7

LTC1756

10kV ESD Protection

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

plane.

Capacitor Selection

The style and value of capacitors used with the LTC1756

determine several parameters such as output ripple volt-

age, charge pump strength, Smart Card switch debounce

Due to the switching nature of a capacitive charge pump,

low equivalent series resistance (ESR) capacitors are

storage capacitor, considerable current flows. The prod-

uct of this high current and the ESR of the output capacitor

These spikes may cause problems with the Smart Card or

may interfere with the regulation loop of the LTC1756.

Therefore, ceramic or tantalum capacitors are recom-

mended rather than higher ESR aluminum capacitors.

Between ceramic and tantalum, ceramic capacitors gener-

ally 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 ripple voltage magnitude and the discharge time of the

Smart Card voltage. To minimize ripple, generally, a large

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

performance, this capacitor should be connected as close

electrostatic discharge (ESD) current on the Smart Card

pins is absorbed by this capacitor.