6

Rev. D

04/17/02

HV302 / HV312

Design Information

Programming Under and Over Voltage Shut Down

The UV and OV pins are connected to comparators with nominal

1.21V thresholds and 100mV of hysteresis (1.21V

± 50mV). They

are used to detect under voltage and over voltage conditions at the

input to the circuit. Whenever the OV pin rises above its high

threshold (1.26V) or the UV pin falls below its low threshold

(1.16V) the GATE voltage is immediately pulled low, the PWRGD

pin changes to its inactive state and the external capacitor

connected to the RAMP pin is discharged.

Calculations can be based on either the desired input voltage

operating limits or the input voltage shutdown limits.

In the

following equations the shutdown limits are assumed.

The under voltage and over voltage shut down thresholds can be

programmed by means of the three resistor divider formed by R1,

R2 and R3. Since the input currents on the UV and OV pins are

negligible the resistor values may be calculated as follows:

R3

R2

R1

R3

R2

V

1.16

V

UV

EEUV(off)

UVL

OFF

+

+

+

×

=

=

=

R3

R2

R1

R3

V

1.26

V

OV

EEOV(off)

OVH

OFF

+

+

×

=

=

=

Where

Down Threshold points.

If we select a divider current of 100

µA at a nominal operating input

voltage of 50 Volts then

500kΩ

100uA

50V

R3

R2

R1

=

=

+

+

From the second equation for an OV shut down threshold of 65V

the value of R3 may be calculated.

500kΩ

R3

65

1.26

OV

OFF

×

=

=

9.69kΩ

65

500KΩ

1.26

R3

=

×

=

The closest 1% value is 9.76k

Ω

From the first equation for a UV shut down threshold of 35V the

value of R2 can be calculated.

()

500KΩ

R3

R2

35

1.16

UV

OFF

+

×

=

=

6.81kΩ

9.76kΩ

35

500kΩ

1.16

R2

=

−

×

=

The closest 1% value is 6.81k

Ω

Then

483kΩ

R3

R2

500KΩ

R1

=

−

−

=

The closest 1% value is 487k

Ω

Under Voltage/Over Voltage Operation

From the calculated resistor values the OV and UV start up

threshold voltages can be calculated as follows:

R3

R2

R1

R3

R2

V

1.26

V

UV

EEUV(on)

UVH

ON

+

+

+

×

=

=

=

R3

R2

R1

R3

V

1.16

V

OV

EEOV(on)

OVL

ON

+

+

×

=

=

=

Where

Up Threshold points.

Then

R3

R2

R3

R2

R1

1.26

V

EEUV(on)

+

+

+

×

=

38.29V

9.76kΩ

6.81kΩ

9.76kΩ

6.81kΩ

487kΩ

1.26

V

EEUV(on)

=

+

+

+

×

=

And

R3

R3

R2

R1

1.16

V

EEOV(on)

+

+

×

=

59.85V

9.76kΩ

9.76kΩ

6.81kΩ

487kΩ

1.16

V

EEOV(on)

=

+

+

×

=

Therefore, the circuit will start when the input supply voltage is in

the range of 38.29V to 59.85V.

Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com