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:
Under Voltage/Over Voltage Operation
From the calculated resistor values the OV and UV start up
threshold voltages can be calculated as follows:
UV
ON
=
V
UVH
=
1.26
=
V
EEUV(on)
×
R2
+
R3
R1
+
R2
+
R3
R3
OV
OFF
=
V
OVH
=
1.26
=
V
EEOV(off)
×
R1
+
R2
+
R3
Where
V
EEUV(off)
and
V
EEOV(off)
are Under & Over Voltage Shut
Down Threshold points.
UV
OFF
=
V
UVL
=
1.16
=
V
EEUV(off)
×
If we select a divider current of 100µA at a nominal operating input
voltage of 50 Volts then
R2
+
R3
R1
+
R2
+
R3
R3
R1
+
R2
+
R3
OV
ON
=
V
OVL
=
1.16
=
V
EEOV(on)
×
Where
V
EEUV(on)
and
V
EEOV(on)
are Under & Over Voltage Start
Up Threshold points.
Then
50V
R1
+
R2
+
R3
=
=
500kΩ
100uA
From the second equation for an OV shut down threshold of 65V
the value of R3 may be calculated.
V
EEUV(on)
=
1.26
×
R1
+
R2
+
R3
R2
+
R3
487kΩ
+
6.81kΩ
+
9.76kΩ
=
38.29V
6.81kΩ
+
9.76kΩ
V
EEUV(on)
=
1.26
×
And
OV
OFF
=
1.26
=
R3
=
65
×
R3
500kΩ
V
EEOV(on)
=
1.16
×
R1
+
R2
+
R3
R3
1.26
×
500KΩ
=
9.69kΩ
65
V
EEOV(on)
=
1.16
×
487kΩ
+
6.81kΩ
+
9.76kΩ
=
59.85V
9.76kΩ
The closest 1% value is 9.76kΩ
From the first equation for a UV shut down threshold of 35V the
value of R2 can be calculated.
Therefore, the circuit will start when the input supply voltage is in
the range of 38.29V to 59.85V.
UV
OFF
=
1.16
=
R2
=
35
×
(
R2
+
R3
)
500KΩ
1.16
×
500kΩ
−
9.76kΩ
=
6.81kΩ
35
The closest 1% value is 6.81kΩ
Then
R1
=
500KΩ
−
R2
−
R3
=
483kΩ
The closest 1% value is 487kΩ
6
Rev. D
04/17/02
Supertex, Inc. 1235 Bordeaux Drive, Sunnyvale, CA 94089 TEL: (408) 744-0100 Fax: (408) 222-4895 www.supertex.com
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