AN804 datasheet(2/4 Pages) VISHAY

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AN804

Vishay Siliconix

www.vishay.com

S FaxBack 408-970-5600

Document Number: 70611

10-Mar-97

If an n-channel, enhancement-mode MOSFET were switching

a positive-polarity voltage to a grounded load, the output would

be limited to VGG – Vth.

The equations describing performance of the n-channel

ground-switching MOSFET with a ground-reference gate drive

are based on the relationship between VDD and VGG:

If VDD ≥ VGG, then VO = VGG – Vth ;

If VDD < VGG – Vth . then VO = VDD – ILrDS(on).

Sustaining a more acceptable gain with an output in direct

relation to VDD would require an isolated gate drive referenced

to the source (Figure 4). The dc bias option rectifies the pulse

of ac from the transformer and stores a “+” voltage on the

gate-to-source capacitance of the MOSFET. The RC

determines the turn off time.

Bootstrapping the n-channel MOSFET (Figure

2) is

satisfactory for short turn-on times of a few milliseconds. In this

arrangement, both MOSFETs must have breakdown voltage

specifications that match or exceed the supply voltage.

Using a p-channel MOSFET may place some severe

restraints on the gate drive, since the gate must be close to

VDD (Figure 1b). To return gate control to a more acceptable

logic format, add an n-channel MOSFET (Figure 3).

Using an n-channel MOSFET in this way simplifies the gate

drive for a high-voltage, high-side, p-channel MOSFET.

Placing a Zener diode between the gate and supply ensures

that V(BR)GSS will not be exceeded. Again, both MOSFETs

must withstand the full rail voltage.

N-Channel

TTL

N-Channel

VDD

FIGURE 2. Bootstrapping for N-Channel

Grounded-Load Switching

N-Channel

TTL

P-Channel

Load

15 V

VDD – ILrDS

FIGURE 3. Using An N-Channel Level-Shifter

Simplifies Driving From Logic

VDD

Load

FIGURE 4. Floating Gate Drive

dc Bias

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