HV320

6

Design Information

Setting Under Voltage and Over Voltage Shut Down

The UV and OV pins are connected to comparators with

typical 1.26V thresholds and 100mV of hysteresis. 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.

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:

UV

OV

Where •V

Over Voltage Shut Down Threshold points.

If we select a divider current of 100

µA at a nominal

operating input voltage of 50 Volts, then

R1+R2+R3 = 50V/100uA = 500k Ohm

From the second equation, for an OV shut down threshold

of 65V, the value of R3 may be calculated.

OV

R3 = (1.26x 500k)/65 = 9.69k

The closest 1% value is 9.76k Ohm.

From the first equation, for a UV shut down threshold of

35V, the value of R2 can be calculated.

UV

R2 = ((1.16 x 500k)/35) – 9.76k = 6.81k

The closest 1% value is 6.81k Ohm.

Then

R1 = 500k – R2 – R3 = 483k Ohm.

The closest 1% value is 487K Ohm.

From the calculated resistor values the OV and UV start

up threshold voltages can be calculated as follows:

UV

OV

Where •V

EEUV(on)

• and •V

EEOV(on)

• are Under and Over

Voltage Start Up Threshold points relative to Vee.

Then

•V

•V

= 38.29V

and

•V

•V

Therefore, the circuit will start when the input supply

voltage is in the range of 38.29V to 59.85V. To overcome

longer bounce time during insertion, POR time must be

extended. An additional cap C1 (Page 1) must be added

from the UV pin to V

The value of this cap can be

calculated accordingly:

VC1 ( t ) = V

)

Where

VC1= 1.26

V

t

Req = R1

⎢⎢ (R2 + R3)

From the above C1 can be calculated:

t

POR

Req x 1.60

For example for t

Values show on Page 1, the C1 calculates to be:

487 x (6.81 + 9.76)

487 + 6.81 + 9.76

10 x 10

-3

16000 x 1.60

C1

≅ .39µF

a .47

µF can be used.

Under Voltage/Over Voltage Operation

GND

UV

OFF

UV

ON

Vin

OV

ON

OV

OFF

Pass

Transistor

ON

OFF

(

Req =

= 16K

C1 =

C1 =

Req x C1

)