Datasheet

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 4µs. 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°C, 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°C.
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 3kΩ 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
100kΩ 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 4µs when a circuit-
breaker event occurs or if the die temperature exceeds
+125°C. 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.
RR
V
V
UVLO
ON OFF
21 1 =× −
⎛
⎝
⎜
⎞
⎠
⎟
/
MAX5902/MAX5903
+72V, SOT23/TDFN, Simple Swapper Hot-Swap
Controllers
8 _______________________________________________________________________________________
HOT-SWAP CONTROLLER
+9V TO +72V
V
S
GND
100kΩ
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 47kΩ RESISTOR AS SHOWN.
47kΩ
MAX5902
MAX5903
GND
ON/OFF
Figure 5. Logic Control