Datasheet

MAX5955/MAX5956
Applications Information
Component Selection
n-Channel MOSFET
Select the external MOSFETs according to the applica-
tion’s current levels. Table 2 lists some recommended
components. The MOSFET’s on-resistance (R
DS(ON)
)
should be chosen low enough to have a minimum volt-
age drop at full load to limit the MOSFET power dissi-
pation. High R
DS(ON)
causes output ripple if there is a
pulsating load. Determine the device power rating to
accommodate a short-circuit condition on the board at
startup and when the device is in automatic-retry mode
(see the MOSFET Thermal Considerations section).
Using the MAX5955B/MAX5956B in latched mode allows
the use of MOSFETs with lower power ratings. A MOSFET
typically withstands single-shot pulses with higher dissi-
pation than the specified package rating. Table 3 lists
some recommended manufacturers and components.
Sense Resistor
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV. Select a sense resistor that causes
a drop equal to the slow-comparator threshold voltage at
a current level above the maximum normal operating
current. Typically, set the overload current at 1.2 to 1.5
times the full load current. The fast-comparator threshold
is four times the slow-comparator threshold in normal
operating mode. Choose the sense-resistor power rating
to be greater than (I
OVERLOAD
)
2
x V
SC,TH
.
Slow-Comparator Threshold, R
LIM
The slow-comparator threshold voltage is adjustable
from 25mV to 100mV, allowing designers to fine-tune
the current-limit threshold for use with standard-value
sense resistors. Low slow-comparator thresholds allow
for increased efficiency by reducing the power dissi-
pated by the sense resistor. Furthermore, the low 25mV
slow-comparator threshold is beneficial when operating
with supply rails down to 1V because it allows a small
percentage of the overall output voltage to be used for
current sensing. The VariableSpeed/BiLevel fault pro-
tection feature offers inherent system immunity against
load transients and noise. This allows the slow-com-
parator threshold to be set close to the maximum nor-
mal operating level without experiencing nuisance
faults. To adjust the slow-comparator threshold, calcu-
late R
LIM
as follows:
where V
TH
is the desired slow-comparator threshold
voltage.
R
VmV
A
LIM
TH
=
− 25
025. µ
Low-Voltage, Dual Hot-Swap Controllers with
Independent On/Off Control
10 ______________________________________________________________________________________
PART
OVERCURRENT
FAULT (V
OUT1
)
OVERCURRENT
FAULT (V
OUT2
)
OVER/UNDER-
VOLTAGE FAULT
(V
OUT1
)
OVER/UNDER-
VOLTAGE FAULT
(V
OUT2
)
PGOOD1/
PGOOD2
GATE1/
GATE2
Yes X X X Low/Low Off/Off
X Yes X X Low/Low Off/Off
X X Yes X Low/Low Off/Off
MAX5955
UV/OV
Protection
X X X Yes Low/Low Off/Off
Yes X X X Low/Low Off/Off
X Yes X X Low/Low Off/Off
X X Yes No Low/High On/On
MAX5956
UV/OV
Monitor
X X No Yes High/Low On/On
Table 1. Status Output Truth Table
PART NUMBER MANUFACTURER DESCRIPTION
IRF7413 11mΩ, 8 SO, 30V
IRF7401 22mΩ, 8 SO, 20V
IRL3502S
International
Rectifier
6mΩ, D2PAK, 20V
MMSF3300 20mΩ, 8 SO, 30V
MMSF5N02H 30mΩ, 8 SO, 20V
MTB60N05H
Motorola
14mΩ, D
2
PAK, 50V
FDS6670A 10mΩ, 8 SO, 30V
NDS8426A 13.5mΩ, 8 SO, 20V
FDB8030L
Fairchild
4.5mΩ, D
2
PAK, 30V
Table 2. Recommended n-Channel
MOSFETs