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HV320LG Datasheet(PDF) 6 Page - Supertex, Inc |
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HV320LG Datasheet(HTML) 6 Page - Supertex, Inc |
6 / 9 page 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 OFF = VUVL = 1.16 = •VEEUV(off)•x (R2+R3)/(R1+R2+R3) OV OFF = VOVL = 1.26 = •VEEOV(off)•x R3/(R1+R2+R3) Where •V EEUV(off)•and •VEEOV(off)• relative to VEE are Under and 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 OFF = 1.26 = (65xR3)/500k 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 OFF = 1.16 = 35 x (R2+R3) / 500k 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 ON = VUVH = 1.26 = •VEEUV(on) •x (R2+R3)/(R1+R2+R3) OV ON = VOVL = 1.16 = •VEEOV(on) •x R3/(R1+R2+R3) Where •V EEUV(on) • and •V EEOV(on) • are Under and Over Voltage Start Up Threshold points relative to Vee. Then •V EEUV(on) • = 1.26 x (R1+R2+R3)/(R2+R3) •V EEUV(on) •= 1.26 x (487k+6.81k+9.76k)/(6.81k+9.76k ) = 38.29V and •V EEOV(on) •= 1.16 x (R1+R2+R3)/R3 •V EEOV(on) •= 1.16 x (487k +6.81k +9.76k)/9.76k = 59.85V 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 EE. The value of this cap can be calculated accordingly: VC1 ( t ) = V IN x ( 1 – e ) Where VC1= 1.26 V IN = 1.60V t POR = desired POR time to overcome the bounce Req = R1 ⎢⎢ (R2 + R3) From the above C1 can be calculated: t POR Req x 1.60 For example for t POR = 10ms and 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 = -tPOR Req x C1 ) |
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