MAX5911/MAX5912
-48V Simple Swapper Hot-Swap Switches
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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 V
IN
≤UVLO, where UVLO is the desired voltage
at which the UVLO fault occurs, FAULT is pulled low
and the internal FET is turned off. If the condition per-
sists for over 350ms (IF
DEL
), the FET is latched off. The
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
brought back high to release FAULT and enable V
OUT
.
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
If |V
IN
- V
OUT
| is more than the power-good threshold
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
release FAULT and enable V
OUT
.
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
