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MAX8215 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX8215 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page ±5V, ±12V (±15V) Dedicated Microprocessor Voltage Monitors 8 _______________________________________________________________________________________ Monitoring the Supply Voltage The supply voltage to these devices can also be moni- tored by the 5V dedicated comparator and the auxiliary comparator. Figure 4 shows a circuit that monitors the voltage connected at VDD for both overvoltage and undervoltage conditions. The +5V comparator checks for undervoltage conditions while the auxiliary compara- tor monitors overvoltage conditions. In general, no extra supply bypassing circuitry (other than the normally rec- ommended 0.1µF capacitor) is required when perform- ing this function. However, using resistor values higher than 100k Ω at the auxiliary comparator’s input requires attention to eliminate potential oscillations. Also, partic- ularly low pull-up resistor values on DOUT contribute to the likelihood of the auxiliary comparator’s oscillation. See the section Eliminating Output Oscillation. Microprocessor Reset ____________Circuit with Time Delay It is often necessary to reset a microprocessor (µP) when its supply voltage drops below a certain level. Figure 5’s circuit generates a low output when the monitored volt- age drops below the 5V monitor’s threshold. Additionally, this output remains low for 200ms after the supply voltage goes above the threshold. µP reset circuits typically include this feature because it gives the µP time to be fully reset after power has been restored, and allows any capacitors in associated circuitry time to charge. Figure 6 shows this circuit’s waveforms and formulas. Figure 7 shows Figure 5’s µP reset circuit, but with the monitored supply also powering the MAX8215. Figure 6’s waveforms and equations also apply to this circuit. The MAX8215/MAX8216 comparator outputs correctly display a low level down to a 0.8V typical supply voltage. Unused Inputs When the uncommitted comparator within the MAX8215/MAX8216 is not used, tie the unused input to either the positive supply or ground. This prevents noise generation due to the comparator output switching from one logic state to another (due to noise at the input). Output Pull-Up Resistors Pull-up resistors are required at the outputs of each comparator. Resistor values should not be less than 2.7k Ω if the outputs are pulled up to VDD. In general, save power by using higher values, e.g., ≥100kΩ. Use of higher-value resistors also minimizes the possibility of oscillations due to a spurious feedback (see the sec- tion Eliminating Output Oscillation). Input Voltage Limitation If the voltages at the various inputs are kept within the absolute maximum ratings, the device is not damaged. However, high input voltages within this range can cause the reference voltage to move. To prevent the reference voltage from changing, limit the +5V input to +17V; the -5V and -15V inputs to +1V; and the +15V input to +60V. Negative input voltages within the MAX8215 MAX8216 14 VDD 8 PGND 0.1 µF OUT1 DOUT 7 13 9 RESET 3 2 +VS GND +5V DIN +5V 680k 680k 1 µF Figure 5. Microprocessor Reset Circuit with 200ms Time Delay VTRIP2 VTRIP1 GND VDD INPUT VOLTAGE (VS) OUTPUT VOLTAGE (OUT1) GND tDLY = -RC In 1 - VREF VCC ( ) VDD GND tDLY OUTPUT VOLTAGE (DOUT) VTRIP1 IS FOR VS DECREASING; VTRIP2 IS FOR VS INCREASING. NOTE: VTH IS THE VOLTAGE AT THE INVERTING PIN OF THE TWO COMPARATORS. IN THIS CASE, IT IS EQUAL TO THE INTERNAL REFERENCE VOLTAGE. Figure 6. Microprocessor Reset with Time Delay Waveforms |
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