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MAX4370

Current-Regulating Hot-Swap Controller with DualSpeed/BiLevel Fault Protection

Maxim Integrated 

Current-Regulating Hot-Swap Controller with

DualSpeed/BiLevel Fault Protection

Detailed Description

The MAX4370 is a circuit-breaker IC designed for hot-

swap applications where a card or board is to be

inserted into a rack with the main system power supply

turned on. Normally, when a card is plugged into a live

backplane, the card is discharged filter capacitors pro-

vide a low impedance, which can momentarily cause

the main power supply to collapse. The MAX4370 is

designed to reside either in the backplane or in the

removable card to provide inrush-current limiting and

short-circuit protection. This is achieved using a charge

pump as gate drive for an external N-channel MOSFET,

an external current-sense resistor, and two on-chip

comparators. Figure 1 shows the device’s functional

diagram.

The slow comparator response time and the start-up

timer can be adjusted with external capacitors. The tim-

ing components are optional; without them the part is

set to its nominal values, as shown in the Electrical

Characteristics.

Start-Up Period

CTIM sets the start-up period. This mode starts when

the power is first applied to VIN if ON is connected to

VIN, or at the rising edge of ON. In addition, the voltage

at VIN must be above the undervoltage lockout for

150ms (see Undervoltage Lockout).

During start-up, the slow comparator is disabled and

current limiting is provided two different ways:

1) Slow ramping of the current to the load by controlling

the external MOSFET gate voltage.

2) Limiting the current to the load by regulating the volt-

age across the external current-sense resistor.

Unlike other circuit-breaker ICs, the MAX4370 hot-swap

controller regulates the current to a preset level instead

of completely turning off if an overcurrent occurs during

start-up.

In start-up mode, the gate drive current is limited to

100µA and decreases with the increase of the gate

voltage (see Typical Operating Characteristics). This

allows the MAX4370 to slowly enhance the MOSFET. If

the fast comparator detects an overcurrent, the gate

voltage is momentarily discharged with a fixed 80µA

current until the load current through the sense resistor

(RSENSE) decreases below its threshold point. This

effectively regulates the turn-on current during start-up.

Figure 2 shows the start-up waveforms. STAT goes

high at the end of the start-up period if no fault condi-

tion is present.

ON

STAT

VGATE

VOUT

IFAST, SET

ILOAD

tSTART

~VIN

VTH

tON

CBOARD = LARGE

CBOARD = 0

VGATE

VOUT

Figure 2. Start-Up Waveforms

Normal Operation (DualSpeed/BiLevel)

In normal operation (after the start-up period has

expired), protection is provided by turning off the exter-

nal MOSFET when a fault condition is encountered.

DualSpeed/BiLevel fault protection incorporates two

comparators with different thresholds and response

times to monitor the load current:

1) Slow Comparator. This comparator has an externally

set response time (20µs to seconds) and a fixed

50mV threshold voltage. The slow comparator

ignores low-amplitude momentary current glitches.

After an extended overcurrent condition, a fault is

detected and the MOSFET gate is discharged.

2) Fast Comparator. This comparator has a fixed

response time and a higher 200mV threshold volt-

age. The fast comparator turns off the MOSFET

immediately after it detects a large amplitude event

such as a short circuit.

In each case, when a fault is encountered, the status

pin (STAT) goes low and the MAX4370 stays latched

off. Figure 3 shows the waveforms after a fault condi-

tion.

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