MAX5904(2003) Ver la hoja de datos (PDF) - Maxim Integrated

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Fabricante

MAX5904
(Rev.:2003)

Low-Voltage, Dual Hot-Swap Controllers/Power Sequencers

Maxim Integrated 

Low-Voltage, Dual Hot-Swap Controllers/Power

Sequencers

ON

PGOOD

tSTART + tPGDLY

4.8V TO 5.8V

VTH

VGATE

VOUT

VSU,TH

RSENSE

ILOAD

tON

Figure 2. Startup Waveforms

CBOARD = LARGE

CBOARD = 0

VGATE

VOUT

PGOOD is pulled low, the MAX5905/MAX5907/

MAX5909 stay latched off and the MAX5904/MAX5906/

MAX5908 automatically restart.

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.

The comparator threshold voltage (VSC,TH) is fixed at

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

sense resistor does not exceed VSU,TH. The startup

fast-comparator threshold voltage (VSU,TH) is scaled to

two times the slow-comparator threshold (VSC,TH).

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

fast-comparator threshold voltage (VFC,TH) is scaled to

four times the slow-comparator threshold (VSC,TH). This

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)

for tD,UVLO. The MAX5904–MAX5909 use power from

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-

cient gate drive voltage. tD,UVLO ensures that the board

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

tD,UVLO and the startup period.

Latched and Auto-Retry 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 auto-retry delay that is internally set

to 64 times tSTART. During the auto-retry delay, toggling

ON below 0.4V does not clear the fault. The auto-retry

can be overridden causing the startup period to begin

immediately by toggling one of the supply voltages

below the UVLO threshold.

Output Voltage Monitor

The MAX5905–MAX5909 monitor the output voltages

with the MON1 and MON2 window comparator inputs.

These voltage monitors are enabled after the startup

period. Once enabled, the voltage monitor detects a

fault if VMON_ is less than 543mV or greater than

687mV. If an output voltage fault is detected PGOOD

pulls low. When the MAX5906/MAX5907 detect an out-

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