Datasheet LT4351 (Analog Devices) - 8

LT4351
OPERATION
Increasingly, system designers have to deal with multiple OV just exceeds the reference, an internal capacitor starts supply sources. The multiplicity may provide parallel, charging, delaying the signal to turn off the MOSFET(s). redundant supplies for increased reliability or provide The second occurs when the OV pin exceeds 330mV. The a means of connecting disparate supplies. In all cases OVF comparator wil immediately trip pul ing GATE to GND. the desire is for behavior like a diode but with no loss or This affords a delay inversely proportional to the amount of voltage drop. overdrive. This also provides for glitch immunity without ORing diodes have been the conventional means of con- compromising response time in the event of a serious necting these supplies. The disadvantage of this approach overvoltage condition. is that diodes introduce efficiency loss because of their forward voltage drop. This variable voltage drop also de- The FAULT output indicates the status of the COV, COVF generates supply tolerance. Additionally, diodes provide and CUV comparators. It pulls low during a fault condi- no information concerning the status of the sourcing tion. It also pulls low when GATE is at compliance and supply. Separate control must also be added to ensure VIN > OUT by more than 0.21V indicating a probable that a supply that is out of range is not allowed to affect nonfunctioning MOSFET. Compliance occurs when GATE the common supply. is at the lesser of OUT + 7.4V or VDD – 2.3V. FAULT derives its drive from the greater of VIN or OUT. It is active if VIN The LT4351 eliminates these problems by using N-channel or OUT is greater than 0.9V. If VIN or OUT is below this MOSFETs as the pass elements. The MOSFET is turned on level, the output state is not guaranteed. when power is being passed, allowing for a low voltage drop from the supply to the load. When the input source The gate drive consists of a high current, wide bandwidth voltage drops below the output common supply voltage it amplifier (driver). When the amplifier is enabled, it attempts turns off the MOSFET, thereby matching the function and to regulate the GATE voltage such that the voltage across performance of an ideal diode. the MOSFET(s) is approximately 15mV. If the MOSFET(s) on resistance is so high as to prevent regulation, then The LT4351 drives either a single MOSFET or dual back- GATE goes to compliance and the MOSFET(s) fully turns to-back MOSFETs. Dual MOSFETs are chosen to eliminate on. The inputs to the amplifier are VIN and OUT. The GATE current flow from the input supply to the output supply pin sources current from VDD and sinks current to GND. when the VIN voltage is greater than OUT. The maximum GATE to VIN voltage is the lesser of VDD – A driver amplifier monitors the input (VIN) and output 2.3V or 7.4V above VOUT or VIN (internal clamp voltage). (OUT) and controls the MOSFETs. If VIN exceeds OUT The STATUS comparator, ST, pulls low when GATE ex- by 15mV, GATE goes high and turns on the MOSFET(s) ceeds V allowing for power passage. IN by 0.7V. This occurs when VIN > OUT + 15mV. The STATUS pin pulls low as an indication that power is Undervoltage and overvoltage comparators CUV, COV passing through the MOSFET(s). and COVF also control power passage. A resistive divider If V in conjunction with the UV and OV pins sets appropriate IN is greater than OUT by 0.21V and GATE > VIN + 7.4V or at compliance (GATE = V thresholds such that the MOSFET(s) is off when the UV DD – 2.3V), STATUS will go high as an indication of a likely open MOSFET. FAULT will pin is below 300mV or OV pin is above 300mV. pull low in this state indicating the probable fault. To help deal with the transients on the supply lines, the UV The gate drive amplifier and STATUS function derive power input has current hysteresis. When the UV voltage drops from V below the 300mV threshold, a 10µA current is pulled from DD. The circuit requires VDD > 2.5V. If VDD is present, the gate drive amplifier and STATUS are active independent the pin. Thus the user can set the hysteresis level through of the state of V appropriate values in the divider. IN. If in a fault, GATE pulls actively low. In the event of VDD collapse there still is an active pull-down Overvoltage shutdown occurs in two stages. The first oc- (though of lesser strength) of GATE powered from OUT, curs when the OV pin exceeds the 300mV reference. When guaranteeing turn off. 4351fd 8 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts