MAX5955/MAX5956
C
BOARD
= 6µF and the load does not draw any current
during the startup period. With no gate capacitor the
inrush current, charge, and discharge times are:
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
SU,TH
. This effectively
regulates 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 2 shows the waveforms and timing diagrams for a
startup transient with current regulation (see
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
MAX5955/MAX5956. ON_ allows independent control
over channel 1 and channel 2. Drive ON1 and ON2
high (> 0.875V) to enable channel 1 and channel 2,
respectively. Pull ON_ low (< 0.875V) to disable the
respective channel. An RC time delay must be added
to the ON_ inputs with delay set to at least 20µs. This
allows the internal circuits to stabilize after application
of a steeply rising V
IN_
.
Using the MAX5955/MAX5956 on the
Backplane
Using the MAX5955/MAX5956 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 5).
Input Transients
The voltage at IN1 or IN2 must be above the UVLO dur-
ing inrush and fault conditions. When a short-circuit con-
dition occurs on the board, the fast comparator trips
causing the external MOSFET gates to be discharged at
3mA. The main system power supply must be able to
sustain a temporary fault current, without dropping below
the UVLO threshold of 2.4V, until the external MOSFET is
completely off. If the main system power supply collapses
below UVLO, the MAX5955/MAX5956 force the device to
restart once the supply has recovered. The MOSFET is
turned off in a very short time resulting 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
normal 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 sce-
nario of a continuous short-circuit fault, consider these
two cases:
1) The single turn-on with the device latched after a
fault (MAX5955B/MAX5956B)
2) The continuous automatic retry after a fault
(MAX5955A/MAX5956A)
MOSFET manufacturers typically include the package
thermal resistance from junction to ambient (R
θJA
) and
thermal resistance from junction to case (R
θJC
), which
determine 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
.
.
.
.
.
.
.
.
.
