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 MAX5920_ features an internal dynam-
ic 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 MAX5920A/MAX5920B
eliminates the need for an external gate-to-source
capacitor. Resistor R3 limits the current into the clamp
circuitry during card insertion.
Power-Supply Ramping
The MAX5920A/MAX5920B can reside either on the
backplane or the removable circuit board (Figure 6a).
Power is delivered to the load by placing an external
N-channel 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
MAX5920A/MAX5920B turn on Q1. The MAX5920A/
MAX5920B gradually turn on the external MOSFET by
charging the gate of Q1 with a 45µA current 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 MAX5920_ 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
GS
to about 18V under any condition.
Board Removal
If the circuit card is removed from the backplane, the volt-
age at the UV pin falls below the UVLO detect threshold,
and the MAX5920_ turns off the external MOSFET.
Current Limit and Electronic
Circuit Breaker
The MAX5920_ provides current-limiting and circuit-
breaker features that protect against excessive load cur-
rent and short-circuit conditions. The load current is
monitored by sensing the voltage across an external
sense resistor connected between V
EE
and SENSE.
MAX5920
-48V Hot-Swap Controller
with External R
SENSE
_______________________________________________________________________________________ 9
*DIODES INC. SMAT70A.
**OPTIONAL.
