Controlling up to three power-supply rails precisely

Sept. 26, 2005
Dual and triple voltage tracker/sequencers ensures proper system power-up/power-down operation.

The MAX687-MAX6883 are dual/triple power supply trackers/sequencers from Maxim Integrated Products. They control up to three power-supply rails to ensure proper power-up and power-down conditions for systems requiring voltage tracking or sequencing. Functions such as voltage thresholds, slew rates and timing parameters, are adjusted with external resistors or capacitors.

To turn on the MOSFETs, the MAX6877-MAX6883 employ independent charge pumps for 5.5V gate-to-source improvement. During initial power up, all monitored voltages must exceed their resistor-set threshold levels before the voltages are switched on through the external n-channel MOSFET. In sequence mode, all MOSFETs are turned on sequentially with a capacitor-set time delay between each rail as they power up. In track mode, the voltage at the source of each MOSFET is monitored and each compared with the others. The gate of each MOSFET is continuously adjusted in this close-loop control system to ensure that the voltage track is within 125mV of the reference ramp.

In the sequencing or tracking mode, if any of the voltages fails to sequence or track within specifications, a fault condition is asserted and all the MOSFETs are turned off quickly. Internal 100 ohm pulldown circuitry is activated to help discharge the capacitance at the output.


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