MAX5909EEE+【PDF 16/24 页】IC HOT-SWAP CTRLR DUAL 16QSOP

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Sheet目录43

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Output Voltage Monitor

The MAX5905MAX5909 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 V

MON

_ 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-

put voltage fault on either MON1 or MON2, the fault is

latched and both external MOSFET gates are dis-

charged at 3mA. When the MAX5908/MAX5909 detect

an output voltage fault the external MOSFET gates are

not affected. The MAX5908/MAX5909 PGOOD goes

high impedance when the output voltage fault is

removed. The voltage monitors do not react to output

glitches of less than 20祍. A capacitor from MON_ to

GND increases the effective glitch filter time. Connect

MON1 to IN1 and MON2 to IN2 to disable the output

voltage monitors.

Status Output (PGOOD)

The status output is an open-drain output that pulls low

in response to one of the following conditions:

" Forced off (ON < 0.8V)

" Overcurrent fault

" Output voltage fault

PGOOD goes high impedance 0.75ms after the device

enters normal operation and no faults are present

(Table 1).

Applications Information

Component Selection

n-Channel MOSFET

Select the external MOSFETs according to the applica-

tions current levels. Table 2 lists some recommended

components. The MOSFETs on-resistance (R

DS(ON)

)

should be chosen low enough to have a minimum volt-

age drop at full load to limit the MOSFET power dissipa-

tion. High R

DS(ON)

causes output ripple if there is a

pulsating load. Determine the device power rating to

accommodate a short-circuit condition on the board at

startup and when the device is in automatic-retry mode

(see the MOSFET Thermal Considerations section).

Using the MAX5905/MAX5907/MAX5909 in latched

mode allows the use of MOSFETs with lower power rat-

ings. A MOSFET typically withstands single-shot pulses

with higher dissipation than the specified package rat-

ing. Table 3 lists some recommended manufacturers

and components.

Sense Resistor

The slow-comparator threshold voltage is set at 25mV

for the MAX5904/MAX5905 and is adjustable from

25mV to 100mV for the MAX5906MAX5909. Select a

sense resistor that causes a drop equal to the slow-

comparator threshold voltage at a current level above

the maximum normal operating current. Typically, set

the overload current at 1.2 to 1.5 times the nominal load

current. The fast-comparator threshold is four times the

slow-comparator threshold in normal operating mode.

Choose the sense resistor power rating to be greater

than (I

OVERLOAD

)

x V

SC,TH

Slow-Comparator Threshold, R

LIM

The slow-comparator threshold voltage of the

MAX5904/MAX5905 is fixed at 25mV and adjustable

from 25mV to 100mV for the MAX5906MAX5909.

The adjustable slow-comparator threshold of the

MAX5906MAX5909 allows designers to fine-tune the

current-limit threshold for use with standard value

sense resistors. Low slow-comparator thresholds allow

for increased efficiency by reducing the power dissipat-

ed by the sense resistor. Furthermore, the low 25mV

Low-Voltage, Dual Hot-Swap Controllers/Power

Sequencers

16 ______________________________________________________________________________________

Table 1. Status Output Truth Table

DEVICE IN

UVLO DELAY

PERIOD

DEVICE IN

STARTUP

PERIOD

OVERCURRENT

FAULT

OVER/UNDER-

VOLTAGE

FAULT

PART IN RETRY-TIMEOUT

PERIOD OR LATCHED OFF

PGOOD

Yes

Low

Yes

Low

Yes

Low

Yes

Low

Yes

Low

High

X = Dont care.

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