1V to 16V, Single-Channel, Hot-Swap Controllers

with Precision Current-Sensing Output

9

Detailed Description

The hot-swap controllers provide electronic circuit-

breaker protection and precision current sensing for a

single-supply voltage from 1V to 16V. Programmable

undervoltage and overvoltage protection qualifies the

supply voltage prior to enhancing the external n-channel

MOSFET with the internal gate driver.

The VariableSpeed/BiLevel fault levels are program-

mable with external resistors providing both slow and

fast circuit-breaker protection. The transconductance

current-sense amplifier provides continuous current

monitoring with high accuracy and features a calibration

mode for production testing.

Programmable Undervoltage

and Overvoltage Protection

The programmable undervoltage and overvoltage pro-

tection enables the hot-swap channel when the voltage

at UV is above 590mV, and the voltage at OV is below

590mV. After the hot-swap channel is enabled, the hot-

swap channel is disabled if the voltage at OV exceeds

the 590mV threshold.

Gate Driver

An integrated 5V charge pump supplies the gate-driver

output of the devices, allowing it to fully enhance the

external n-channel MOSFET during normal operation.

The 5FA (typ) current source at GATE slowly charges the

gate-to-source capacitance of the external n-channel

MOSFET to 5V (typ) relative to the SOURCE input.

Programmable Fast-Trip and Slow-Trip

Overcurrent Circuit Breaker

During normal operation with the channel turned on, two

analog comparators are used to detect an overcurrent

condition by comparing the voltage across the external

SENSE to the voltages across the respective external

overcurrent circuit-breaker threshold set resistors con-

nected from IN to FCOMP and SCOMP. Precision cur-

rent sources at the FCOMP and SCOMP inputs establish

these thresholds.

If the voltage across the sense resistor is less than

the fast-trip and slow-trip overcurrent circuit-breaker

thresholds, the GATE output remains high. If either of

the thresholds is exceeded due to an overcurrent condi-

tion, the GATE output is pulled down to SOURCE by a

500mA current sink, and the FAULT and PG outputs are

asserted low.

If the sense voltage rises above the fast circuit-breaker

threshold, the devices turn off the external MOSFET in

200ns (typ).

If the sense voltage rises above the slow circuit-breaker

threshold, the internal timer begins counting. If the sense

voltage remains above the slow circuit-breaker threshold

until the timer expires, the devices turn off the external

MOSFET. The slow circuit-breaker timer occurs in 1ms

(typ) when the slow-current comparator threshold is over-

driven by 1mV and 130Fs (typ) when overdriven by 50mV.

Current-Sense Amplifier

The integrated transconductance current-sense ampli-

fier features high accuracy with less than 1% error over

its 10mV to 50mV input range, and provides continu-

ous current monitoring into the load. The sense voltage

of the external sense resistor connected between IN

and SENSE is multiplied by the transconductance gain

(2500FS typ) of the amplifier with the resulting current

output at CSOUT.

Calibration Mode

The devices' calibration mode bypasses the trans-

conductance amplifier inputs to measure the voltage

between IN and CALSENSE when the calibration mode

select input CAL is high.

This enables the user to apply a known calibration

voltage across the current-sense amplifier input. This

voltage corresponds to a full scale for the actual sense

voltage. During the calibration mode, the current-sense

amplifier only measures the calibration voltage between

IN and CALSENSE.

The calibration mode is completely asynchronous and

does not disrupt the circuit-breaker threshold compari-

son. Once in calibration mode there is no expiration until

the CAL input is brought low. This allows the calibration

to occur at multiple voltages by applying various calibra-

tion voltages during the calibration mode.

Fault Output

The FAULT output goes low when a slow or fast com-

parator current-limit fault has occurred.

On the MAX5977A, the device is latched in fault mode

until it is reset either by initiating a full power-on reset or

pulling UV below 590mV.

On the MAX5977B, the device reenables the hot-swap

output after the autoretry timer has expired in 175ms

and FAULT is pulled high if the fault condition has been

removed and startup conditions are met.