MAX5903LAETT+T【PDF 8/14 页】IC HOT-SWAP CONTROLLER 6-TDFN

Turn-On and Turn-Off Delays

After power is applied, or ON/OFF is released, there is

a 150ms delay (t

ON

) before the gate ramp is started.

This delay is also the automatic restart time delay.

In the event of a circuit-breaker condition or an over-

temperature fault condition, the turn-off delay is less

than 4祍. An undervoltage condition must exist for at

least 10ms (t

OFF

) before the MAX5902/MAX5903 turns

off the external MOSFET. ON/OFF must be held low for

at least 10ms (t

OFF

) before the MAX5902/MAX5903

turns off the external MOSFET. Turn-off delay minimizes

spurious shutdowns due to noisy signals or momentary

voltage spikes, as well as preventing accidental reset-

ting of the circuit-breaker latch (MAX5902L/MAX5903L).

Thermal Shutdown

A thermal shutdown feature helps protect the external

MOSFET. If the die temperature of the MAX5902/

MAX5903 exceeds +125癈, the MOSFET is turned off.

For accurate performance the MAX5902/MAX5903

must be in close thermal contact with the external MOS-

FET. See the Layout Guidelines section for information.

Due to the low power dissipation of the

MAX5902/MAX5903, its junction temperature will typi-

cally be within a few degrees of the MOSFET. All ver-

sions of the MAX5902/MAX5903 automatically restart

from a temperature fault when the junction temperature

drops below +110癈.

Undervoltage Lockout

The MAX5902/MAX5903 turns off the external MOSFET

if the magnitude of the input voltage is below the level

set by ON/OFF for longer than 10ms (t

OFF

). If ON/OFF

is left unconnected, the lockout voltage (V

UVLO

)

defaults to 31.5V. V

UVLO

may also be set to any value

within the power-supply range by using external resis-

tors. To set the lockout voltage to a value between +9V

and +72V use a resistor divider connected between V

S

and GND, with the center node of the divider connect-

ed to ON/OFF. For example, use a 3k& resistor (R1 in

Figure 3) from ON/OFF to GND and calculate the other

resistor, R2, using:

where V

UVLO

is the desired lockout voltage, and

V

ON/OFF

is the ON/OFF reference threshold specified in

the Electrical Characteristics table (typically 1.26V).

Figure 3 shows an example circuit with V

UVLO

set for

+18V. To defeat the UVLO simply connect a single

100k& resistor between ON/OFF and V

S

, as shown in

Figure 6.

The Power-Good Output

The power-good output, PGOOD (PGOOD), is open-

drain and asserts when the external MOSFET is fully

enhanced and V

SD

is less than the circuit-breaker

threshold (V

CB

). For versions without the circuit-breaker

function (MAX5902N/MAX5903N), PGOOD (PGOOD)

asserts when the external MOSFET is fully enhanced.

PGOOD (PGOOD) deasserts within 4祍 when a circuit-

breaker event occurs or if the die temperature exceeds

+125癈. PGOOD (PGOOD) deasserts if V

S

< V

UVLO

for

longer than 10ms or ON/OFF is held low for longer than

10ms.

The MAX5902 PGOOD is active-low and the MAX5903

PGOOD is active-high. Both are open-drain n-channel

MOSFETs with their sources connected to GND, and

can withstand up to +72V.

R

V

UVLO

ON OFF

2

1

=

?/DIV>

/

+72V, SOT23/TDFN, Simple Swapper Hot-Swap

Controllers

8 _______________________________________________________________________________________

HOT-SWAP CONTROLLER

+9V TO +72V

V

S

GND

100k&

MAX5902

GND

ON/OFF

Figure 6. Defeating Undervoltage Lockout

HOT-SWAP CONTROLLER

5V LOGIC

GND

LOGIC CONTROL OF 3.3V OR LESS MAY BE CONNECTED

DIRECTLY TO THE MAX5902/MAX5903. FOR 5V LOGIC

CONTROL INSERT A SERIES 47k& RESISTOR AS SHOWN.

47k&

MAX5902

MAX5903

GND

ON/OFF

Figure 5. Logic Control

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