MAX5923EUP+T【PDF 10/12 页】IC HOT-SWAP SWITCH +60V 20-TSSOP

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Sheet目录43

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+60V Simple Swapper Hot-Swap Switch

10 ______________________________________________________________________________________

Output Clamping During Short Circuit

During an output short-circuit condition, the energy stored

in the output capacitor is dumped onto the short and

builds up a large current in the power path and its para-

sitic inductance. The larger the output capacitor, the larg-

er the current buildup. The parasitic inductive current

circulates through the short circuit and brings OUT below

ground level. A Schottky diode is needed at the output to

provide the path for this circulating current and to prevent

OUT from going too far below ground and exceeding the

MAX5923s absolute maximum ratings.

For an output capacitor with low ESR (ceramic and film

type), and with values of 0.47礔 or less, a B1100LB

(100V, 1A Schottky) is required as shown in the Typical

Application Circuit. For higher values of capacitance,

more robust clamping is required. For applications using

aluminum electrolytic capacitors, which have relatively

high ESR, the requirement of the clamp diode is reduced.

Good layout with less parasitic trace inductance also

helps in reducing the clamping diode requirement and

should be examined on a case-by-case basis.

Power-OK (POK)

POK goes open-drain t

POK_HIGH

(88ms) after V

OUT

rises to within 0.75V (V

THPOK

) from V

. POK goes low

POK_LOW

(1.4ms) after V

OUT

drops 0.75V below V

Zero-Current Detection

Zero-current detection is enabled if ZC_EN is high and

only after the startup period has finished (indicated by

POK going high). When V

SENSE

falls below the zero-

current threshold (V

ZCTH

) for a continuous t

ZCDEL

350ms, a zero-current fault is generated. The MOSFET

is turned off and ZC is latched low. ZC is open drain

during initial power-up. After a zero-current fault has

occurred, the MAX5923 is latched off and is restarted

by applying a low-to-high transition on the EN pin.

At any time during a zero-current condition, if V

SENSE

goes above V

ZCTH

for the zero-current deglitch time

ZC_DEG

= 10ms), the zero-current counter resets to

zero and a zero-current fault is not generated. Bring

ZC_EN low to disable the zero-current detection func-

tion. ZC stays high impedance in this mode.

Thermal Shutdown

If the MAX5923 die temperature reaches +150癈, an

overtemperature fault is generated. The MOSFET turns

off and FAULT goes low. The MAX5923 die tempera-

ture must cool down below +120癈 before the overtem-

perature fault condition is removed (see the Fault

Management section).

Fault Report (FAULT)

FAULT goes low when there is an overcurrent fault and/or

an overtemperature fault. FAULT is open drain otherwise.

After a fault, the FAULT signal is latched low. FAULT is

unlatched at the beginning of the next power mode.

Fault Management

The MAX5923 offers either latched-off or autoretry fault

management configurable by the LATCH input.

Bringing LATCH high puts the device into latch mode

while pulling LATCH low selects the autoretry option.

In latch mode, the MAX5923 turns the MOSFET off and

keeps it off after an overcurrent fault or an overtemper-

ature fault. After the fault condition goes away, recycle

the power supplies or toggle the EN pin low and high

again to unlatch the part. However, the part waits a

RESTART

period (1.92s) before recovering from a fault

condition and resuming normal operation.

In autoretry mode, MAX5923 turns the MOSFET off after

an overcurrent or overtemperature fault. After the fault

condition is removed, the device waits a t

RESTART

period

(1.92s) and then automatically restarts. If the fault was

due to an overtemperature condition, the MAX5923 waits

for its die temperature to cool down below the hysteresis

level before starting the t

RESTART

time.

RESTART

FAULT

OUT

LIMIT

Figure 3. Periodic Overcurrent Without Generating a Fault

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