MIC2589/MIC2595 Single-Channel, Negative High-Voltage Hot Swap Power Controllers/Sequencers - Micrel - #19

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Micrel MIC2589/MIC2595 December 2005 19 M9999-120505 (408) 955-1690 following as a starting point: 12.23kÙ100ìA1.223V100ìA(typ)VR3OVH=== The closest standard 1% value for R3 = 12.4kÙ. Solving for R2 and R1 yields: 11.73kÙ137V72V12.4kÙR21VVR3R2UVOV=....-....×=......-......×= The closest standard 1% values for R2 = 11.8kÙ. Lastly, the value for R1 is calculated: ()()705.81kÙR1kÙ8.111.223V1.223V72V12.4kÙR1R21.223V1.223VVR3R1OV=-....�|×=-....�|×= The closest standard 1% value for R1 = 698kÙ. Using standard 1% resistor values, the circuit’s nominal UV and OV thresholds are: VUV = 36.5V VOV = 71.2V Good general engineering design practices must consider the tolerances associated with these parameters, including but not limited to, power supply tolerance, undervoltage and overvoltage threshold tolerances, and the tolerances of the external passive components. Programmable UVLO Hysteresis (MIC2595 and MIC2595R) The MIC2595 and the MIC2595R devices have user-programmable hysteresis by means of the ON and OFF pins (Pins 4 and 3, respectively). This allows setting the MIC2595/MIC2595R to turn on at a voltage V1, and not turn off until a second voltage V2, where V2 < V1. This can significantly simplify dealing with source impedances in the supply buss while at the same time increasing the amount of available operating time from a loosely regulated power rail (for example, a battery supply). The MIC2595/MIC2595R holds the output off until the voltage at the ON pin is above its VONH threshold value given in the “Electrical Characteristics” table. Once the output has been enabled by the ON pin, it will remain on until the voltage at the OFF pin falls below its respective VOFFL threshold value, or the part turns off due to an external fault condition. Should either event occur, the GATE pin is immediately pulled low and will remain low until the ON pin voltage once again rises above its VONH threshold. The circuit’s turn-on and turn-off voltage levels are set using the resistor divider R1, R2, and R3 similar to the “Typical Application” circuit and the equations to set the trip points are shown below. For the following example, the circuit’s ON threshold is set to VON = 40V and the circuit’s OFF threshold is set to VOFF = 35V. ()()R3R2R3R2R1(typ)VVR3R3R2R1(typ)VVOFFLOFFONHON+++×=++×= Given VOFF, VON, and any one of the resistor values, the remaining two resistor values can be readily determined. A suggested value for R3 is selected to provide approximately 100ìA (or more) of current through the voltage divider chain at VDD = VOFF. This yields the following as a starting point: 12.23kÙ100ìA1.223V100ìA(typ)VR3OFFL=== The closest standard 1% value for R3 = 12.4kÙ. Solving for R2 and R1 yields: 1.77kÙ135V40V12.4kÙR21VVR3R2OFFON=........�|×=............-×= The closest standard 1% value for R2 = 1.78kÙ. Lastly, the value for R1 is calculated: ()391.38kÙR11.78kÙ1.223V1.223V40V12.4kÙR1R21.223V1.223VVR3R1ON=--×=--×= The closest standard 1% value for R1 = 392kÙ. Using standard 1% resistor values, the circuit’s nominal ON and OFF thresholds are: VON = 40.1V VOFF = 35V Good general engineering design practices must consider the tolerances associated with these parameters, including but not limited to, power supply tolerance, undervoltage and overvoltage threshold tolerances, and the tolerances of the external passive components.