MAX5921/MAX5939
-48V Hot-Swap Controllers with External
R
SENSE
and High Gate Pulldown Current
_______________________________________________________________________________________ 9
Board Insertion
Figure 6a shows a typical hot-swap circuit for -48V sys-
tems. When the circuit board first makes contact with
the backplane, the DRAIN to GATE capacitance (C
gd
)
of Q1 pulls up the GATE voltage to roughly IV
EE
x
(C
gd
/C
gd
+ C
gs
)I. The MAX5921/MAX5939 feature an
internal dynamic clamp between GATE and V
EE
to
keep the gate-to-source voltage of Q1 low during hot
insertion preventing Q1 from passing an uncontrolled
current to the load. For most applications, the internal
clamp between GATE and V
EE
of the MAX5921/
MAX5939 eliminates the need for an external gate-to-
source capacitor. The resistor R3 limits the current into
the clamp circuitry during card insertion.
Power-Supply Ramping
The MAX5921/MAX5939 can reside either on the back-
plane or the removable circuit board (Figure 6a). Power
is delivered to the load by placing an external N-chan-
nel MOSFET pass transistor in the power-supply path.
After the circuit board is inserted into the backplane,
and the supply voltage at V
EE
is stable and within the
undervoltage and overvoltage tolerance, the
MAX5921/MAX5939 gradually turn on the external
MOSFET by charging the gate of Q1 with a 45µA cur-
rent source. Capacitor C2 provides a feedback signal
to accurately limit the inrush current.
The inrush current can be calculated:
I
INRUSH
= I
PU
x C
L
/ C2
where C
L
is the total load capacitance, C3 + C4, and
I
PU
is the gate pullup current.
Figure 6b shows the inrush current waveform. The cur-
rent through C2 controls the GATE voltage. At the end
of the DRAIN ramp, the GATE voltage is charged to its
final value. The GATE-to-SENSE clamp limits the maxi-
mum ∆V
GATE
to 18V.
Board Removal
If the circuit card is removed from the backplane, the volt-
age at the UV falls below the UVLO detect threshold, and
the MAX5921/MAX5939 turn off the external MOSFET.
Current Limit and Electronic Circuit
Breaker
The MAX5921/MAX5939 provide current-limiting and cir-
cuit-breaker features that protect against excessive load
current and short-circuit conditions. The load current is
monitored by sensing the voltage across an external
sense resistor connected between V
EE
and SENSE.
