Slow-Comparator Startup Period

The slow comparator is disabled during the startup

period while the external MOSFETs are turning on.

Disabling the slow comparator allows the device to

ignore the higher-than-normal inrush current charging

the board capacitors when a card is first plugged into a

live backplane.

Slow-Comparator Normal Operation

After the startup period is complete the slow compara-

tor is enabled and the device enters normal operation.

25mV for the MAX5904/MAX5905 and is adjustable

from 25mV to 100mV for the MAX5906–MAX5909. The

slow-comparator response time decreases to a mini-

mum of 110µs with a large overdrive voltage (Figure 9).

Response time is 3ms for a 1mV overdrive. The variable

speed response time allows the MAX5904–MAX5909 to

ignore low-amplitude momentary glitches, thus increas-

ing system noise immunity. After an extended overcur-

rent condition, a fault is generated, PGOOD is pulled

low, and the MOSFET gates are discharged with a

strong 3mA pulldown current.

Fast-Comparator Startup Period

During the startup period the fast comparator regulates

the gate voltage to ensure that the voltage across the

Fast-Comparator Normal Operation

In normal operation, if the load current reaches the fast-

comparator threshold, a fault is generated, PGOOD is

pulled low, and the MOSFET gates are discharged with

a strong 3mA pulldown current. This happens in the

event of a serious current overload or a dead short. The

comparator has a fast response time of 260ns (Figure 9).

Undervoltage Lockout (UVLO)

The undervoltage lockout prevents the MAX5904–

MAX5909 from turning on the external MOSFETs until

one input voltage exceeds the UVLO threshold (2.4V)

the higher input voltage rail for the charge pumps. This

allows for more efficient charge-pump operation. The

UVLO protects the external MOSFETs from an insuffi-

is fully inserted into the backplane and that the input

voltages are stable. Any input voltage transient on both

supplies below the UVLO threshold will reinitiate the

Latched and Autoretry Fault Management

The MAX5905/MAX5907/MAX5909 latch the external

MOSFETs off when a fault is detected. Toggling ON

below 0.4V or one of the supply voltages below the

UVLO threshold for at least 100µs clears the fault latch

and reinitiates the startup period. Similarly, the

MAX5904/MAX5906/MAX5908 turn the external

MOSFETs off when a fault is detected then automatical-

ly restart after the autoretry delay that is internally set to

ON below 0.4V does not clear the fault. The autoretry

can be overridden causing the startup period to begin

immediately by toggling one of the supply voltages

below the UVLO threshold.

Low-Voltage, Dual Hot-Swap Controllers/Power

Sequencers

______________________________________________________________________________________

11

4.8V TO 5.8V

PGOOD

ON

Figure 2. Startup Waveforms