6

www.ixysic.com

R03

AN-500

3.5

Voltage Ramp Generator

Applications such as high voltage sweep circuits and automatic test equipment require high voltage ramps with a

linear relationship between output voltage and time. The circuit shown in

Figure 7 utilizes one depletion-mode

MOSFET to design a voltage-ramp generator circuit.

Figure 7

High Voltage Ramp Generator with Depletion-Mode and Enhancement-Mode N-Channel MOSFETs

Q1 is configured as a constant current source charging a capacitor, C1; R1 provides negative feedback to regulate

and set the desired current value. The constant current source charges the capacitor C1, and generates a voltage

voltage by discharging the capacitor to ground through R2. Resistor R2 is used to limit the discharge current for Q2

to operate within its SOA rating.

Assume the ramp voltage:

The value of capacitor C1 should be small enough to reduce excessive charging and discharging of energy, but

large enough that output loads and stray capacitances will not introduce significant errors. C1 is chosen to be 1nF.

The charging current is defined as:

The value of R1 for a 100

A current source can be approximated:

Where:

Time (ms)

0123456789

10

0

100

200

300

400

500

600

V

OUT

0

10

20

30

40

50

60

V

IN

+V

DD

Q1

CPC3980

D

S

R1

17k

Ω

C1

1nF

I

D

V

OUT

R2

25k

Ω

Q2

IXTP02N120P

V

IN

G

dV

dt

-------0.1V/

s

=

IC1

dV

dt

-------



=

Equation (3)

I

1nF 0.1V/

s



100

A

==

R1

---------

---------- 1

–









