MAX5948BESA+T【PDF 14/18 页】IC CNTRLR HOT SWAP 8-SOIC

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Sheet目录43

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When the DRAIN voltage of the MAX5948B is high with

respect to V

(Figure 12), the internal MOSFET Q3 is

turned off so that I1 and the internal MOSFET Q2 clamp

the PWRGD pin to the DRAIN pin. MOSFET Q2 sinks

the modules pullup current, and the module turns off.

When the DRAIN voltage drops below V

, MOSFET Q3

turns on, shorting I1 to V

and turning Q2 off. The

pullup current in the module pulls PWRGD high,

enabling the module.

GATE Voltage Regulation

GATE goes high when the following startup conditions

are met: UV is high, OV is low, the supply voltage is

above V

UV,LH

, and (V

SENSE

- V

) is less than 50mV.

GATE is pulled up with a 45礎 current source and is

regulated at 13.5V above V

. The MAX5948A/

MAX5948B include an internal clamp that ensures the

GATE voltage of the external MOSFET never exceeds

18V. During a fast-rising V

, the clamp also keeps the

GATE and SENSE potentials as close as possible to

prevent the FET from accidentally turning on. When a

fault condition is detected, GATE is pulled low with a

50mA current.

DRAIN Pin Protection

The MAX5948s DRAIN pin withstands negative volt-

ages (referenced to V

); no external diode is required.

When the -48V backplane shorts to ground and V

becomes 0V, the DRAIN pin is held at less than 1.5V

(sum of Q1s body diode and voltage drop across R1)

below V

due to the storage capacitor C3 (Figure 13).

The -1.5V results in a 50mA reverse DRAIN current,

which is within the capability of the MAX5948. A design

with R1 larger than 0.1& may require a resistor in series

with the DRAIN pin to avoid exceeding the 50mA drain

current maximum.

-48V Hot-Swap Controllers

with External R

SENSE

14 ______________________________________________________________________________________

PWRGD

-48V

MAX5948A

IN+

IN-

OUT+

OUT-

ON/OFF

ACTIVE-HIGH

ENABLE MODULE

DRAIN

GATE

SENSE

*DIODES INC. SMAT70A.

-48V RTN

(SHORT PIN)

Figure 13. Active-Low Enable Module

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