DESCRIPCIÓN

The LTC4211 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 ranging from 2.5V to 16.5V. The LTC4211 provides soft-start and inrush current limiting during the start-up period which has a programmable duration.

Two on-chip current limit comparators provide dual level overcurrent circuit breaker protection. The slow comparator trips at VCC – 50mV and activates in 20µs (or is programmed by an external filter capacitor, MS only). The fast comparator trips at VCC – 150mV and typically responds in 300ns.

The FB pin monitors the output supply voltage and signals the RESET output pin. The ON pin signal turns the chip on and off and can also be used for the reset function. The  MS package has FAULT and FILTER pins to provide additional functions like fault indication, autoretry or latch-off modes, programmable current limit response time and programmable overvoltage protection using an external Zener diode clamp.

 

CARACTERÍSTICAS

Allows Safe Board Insertion and Removal from a Live Backplane

Controls Supply Voltages from 2.5V to 16.5V

Programmable Soft-Start with Inrush Current Limiting, No External Gate Capacitor Required

Faster Turn-Off Time Because No External Gate Capacitor is Required

Dual Level Overcurrent Fault Protection

Programmable Response Time for Overcurrent Protection (MS)

Programmable Overvoltage Protection (MS)

Automatic Retry or Latched Mode Operation (MS)

High Side Drive for an External N-Channel FET

User-Programmable Supply Voltage Power-Up Rate

FB Pin Monitors VOUT and Signals RESET

Glitch Filter Protects Against Spurious RESET Signa

 

APLICACIONES

Electronic Circuit Breaker

Hot Board Insertion and Removal (Either On Backplane or On Removable Card)

Industrial High Side Switch/Circuit Breaker

 

OPERACIÓN

HOT CIRCUIT INSERTION

When circuit boards are inserted into or removed from live backplanes, the supply bypass capacitors can draw huge transient currents from the backplane power bus as they charge. The transient current can cause permanent damage to the connector pins as well as cause glitches on the system supply, causing other boards in the system to reset.

The LTC4211 is designed to turn a printed circuit board’s supply voltages ON and OFF in a controlled manner, allowing the circuit board to be safely inserted or removed from a live backplane. The device provides a system reset signal to indicate when board supply voltage drops below a predetermined level, as well as a dual function fault monitor.

OUTPUT VOLTAGE MONITOR

The LTC4211 uses a 1.236V bandgap reference, precision voltage comparator and an external resistive divider to monitor the output supply voltage as shown in Figure 1.

The operation of the supply monitor in normal mode is illustrated in Figure 2. When the voltage at the FB pin drops below its reset threshold (1.236V), the comparator COMP2 output goes high.After a glitch filter delay, RESET  is pulled low (Time Point 1). When the voltage at the FB pin rises above its reset threshold (1.239V), COMP2’s output goes low and a timing cycle starts (Time Point 4). After a complete timing cycle, RESET is pulled high by the external pull-up resistor. If the FB pin rises above the reset threshold for less than a timing cycle,the RESET output remains low (Time Points 2 to 3).

 

UNDERVOLTAGE LOCKOUT

The LTC4211’s power-on reset circuit initializes the startup procedure and ensures the chip is in the proper state if the input supply voltage is too low. If the supply voltage falls below 2.18V, the LTC4211 is in undervoltage lockout (UVLO) mode, and the GATE pin is pulled low. Since the UVLO circuitry uses hysteresis, the chip restarts after the supply voltage rises above 2.3V and the ON pin goes high.

 

USING AN EXTERNAL GATE CAPACITOR

In addition to reducing the inrush current, an external gate capacitor may also be useful to

decrease or eliminate current spikes through the MOSFET when power is first applied. At power-up, the instantaneous input voltage step attempts to pull the MOSFET gate up through the MOSFET’s drain-to-gate capacitance. If the MOSFET’s CGS is small, the gate can be pulled up high enough to turn on the MOSFET, thereby allowing a current spike to the output. This event occurs during the time that the LTC4211 is coming out of UVLO and getting its intelligence to hold the GATE pin low. An external capacitor attenuates the voltage to which the GATE is pulled up and eliminates the current spike. The value required is dependent on the MOSFET capacitance specifications. In typical applications, this capacitor is not required.