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EDN, May 26, 2011

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designideas of the circuit is a slight modification of the circuit in an earlier Design Idea (Reference 3). VCOMP As the ramp-up/ramp-down cycle repeats, the integrator produces a symmetrical, triangular waveform of VINT , and a rectangular waveform, , appears at the output of the comparator. The amplitude of VINT is approximately (VREF+−VREF− )/2. The duty cycle of the rectangular waveform is close to 50%. The thresholds of the comparator are independent of the out- put load, and you derive them from a precision source of reference voltages. Thus, the circuit has low sensitivity of the repetition frequency of its output to supply-voltage variations and to load variations. In a simplistic model of the generator, the amplitude of the triangu- lar waveform at the output of the inte- grator no longer depends on variations of supply voltage. Experimentally, increasing sup- ply voltage VS from 5.0365 to 5.437V increases the amplitude of the triangu- lar waveform by 2.85 mV, representing 0.285% of full-scale. Under the same conditions, a classic triangular/rect- angular-waveform generator typically shows an 8% increase in amplitude. Thus, this circuit reduces the depen- dence of amplitude on supply-voltage variation by a factor of about 28. In testing this circuit, you can expect an output frequency of 1.366 MHz with a supply voltage of 5.0365V. When the supply voltage is 5.437V, the output frequency will be 1.368 MHz. The time constant sets the repetition rate. In this case, the repetition rate is one divided by four times the time con- stant for an ideal comparator and ideal switches. The comparator's propaga- tion delay and the on/off times of the switches lower the repetition rate to lower than the ideal value.EDN REFERENCES 1 Štofka, Marián, "Circuit uses two ref- erence voltages to improve hysteresis accuracy," EDN, Jan 7, 2010, pg 43, http://bit.ly/g9GmAc. 2 Štofka, Marián, "Schottky diodes improve comparator's transient response," EDN, Jan 21, 2010, pg 32, http://bit.ly/el3N0d. 3 Štofka, Marián, "Build an accurate bipolar reference," EDN, May 12, 2011, pg 42, http://bit.ly/knjieC. Produce current from positive or negative high-voltage supplies Kurt Nell, Sankt Pölten, Austria ↘ You sometimes need a current source for supply voltages as high as 1000V or more. This current source can be useful for ripple-voltage reduction when the current source's high-impedance node feeds an electro- lytic capacitor to effectively short the ripple voltage. The circuit in Figure 1 does the job with a temperature-stable and exact-output current. The circuit uses N-channel MOSFET Q1 , which has a drain-to-source voltage of 1000V. Zener diode D2 , a ZR431LF01 shunt and the voltage across it deter- mine the output current. In this case, 1.25V/220Ω=5.6 mA. D2 reference voltage. R1 erence voltage, which is temperature stable and accurate, making the cur- equals D2 BUZ50 Q1 950V BZV55CIZ D1 330k R2 330k R3 330k R4 ZR431LF01 D2 Figure 1 This circuit produces current using positive voltages. 54 EDN | MAY 26, 2011 10k R5 10 μF C2 10 μF C1 + + 10M R7 R1 10M MOSFET's gate-to-source voltage until the voltage across R1 regulates the 's ref- regulator, stabilizes and regulates the output current. The threshold voltage of Q1 must be higher than D2 's 1.25V rent source stable and accurate, as well. Zener diode D1 function of the current regulation with zener diode D2 voltages.EDN limits the gate-to-source voltage if no load connects to the current source. You can use a similar circuit to get constant current from negative high- voltage supplies even if a P-channel MOSFET with a high drain-to-source voltage is unavailable. To make the cir- cuit work with negative supplies with an N-channel MOSFET, you must modify the circuit in Figure 1. You can use the N-channel MOSFET by changing the source and drain connections of MOSFET Q1 protects Q1 in Figure 2 (pg 55). The is the same as for positive 's gate and 220 R1 OUT+

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