With a 22nF gate capacitor, the inrush current, charge,
and discharge times are:
Case B: Fast Turn-On (With Current Limit)
In applications where the board capacitance (C
BOARD
)
is high, the inrush current causes a voltage drop across
R
SENSE
that exceeds the startup fast-comparator
threshold. The fast comparator regulates the voltage
across the sense resistor to V
FC,TH
. This effectively reg-
ulates the inrush current during startup. In this case,
the current charging C
BOARD
can be considered con-
stant and the turn-on time is:
The maximum inrush current in this case is:
Figure 6 shows the waveforms and timing diagrams for
a startup transient with current regulation (see the
Typical Operating Characteristics
section). When oper-
ating under this condition, an external gate capaci-
tor is not required.
ON Comparators
The ON comparators control the on/off function of the
MAX5930A/MAX5931A/MAX5931B. ON_ is also used to
reset the fault latch (latch mode). Pull V
ON_
low for
100µs, t
UNLATCH
, to reset the shutdown latch. ON_ also
programs the UVLO threshold (see Figure 10). A resis-
tive divider between V
IN_
, V
ON_
, and GND sets the
user-programmable turn-on voltage. In power-sequenc-
ing mode, an RC circuit can be used at ON_ to set the
delay timing (see Figure 11).
Using the MAX5930A/MAX5931A/
MAX5931B on the Backplane
Using the MAX5930A/MAX5931A/MAX5931B on the
backplane allows multiple cards with different input
capacitance to be inserted into the same slot even if
the card does not have on-board hot-swap protection.
The startup period can be triggered if IN_ is connected
to ON_ through a trace on the card (Figure 12).
Input Transients
The voltage at IN1, IN2, or IN3 must be above V
UVLO
dur-
ing inrush and fault conditions. When a short-circuit con-
dition occurs on the board, the fast-comparator trips
cause the external MOSFET gates to be discharged at
50mA according to the mode of operation (see the
Mode
section). The main system power supply must be able to
sustain a temporary fault current, without dropping below
the UVLO threshold of 2.45V, until the external MOSFET is
completely off. If the main system power supply collapses
below UVLO, the MAX5930A/MAX5931A/MAX5931B
force the device to restart once the supply has recov-
ered. The MOSFET is turned off in a very short time result-
ing in a high di/dt. The backplane delivering the power to
the external card must have low inductance to minimize
voltage transients caused by this high di/dt.
