DESCRIPTION

The LTC4218 is a Hot Swap controller that allows a board to be safely inserted and removed from a live backplane. An internal high side switch driver controls the gate of an external N-channel MOSFET for supply voltages from 2.9V to 26.5V. A dedicated 12V version (LTC4218-12) contains preset 12V specific thresholds, while the standard LTC4218 allows adjustable thresholds.
The LTC4218 provides an accurate (5%) current limit with current foldback limiting. The current limit threshold can be adjusted dynamically using an external pin. Additional features include a current monitor output that amplifies the sense voltage for ground referenced current sensing. Overvoltage, undervoltage and power good monitoring are also provided.

 

CARACTÉRISTIQUES

-Wide Operating Voltage Range: 2.9V to 26.5V
-Adjustable, 5% Accurate 15mV Current Limit
-Current Monitor Output
-Adjustable Current Limit Timer Before Fault
-Power Good and Fault Outputs
-Adjustable Inrush Current Control
-2% Accurate Undervoltage and Overvoltage Protection
-Available in 16-Lead SSOP and 16-Pin 5mm × 3mm DFN Packages

 

CANDIDATURES

-RAID Systems
-ATCA, AMC, µTCA Systems
-Server I/O Cards
-Industrial

 

FONCTIONNEMENT

The Functional Diagram displays the main circuits of the device. The LTC4218 is designed to turn a board’s supply voltage on and off in a controlled manner, allowing the board to be safely inserted and removed from a live backplane. During normal operation, the charge pump and gate driver turn on the external N-channel pass FET’s gate to provide power to the load.
The current sense (CS) amplifier monitors the load current using the voltage sensed across the current sense resistor. The CS amplifier limits the current in the load by reducing the GATE-to-SOURCE voltage in an active control loop. It is simple to adjust the current limit threshold using the current limit adjustment (ISET) pin. This allows a different threshold during other times such as startup.
A short circuit on the output to ground causes significant power dissipation during active current limiting. To limit this power, the foldback amplifier reduces the current limit value from 15mV to 3.75mV (referred to the SENSE+ minus SENSE– voltage) in a linear manner as the FB pin drops below 0.6V .
If an overcurrent condition persists, the TIMER pin ramps up with a 100µA current source until the pin voltage exceeds 1.2V . This indicates to the logic that it is time to turn off the MOSFET to prevent overheating. At this point the TIMER pin ramps down using the 2µA current source until the voltage drops below 0.2V which tells the logic to start an internal 100ms timer. At this point, the pass transistor has cooled and it is safe to turn it on again. Latchoff is the normal operating condition following overcurrent turn-off. Retry is initiated by pulling the UV pin low for a minimum of 1µs then high. Autoretry is implemented by tying the FLT to the UV pin.
The fixed 12V version, LTC4218-12, uses two separate internal dividers from VDD to drive the UV and OV pins. This version also features a divider from the SOURCE pin to drive the FB pin. The LTC4218-12 is available in a DFN package while the LTC4218 (adjustable version) is in a SSOP package.
The output voltage is monitored using the FB pin and the PG comparator to determine if the power is available for the load. The power good condition is signaled by the PG pin using an open-drain pull-down transistor.
The Functional Diagram shows the monitoring blocks of the LTC4218. The comparators on the left side include the UV and OV comparators. These comparators are used to determine if the external conditions are valid prior to turning on the MOSFET. But first, the undervoltage lockout circuits must validate the input supply and internally generated 3.1V supply (INTVCC) and generate the power up initialization to the logic circuits. If the external conditions remain valid for 100ms the MOSFET is allowed to turn on.
Other monitoring features include the IMON current monitor. The current monitor (CM) outputs a current proportional to the sense resistor current. This current can drive an external resistor or other circuits for monitoring purposes.