2.1.7 Charge Pump Effect Considerations

The on-resistance of a MOSFET is inversely proportional to the overdrive voltage in the region where MOSFETS are used as switches.

Conventional analog switch/multiplexers derive their overdrive voltage directly from the supply voltage and common mode input volt-

age; hence, the on-resistance varies with the supply voltage or common mode input voltage.

The TS4100-TS4102 maintains a flat on-resistance that is independent of the supply voltage or common mode input voltage. To ach-

ieve this, a charge pump scheme is implemented where a constant overdrive voltage is applied across the MOSFET. The charge pump

is refreshed at a period of 40 μs with a time period variation of up to 2X.

In applications where input and output impedance is high in the order of MΩs, transients generated by the charge pump can couple to

the input and output of the device. The pulse width of the spikes is 10-s of nanoseconds. The amplitudes of the spikes are independent

of the operating conditions, such as temperature, common mode input voltage and supply voltage.

The figures below show a scope capture of these spikes with an input/output impedance of 1 MΩ and 10 MΩ. With an input/output

impedance of 1 MΩ and 10 MΩ, the amplitude of the spikes is less than 200 μV and 500 μV, respectively.

If these spikes are of a concern in the application, placing a 500 pF capacitor to ground at the input or output will suppress the spikes.

TS4100/01/02 Data Sheet

System Overview

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