01 VREF BIDIRECTIONAL PIN VCC OR DVCC APPLICATIO S I FOR ATIO |
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