Turn-On and Turn-Off Delays
After power is applied, or ON/
OFF
is released, there is
a 300ms 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 2μs. An undervoltage condition must exist for at
least 20ms (t
OFF
) before the MAX5900/MAX5901 turn
off the external MOSFET. ON/
OFF
must be held low for
at least 20ms (t
OFF
) before the MAX5900/MAX5901 turn
off the external MOSFET. Turn-off delay minimizes spu-
rious shutdowns due to noisy signals or momentary
voltage spikes, as well as preventing accidental
resetting of the circuit breaker latch (MAX5900L/
MAX5901L).
Thermal Shutdown
A thermal shutdown feature helps protect the external
MOSFET. If the die temperature of the MAX5900/
MAX5901 exceeds +125
°
C, the MOSFET is turned off.
For accurate performance, the MAX5900/MAX5901
must be in close thermal contact with the external
MOSFET (see
Layout Guidelines
section). Due to the
low power dissipation of the MAX5900/MAX5901, its
junction temperature will typically be within a few
degrees of the MOSFET. All versions of the MAX5900/
MAX5901 automatically restart from a temperature fault
when the junction temperature drops below +110
°
C.
Undervoltage Lockout
The MAX5900/MAX5901 turn off the external MOSFET if
the magnitude of the input voltage is below the level set
by ON/
OFF
for longer than 20ms (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 resistors. To
set the lockout voltage to a value between -9V and
-100V, use a resistor-divider connected between GND
and V
EE
, with the center node of the divider connected
to ON/
OFF
. For example, use a 3k
resistor (R1 in
Figure 2) from ON/
OFF
to V
EE
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 2 shows an example circuit with V
UVLO
set for
-20V. To defeat the UVLO, simply connect a single 100k
resistor between ON/
OFF
and GND, as shown in Figure 4.
R
R
V
UVLO
1 26
.
2
1
1
=
×
M
-100V SOT23 Simple Swapper Hot-Swap
Controllers
_______________________________________________________________________________________
7
HOT-SWAP CONTROLLER
OPTIONAL
ON/OFF
-48V
V
EE
GND
R2
47k
R1
3k
MAX5900
MAX5901
GND
ON/OFF
DGND
HOT-SWAP CONTROLLER #1
V
EE
GND
ON/OFF
C
2C
MAX5900
MAX5901
GND
HOT-SWAP CONTROLLER #2
V
EE
V
EE
GND
ON/OFF
MAX5900
MAX5901
Figure 2. Programmed -20V Lockout With Optional
Optocoupler On/Off Control
Figure 3. Power-Supply Sequencing
Figure 4. Defeating Undervoltage Lockout
HOT-SWAP CONTROLLER
-48V
V
EE
GND
100k
MAX5900
MAX5901
GND
ON/OFF