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
). When operating
under this condition, an external gate capacitor is
not required.
ON_ Comparators
The ON_ comparators control the on/off function of the
MAX5927A/MAX5929A–MAX5929D. 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 MAX5927A/
MAX5929A–MAX5929D on the Backplane
Using the MAX5927A/MAX5929A–MAX5929D 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, IN3, or IN4 must be above V
UVLO
during inrush and fault conditions. When a short-circuit
condition 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 MAX5927A/MAX5929A–MAX5929D
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.
MOSFET Thermal Considerations
During normal operation, the external MOSFETs dissi-
pate little power. The MOSFET R
DS(ON)
is low when the
MOSFET is fully enhanced. The power dissipated in nor-
mal operation is P
D
= I
LOAD
2
x R
DS(ON)
. The most
power dissipation occurs during the turn-on and turn-off
transients when the MOSFETs are in their linear regions.
Take into consideration the worst-case scenario of a
continuous short-circuit fault, consider these two cases:
1) The single turn-on with the device latched after a
fault: MAX5927A (LATCH = high or unconnected) or
MAX5929A/MAX5929B.
2) The continuous autoretry after a fault: MAX5927A
(LATCH = low) or MAX5929C/MAX5929D.
MOSFET manufacturers typically include the package
thermal resistance from junction to ambient (R
θJA
) and
thermal resistance from junction to case (R
θJC
), which
determines the startup time and the retry duty cycle
(d = t
START
/(t
START
+ t
RETRY
). Calculate the required
transient thermal resistance with the following equation:
where I
START
= V
SU,TH
/R
SENSE
.
