DESCRIPTION

The LTC4226 dual Hot Swap™ controller allows two power paths to be safely inserted and removed from a live backplane or powered connector. Using N-channel pass transistors, supply voltages ranging from 4.5V to 44V are ramped up at an adjustable rate.

Three selectable ratios of current limit to circuit breaker threshold accommodate noisy loads and momentary high peak currents without interruption, while a dual-rate fault timer protects the MOSFET from extended output over-current events. FAULT outputs indicate the circuit breaker status. The LTC4226-1 remains off after a fault while the LTC4226-2 automatically retries after a 0.5s delay.

The LTC4226 can also be configured as a bidirectional current limiter/circuit breaker. For high current applications, two channels may be configured as parallel power paths.

 

CARACTÉRISTIQUES

Allows Safe Board Insertion into Live Backplane

Selectable Current Limit and Dual-Rate Timer Accommodate Load Surges

Fast Response Limits Peak Fault Current

Wide Operating Voltage Range: 4.5V to 44V

Optional Auto-Retry or Latchoff after Overcurrent Fault

High Side Drive for External N-Channel MOSFET

Allows Parallel Power Paths for High Current Applications

Available in 16-Pin QFN (3mm × 3mm) and MSOP Packages

 

CANDIDATURES

Apple FireWire/IEEE 1394

Disques durs

Rugged 12V, 24V Applications

Hot Board/Connector Insertion

Uni/Bidirectional Current Limiter/Circuit Breaker

 

FONCTIONNEMENT

The LTC4226 controls two independent Hot Swap channels. It is designed to turn each supply voltage on and off in a controlled manner, allowing live insertion into a powered connector or backplane.

The LTC4226 powers-up the output of a channel when that channel’s VCC pin has remained above the 3.7V undervoltage lockout threshold VCC(UVL) for more than 50ms and its ON pin has remained above the VON threshold for more than 10ms. During normal operation, a charge pump turns on the external N-channel MOSFET providing power to the load. Each channel’s charge pump derives its power from its own VCC supply pin. To protect the MOSFET, the GATE voltage is clamped at about 12V above the OUT pin. It is also clamped a diode voltage below the OUT pin and a diode voltage below GND.

The current flowing through the MOSFET is measured by the external sense resistor. The sense voltage across the sense resistor is measured between the VCC and SENSE pins. The LTC4226 has a circuit breaker (CB) comparator to detect the sense current above circuit breaker threshold and a current limit (CL) amplifier to actively clamp the sense current at the current limit threshold. Both the CB comparator and the CL amplifier monitor the sense resistor voltage between the VCC and SENSE pins. When the sense voltage exceeds VCB but is below VLIMIT, the CB comparator enables a 2μA IFTMR(CB) current source that ramps up the voltage on the FTMR pin. If the sense resistor voltage exceeds VLIMIT, the CL amplifier limits the current in the MOSFET by reducing the GATE-to-OUT voltage with an active control loop. The fast response CL amplifier can quickly gain control of the GATE-to-OUT voltage in the event of an OUT-to-GND short circuit. The FTMR pin is ramped up by the larger IFTMR(CL) current source during active current limiting. If the sense voltage falls below VCB, the FTMR is ramped down by the default 2μA IFTMR(DEF) pull-down current.

 

INFORMATIONS SUR LES APPLICATIONS

The typical LTC4226 application is in high availability systems that distribute positive voltage supplies between 4.5V to 44V to hot-swappable ports or cards. It can also be used in daisy chain port applications like FireWire to provide instant current limit.

Each Hot Swap channel has a power path controlled by an external MOSFET switch and a sense resistor for monitoring current.

Overcurrent Fault

The LTC4226 manages overcurrent faults by differentiating between circuit breaker faults and current limit faults. Typical applications have a load capacitor to filter the load current. A large load capacitor is an effective filter, but it can increase MOSFET switch power dissipation at start-up or during step up supply transients.