-48V Simple Swapper Hot-Swap Switches

6

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Detailed Description

Figure 1 shows a functional block diagram. The

MAX5911/MAX5912 are hot-swap switches for negative

supply rails. The devices allow the safe insertion and

removal of circuit cards into live backplanes or ports

without causing undue glitches on the negative power-

supply rail. They operate over the -16V to -65V voltage

range. The devices act as a current regulator, using

their on-board FET to limit the amount of current drawn

from the backplane to 280mA for MAX5911 and 415mA

for MAX5912. If the load current exceeds that current

limit, the internal switch behaves like a constant current

source.

The MAX5911/MAX5912 monitor the input voltage, the

output voltage, the output current, and the die tempera-

ture. They assert their FAULT output if they detect an

error condition.

A zero-current load disconnect feature activates if the

load current drops below 8mA for over 350ms. They also

include an enable input that responds to positive logic,

allowing the host system to turn the FET switch on or off.

The enable input is also used to reset the devices after

an error condition has latched the output FET off.

Normal Operation

Startup

When power is first applied with ENABLE high, or when

ENABLE is asserted, the MAX5911/MAX5912 limit the

current to the load to 280mA/415mA. If the output volt-

age has not reached the power-good threshold within

the 14ms power-good delay, FAULT is pulled low.

FAULT stays low until the output voltage exceeds the

power-good threshold, at which time it is released and

becomes high impedance.

Undervoltage Lockout Fault

If

at which the UVLO fault occurs, FAULT is pulled low

and the internal FET is turned off. If the condition per-

default UVLO voltage is -28V, but it can be adjusted

using an external resistive divider (see the Changing

the Undervoltage Lockout Setting section). Once the

FET is latched off, ENABLE must be pulsed low then

Thermal Shutdown Fault

The MAX5911/MAX5912 monitor their internal die tem-

peratures. If the temperature of the die exceeds

+140°C, FAULT is pulled low and the internal FET is

latched off. ENABLE must be pulsed low and then

brought high again to reset the FET latch, and release

FAULT, provided the die is below the thermal shutdown

temperature by 5°C (typ).

Power-Not-Good Fault

for over 14ms, FAULT is pulled low. When the output

voltage exceeds the power-good threshold, FAULT is

released. FAULT output is not latched during a power-

not-good fault.

Zero-Current Detection

If the load current drops below 8mA for over 350ms,

FAULT is pulled low and the FET is latched off. ENABLE

must be pulsed low and then brought high again to

Applications Information

Chosing A Device

The MAX5911 will output up to 280mA (typ) and the

MAX5912 up to 415mA (typ). In applications where

high heat is expected, use the MAX5912 for its superior

UVLO

AGND

ENABLE

FAULT

DGND

LEVEL

SHIFTER

CONTROL

LOGIC

2.11M

Ω

99.5k

Ω

MAX5911/MAX5912

GATE

0

POWER GOOD

POWER-NOT-GOOD DELAY

POWER-GOOD

HYSTERESIS

Figure 1. Functional Block Diagram