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MAX6887AETE Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX6887AETE Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 14 page Hex/Quad, Power-Supply Supervisory Circuits _______________________________________________________________________________________ 7 Detailed Description The MAX6887/MAX6888 provide several supply-detector inputs, one watchdog input, and three outputs for power- supply monitoring applications. The MAX6887 offers six voltage-detector inputs, while the MAX6888 offers four. Each voltage-detector input offers both an undervoltage and overvoltage threshold. The undervoltage and overvoltage thresholds are facto- ry-set for monitoring standard supply voltages (see the Selector Guide). Inputs in the Selector Guide that con- tain “Adj” allow an external voltage-divider to be con- nected to set a user-defined threshold. RESET goes low when any input voltage drops below its undervoltage threshold or when MR is brought low. RESET stays low for 200ms after all assertion-causing conditions have been cleared. OV goes low when an input voltage rises above its overvoltage threshold. OV typically stays low for 25µs (typ) after all inputs fall back under their overvoltage thresholds. The MAX6887/MAX6888 offer a watchdog timer with initial and normal timeout periods of 102.4s and 1.6s, respectively. WDO goes low when the watchdog timer expires and deasserts when WDI transitions from low- to-high or high-to-low. Powering the MAX6887/MAX6888 The MAX6887/MAX6888 derive power from the voltage- detector inputs IN1–IN4 or through an externally sup- plied VCC. A virtual diode-ORing scheme selects the positive input that supplies power to the device (see the Functional Diagram). The highest input voltage on IN1–IN4 supplies power to the device. One of IN1–IN4 must be at least 2.7V to ensure proper operation. Internal hysteresis ensures that the supply input that initially powered the device continues to power the device when multiple input voltages are within 50mV of each other. VCC powers the analog circuitry and is the bypass con- nection for the MAX6887/MAX6888 internal supply. Bypass VCC to GND with a 1µF ceramic capacitor installed as close to the device as possible. The inter- nal supply voltage, measured at VCC, equals the maxi- mum of IN1–IN4. If VCC is externally supplied, VCC must be at least 200mV higher than any voltage applied to IN1–IN4 and VCC must be brought up first. VCC always powers the device when all IN_ are factory set as “Adj.” Do not use the internally generated VCC to provide power to external circuitry. The MAX6887/MAX6888 generate a supply voltage at BP for the internal logic circuitry. Bypass BP to GND with a 1µF ceramic capacitor installed as close to the device as possible. The nominal BP output voltage is +2.55V. Do not use BP to provide power to external circuitry. Inputs The MAX6887 offers six voltage-detector inputs, while the MAX6888 offers four voltage-detector inputs. Each voltage-detector input offers an undervoltage and over- voltage threshold set at the factory to monitor standard supply voltages (see the Selector Guide). The 5% and 10% tolerances are based on maximum and minimum threshold values. Actual thresholds for the MAX6887/MAX6888 are shown in Tables 1 and 2. Inputs in the Selector Guide listing “Adj” allow an exter- nal voltage-divider to be connected to set a user- defined threshold. Adjustable Threshold Inputs Inputs listed in the Selector Guide containing “Adj” for inputs allow external resistor voltage-dividers to be connected at the voltage-detector inputs. These inputs monitor any voltage supply higher than 0.6V (see Figure 1). Use the following equation to set a voltage- MAX6887 MAX6888 VIN IN_ R1 R2 *VREFUV *VREFOV AND VREFUV ARE REFERENCED TO 0.6V ACCORDING TO THE DEVICE'S TOLERANCE *VREFOV Figure 1. Adjusting the Monitored Threshold |
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