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ADM1176EBZ Datasheet(PDF) 13 Page - Analog Devices |
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ADM1176EBZ Datasheet(HTML) 13 Page - Analog Devices |
13 / 24 page ADM1176 Rev. 0 | Page 13 of 24 OVERVIEW OF THE HOT SWAP FUNCTION When circuit boards are inserted into a live backplane, discharged supply bypass capacitors draw large transient currents from the backplane power bus as they charge. Such transient currents can cause permanent damage to connector pins, as well as dips on the backplane supply that can reset other boards in the system. The ADM1176 is designed to turn a circuit board supply voltage on and off in a controlled manner, allowing the circuit board to be safely inserted into or removed from a live backplane. The ADM1176 can reside either on the backplane or on the circuit board itself. The ADM1176 controls the inrush current to a fixed maximum level by modulating the gate of an external N-channel FET placed between the live supply rail and the load. This hot swap function protects the card connectors and the FET itself from damage and limits any problems that can be caused by high current loads on the live supply rail. The ADM1176 holds the GATE pin down (and, thus, the FET is held off) until a number of conditions are met. An undervoltage lockout circuit ensures that the device is provided with an adequate input supply voltage. Once the input supply voltage has been successfully detected, the device goes through an initial timing cycle to provide a delay before it attempts to hot swap. This delay ensures that the board is fully seated in the backplane before the board is powered up. Once the initial timing cycle is complete, the hot swap function is switched on under control of the ON pin. When the ON pin is asserted high, the hot swap operation starts. The ADM1176 charges up the gate of the FET to turn on the load. It continues to charge up the GATE pin until the linear current limit (set to 100 mV/RSENSE) is reached. For some combi- nations of low load capacitance and high current limit, this limit may not be reached before the load is fully charged up. If current limit is reached, the ADM1176 regulates the GATE pin to keep the current at this limit. For currents above the overcurrent fault timing threshold, nominally 100 mV/RSENSE, the current fault is timed by sourcing a current out to the TIMER pin. If the load becomes fully charged before the fault current limit time is reached (when the TIMER pin reaches 1.3 V), the current drops below the overcurrent fault timing threshold. The ADM1176 then charges the GATE pin higher to fully enhance the FET for lowest RON, and the TIMER pin is pulled down again. If the fault current limit time is reached before the load drops below the current limit, a fault has been detected, and the hot swap operation is aborted by pulling down on the GATE pin to turn off the FET. The ADM1176-2 latches off at this point and attempts to hot swap again only when the ON pin is deasserted and then asserted again. The ADM1176-1 retries the hot swap operation indefinitely, keeping the FET in its safe operating area (SOA) by using the TIMER pin to time a cool-down period in between hot swap attempts. The current and voltage threshold combinations on the TIMER pin set the retry duty cycle to 3.8%. The ADM1176 is designed to operate over a range of supplies from 3.15 V to 16.5 V. UNDERVOLTAGE LOCKOUT An internal undervoltage lockout (UVLO) circuit resets the ADM1176 if the VCC supply is too low for normal operation. The UVLO has a low-to-high threshold of 2.8 V, with 80 mV hysteresis. Above 2.8 V supply voltage, the ADM1176 starts the initial timing cycle. ON FUNCTION The ADM1176-1 has an active-high ON pin. The ON pin is the input to a comparator that has a low-to-high threshold of 1.3 V, a 50 mV hysteresis, and a glitch filter of 3 μs. A low input on the ON pin turns off the hot swap operation by pulling the GATE pin to ground, turning off the external FET. The TIMER pin is also reset by turning on a pull-down current on this pin. A low-to- high transition on the ON pin starts the hot swap operation. A 10 kΩ pull-up resistor connecting the ON pin to the supply is recommended. Alternatively, an external resistor divider at the ON pin can be used to program an undervoltage lockout value higher than the internal UVLO circuit, thereby setting a voltage level at the VCC supply where the hot swap operation is to start. An RC filter can be added at the ON pin to increase the delay time at card insertion if the initial timing cycle delay is insufficient. TIMER FUNCTION The TIMER pin handles several timing functions with an external capacitor, CTIMER. There are two comparator thresholds: VTIMERH (0.2 V) and VTIMERL (1.3 V). The four timing current sources are a 5 μA pull-up, a 60 μA pull-up, a 2 μA pull-down, and a 100 μA pull-down. The 100 μA pull-down is a non-ideal current source approximating a 7 kΩ resistor below 0.4 V. These current and voltage levels, together with the value of CTIMER chosen by the user, determine the initial timing cycle time, the fault current limit time, and the hot swap retry duty cycle. |
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