max5956 Maxim Integrated Products, Inc., max5956 Datasheet - Page 12

• Manufacturer Part Number: max5956
• Description: Low-voltage, Dual Hot-swap Controllers With Independent On/off Control
• Manufacturer: Maxim Integrated Products, Inc.

C
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
is high, the inrush current causes a voltage drop across
R
threshold. The fast comparator regulates the voltage
across the sense resistor to V
regulates the inrush current during startup. In this case,
the current charging C
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.
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
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
12
BOARD
SENSE
I
______________________________________________________________________________________
t
t
INRUSH
CHARGE
DISCHARGE
I
t
t
INRUSH
CHARGE
DISCHARGE
= 6µF and the load does not draw any current
that exceeds the startup fast-comparator
t
ON
=
=
600
=
22
=
=
=
600
I
INRUSH
C
pF
nF
0 10 4
22
6
BOARD
=
6
×
µ
pF
µ
+
×
nF
0 10 4
F
F
100
BOARD
10 4
22
×
100
+
.
×
nF
.
0
=
10 4
V
µ
V
3
µ
V
×
.
×
3
A
SU TH
R
+
mA
.
V
A
×
mA
V
100
+
V
SENSE
SU TH
60
IN
100
,
V
can be considered con-
60
+
SU,TH
,
nC
+
×
ON_ Comparators
60
µ + =
nC
µ + =
60
R
A
nC
A
SENSE
=
nC
=
0 6
. This effectively
0 1
2 89
0 26 5
.
=
.
=
ms
0 02
0 096
.
A
ms
.
.
ms
mA
ms
BOARD
)
allows the internal cicuits to stablilize after application
of a steeply rising V
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).
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.
During normal operation, the external MOSFETs dissi-
pate little power. The MOSFET R
MOSFET is fully enhanced. The power dissipated in
normal operation is P
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
2) The continuous automatic retry after a fault
MOSFET manufacturers typically include the package
thermal resistance from junction to ambient (R
thermal resistance from junction to case (R
determine the startup time and the retry duty cycle
(d = t
transient thermal resistance with the following equation:
where I
fault (MAX5955B/MAX5956B)
(MAX5955A/MAX5956A)
START
START
Using the MAX5955/MAX5956 on the
/t
MOSFET Thermal Considerations
START
= V
Z
θ
SU,TH
JA MAX
IN_
(
+ t
D
.
RETRY
/ R
= I
)
≤
SENSE
LOAD
V
T
IN
JMAX
). Calculate the required
×
2
DS(ON)
I
START
Input Transients
x R
−
T
DS(ON)
A
is low when the
Backplane
θJC
. The most
θJA
), which
) and