MAX8533EUB+T【PDF 8/10 页】IC HOT-SWAP CNTRLR 12V 10-UMAX

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Sheet目录44

4页 5页 6页 7页 [8页]9页 10页

undervoltage reset monitor at full load. The maximum

gate voltage (V

) rating must be at least ?0V. Low

MOSFET gate capacitance is not necessary for the

inrush current limiting because it is achieved by limiting

the GATE dV/dt. However, higher gate capacitance

increases the turn-off time of the MOSFET under fault

conditions.

Current-Limit and Overload Protection

The MAX8533 features a dual overcurrent protection

circuit that turns off the MOSFET in overcurrent situa-

tions. When an overload event is sensed, the IC limits

the current to a level set by ISET. Continuous overload

for a period set by the user (t

CTIM

) latches off the

MOSFET. The severe overcurrent protection immediate-

ly shuts down the external MOSFET and latches it off.

R3 sets the current-limit threshold voltage. This voltage

is generated from an internal 20礎 source driven

through R3. Therefore:

ILIM

= R3 x 20礎

The current-sense signal is sensed across resistor R1.

With no load, the voltage at ISET is the input voltage

plus V

ILIM

. As the load current increases, the voltage

drop across R1 increases and reduces the voltage at

ISET. Once V

ISET

is lower than V

, the overcurrent

comparator (Figure 1) is tripped and the MAX8533

enters current regulation mode. During current regula-

tion mode, the gate voltage of the MOSFET is

decreased to limit the current to the output. The maxi-

mum time period for the current regulation mode is set

by the external capacitor at CTIM (C3). This feature

allows transient currents that exceed the current limit to

pass without shutting down the circuit. The current regu-

lation time period is determined as:

IREG

= C3 x (1.8V/20礎)

If t

IREG

expires and the overcurrent condition still

exists, the MOSFET is latched off.

The severe overcurrent comparator (Figure 1) trips if

the drop across the current-sense resistor (R1) is

150mV higher than the current-limit threshold (V

exceeds V

ISET

by 150mV). During a severe overcurrent

event, the gate of the external MOSFET is pulled down

with a 350mA current source and latched immediately.

Toggle EN, LPEN, or input power to clear the latched

fault condition.

Overvoltage Protection

The MAX8533 has an adjustable overvoltage protection

feature that latches the IC off in case of an overvoltage

event. An external resistor-divider (R4 and R5, Figure 2)

from OUT to RET with OVP connected to the center,

sets the overvoltage threshold. Use 4.99k& for R4. R5

is determined using the following equation:

R5 = 4.99 x 10

x ((V

OVT

/ V

OVP

) - 1)

OVT

is the desired overvoltage threshold and V

OVP

2V (typ). OVP latches off the MAX8533 if an overvoltage

condition exists for 1.5ms. Toggle EN, LPEN, or input

power to clear the latched fault condition.

Fault Reset

Overcurrent, severe overcurrent, and overvoltage con-

ditions result in the MAX8533 entering a latched fault

condition. Toggle LPEN, EN, or input power to reset the

latched fault condition and return to normal operation.

Power-Good Output (POK)

POK is an open-drain output used to enable the on-

board DC-to-DC converter. The POK output turns high

impedance when the output rail reaches 9.6V. POK

must be pulled up to the users logic level using a

pullup resistor.

Smallest, Most Reliable, 12V, Infiniband-

Compliant Hot-Swap Controller

8 _______________________________________________________________________________________

REF

DESIGNATOR

DESCRIPTION

MANUFACTURER

PART NO.

470礔, 25V aluminum

electrolytic capacitor

Sanyo

25MV470HC

0.1礔 ?0%, 25V X5R

ceramic capacitor

Taiyo Yuden

TMK107BJ104KA

0.01礔 ?0% X7R

ceramic capacitor

Kemet

C0606C103M4RAC

0.22礔 ?0%, 25V X5R

ceramic capacitor

TDK

C1608X5R1A224K

R1*

10m& ?%, 0.5W

current-sense resistor

Dale

LRF1206- 01- R010- F

20& ?% resistor

Panasonic

3.09k& ?% resistor Panasonic

4.99k& ?% resistor Panasonic

31.6k& ?% resistor Panasonic

N-channel MOSFET,

30V, 6m&

Siliconix

Si4842DY

Table 2. External Component List for 50W

Output

*Use a 20m& ?/SPAN>1%, 0.25W current-sense resistor for 25W

applications.

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