MAX5963UTL+T【PDF 17/20 页】IC HOT SWAP DIO ORING CTL 40TQFN

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Sheet目录43

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The MAX5963 is capable of being powered through

PWR or through channel A or B. Pull ENZ low and apply

a voltage to either channel to power the device. In this

mode, BP is powered by V

according to Table 3.

Fast-Off/Slow-On (FOSO) Protection

GATE1A, GATE2A, GATE1B, and GATE2B immediately

pull low when FOSO exceeds the 1.24V (typ) threshold.

The MAX5963 waits for the 1s (typ) turn-on delay once

FOSO falls below the threshold hysteresis before allow-

ing either channel to turn back on. See the FireWire

Power Management section for more information.

Applications Information

Startup Considerations

Set the appropriate current-limit threshold for success-

ful startup. A successful startup is dependent on the

MAX5963 current-limit threshold and timeout period. A

large capacitor at the output results in a charging cur-

rent equivalent to the current-limit threshold and may

cause the MAX5963 to exceed its 1ms (typ) default

timeout period if the current-limit threshold is set too

low. Use the following formula to compute the minimum

current-limit setting:

where I

CL_LIMIT

is the programmed current limit, C

OUT

is the capacitor at OUT_, V

is the supply voltage, t

is the adjustable current-limit timeout period, and I

LOAD

is the load current during startup. With V

= 12V,

OUT

= 220礔, t

= 1ms, and I

LOAD

= 0, set the

MAX5963 current limit greater than 2.6A. This calcula-

tion does not include tolerances.

Choosing R

SENSE_

Select sense resistors, R

SENSEA

and R

SENSEB

, which

cause the circuit-breaker voltage drop at a current-

limit/circuit-breaker level above the maximum normal

operating current. Typically, set the current limit at 1.2

to 1.5 times the nominal load current.

Choose the sense resistor power rating to accommo-

date a current-limit condition:

SENSE_

= (V

CB_TH

)

SENSE_

where P

SENSE_

is the power dissipated across R

SENSE_

during a current-limit/circuit-breaker fault.

MOSFET Selection

Select external MOSFETs according to the application

current level. The MOSFETs on-resistance (R

DS(ON)

)

should be chosen low enough to have minimum voltage

drop at full load to limit the MOSFET power dissipation.

High R

DS(ON)

also causes large 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 autoretry mode. During

normal operation, the external MOSFETs dissipate little

power. The power dissipated in normal operation is:

P = I

LOAD

x R

DS(ON)

The most power dissipation occurs during a current-

limit event, resulting in high power dissipated in Q_ dur-

ing the current-limit period for the MAX5963. Calculate

the power dissipated across Q_ during this period

using the following equation:

Q__

= (V

- V

SENSE_

) x I

CL_LIMIT

where V

SENSE_

is the voltage across the current-sense

resistor, R

SENSE_

, V

is the input voltage and I

CL_LIMIT

is the programmed current limit. Though there are two

MOSFETs in series at the outputs, the safest assump-

tion is that all of the current-limiting power dissipation

occurs in one of the two MOSFETs; perfect sharing of

the current-limit voltage drop is unlikely.

C x V

CL_LIMIT

OUT PS

LOAD

Dual, 7.5V to 76V, Hot-Swap and

Diode ORing Controller

______________________________________________________________________________________ 17

Table 3. BP Voltage in Standby Mode

PWR AND PS CONFIGURATION

ENZ

PS_UVLO

(V)

> V

PWR

- 0.1V

Low

High

- 0.7V

> V

PWR

- 0.1V

Low

< V

PWR

- 0.1V

Low

< V

PWR

- 0.1V

High*

Low

< V

PWR

- 0.1V

High*

High

PWR

*ENX or ENY must be high to turn on GATEPS.

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