What causes Iib spikes

AN4587

4/14

DocID026923 Rev 1

Two things cause these current spikes on the inputs:

•

Charge injection

•

Clock feed through

They are described below.

1.1

Charge injection

Figure 3 shows that when the transistor MOS is ON, the electrons can travel through the

channel from the source to the drain (in this example the MOS is a NMOS).

But, when the transistor MOS is OFF, the channel is closed, and all charges are dispersed

into the source and drain as shown in Figure 4.

The total charge in the channel of a MOS switch is given by Equation 1.

Equation 1

Where:

•

Am is the area of the MOS gate

•

Cox is the oxide capacitance per unit area

•

Vgs can also be defined as Vcc-Vin where Vcc is directly linked to the power supply of

the TSZ121 and Vin is the input common mode voltage (gate voltage is at Vcc when

the transistor is ON).

When the transistor is switched OFF the charges are evacuated resulting in a current.

So, the spikes appearing on the input pins of the TSZ121 are also linked to the power

supply.

Figure 3. MOS ON

Figure 4. MOS OFF

QAm

Cox Vgs Vth

–

()

⋅

=

(1)