MAX5921/MAX5939
-48V Hot-Swap Controllers with External
R
SENSE
and High Gate Pulldown Current
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If the voltage between V
EE
and SENSE reaches the cur-
rent-limit trip voltage (V
CL
), the MAX5921/MAX5939 pull
down the GATE and regulate the current through the
external MOSFET such that V
SENSE
- V
EE
< V
CL
. If the
current drawn by the load drops below V
CL
/ R
SENSE
limit, the GATE voltage rises again. However, if the load
current is at the regulation limit of V
CL
/ R
SENSE
for a peri-
od of t
PHLCL
, the electronic circuit breaker trips, causing
the MAX5921/MAX5939 to turn off the external MOSFET.
After an overcurrent fault condition, the MAX5921 auto-
matically restarts after t
OFF
has elapsed. The MAX5939
circuit breaker is reset by toggling UV or by cycling
power. Unless power is cycled to the MAX5939, the
device waits until t
OFF
has elapsed before turning on the
gate of the external FET.
Load-Current Regulation
The MAX5921/MAX5939 accomplish load-current regu-
lation by pulling current from GATE whenever V
SENSE
-
V
EE
> V
CL
. This decreases the gate-to-source voltage of
the external MOSFET, thereby reducing the load current.
When V
SENSE
- V
EE
< V
CL
, the MAX5921/MAX5939 pulls
GATE high by a 45µA (I
PU
) current.
Exponential Current Regulation
The MAX5921/MAX5939 provide an exponential pull-
down current to turn off the external FET in response to
overcurrent conditions. The GATE pulldown current
increases (see Typical Operating Characteristics) in
response to V
SENSE
- V
EE
potentials greater than 50mV
(V
CL
).
Load Current Regulation
(Short-Circuit Condition)
The MAX5921/MAX5939 devices also include a very
fast high-current pulldown source connected to GATE
(see Typical Operating Characteristics). The high-cur-
rent pulldown activates if V
SENSE
exceeds V
EE
by
650mV (typ) during a catastrophic overcurrent or short-
circuit fault condition. The high-current pulldown circuit
sinks as much as 450mA from GATE to turn off the
external MOSFET.
Immunity to Input Voltage Steps
The MAX5921/MAX5939 guard against input voltage
steps on the input supply. A rapid increase in the input
supply voltage (V
DD
- V
EE
increasing) causes a current
step equal to I = C
L
x ∆V
IN
/ ∆t, proportional to the input
voltage slew rate (∆V
IN
/ ∆t). If the load current exceeds
V
CL
/ R
SENSE
during an input voltage step, the MAX5921/
MAX5939 current limit activates, pulling down the gate
voltage and limiting the load current to V
CL
/ R
SENSE
. The
DRAIN voltage (V
DRAIN
) then slews at a slower rate than
the input voltage. As the drain voltage starts to slew
down, the drain-to-gate feedback capacitor C2 pushes
back on the gate, reducing the gate-to-source voltage
(V
GS
) and the current through the external MOSFET.
Once the input supply reaches its final value, the DRAIN
slew rate (and therefore the inrush current) is limited by
the capacitor C2 just as it is limited in the startup condi-
tion (see the Power-Supply Ramping section). To ensure
correct operation, R
SENSE
must be chosen to provide a
current limit larger than the sum of the load current and
the dynamic current into the load capacitance in the
slewing mode.
If the load current plus the capacitive charging current is
below the current limit, the circuit breaker does not trip.
Undervoltage and Overvoltage Protection
Use UV and OV to detect undervoltage and overvoltage
conditions. UV and OV internally connect to analog com-
parators with 130mV (UV) and 50mV (OV) of hysteresis.
When the UV voltage falls below its threshold or the OV
voltage rises above its threshold, GATE pulls low. GATE
is held low until UV goes high and OV is low, indicating
that the input supply voltage is within specification. The
MAX5921/MAX5939 includes an internal lockout (UVLO)
that keeps the external MOSFET off until the input supply
voltage exceeds 15.4V, regardless of the UV input.
UV is also used to reset the circuit breaker after a fault
condition has occurred. Pull UV below V
UVL
to reset the
circuit breaker.
