Datasheet
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. Typically, set the overload current at 1.2 to 1.5
times the nominal load current. To adjust the slow-com-
parator threshold calculate R
LIM
as follows:
where V
TH
is the desired slow-comparator threshold
voltage.
Setting the Startup Period, R
TIM
The startup period (t
START
) of the MAX5904/MAX5905 is
fixed at 9ms, and adjustable from 0.4ms to 50ms for the
MAX5906–MAX5909. The adjustable startup period of
the MAX5906–MAX5909 systems can be customized for
MOSFET gate capacitance and board capacitance
(C
BOARD
). The startup period is adjusted with the resis-
tance connected from TIM to GND (R
TIM
). R
TIM
must be
between 4kΩ and 500kΩ. The MAX5906–MAX5909 start-
up period has a default value of 9ms when TIM is left
floating. Calculate R
TIM
with the following equation:
where t
START
is the desired startup period.
There are two ways of completing the startup
sequence. Case A describes a startup sequence that
slowly turns on the MOSFETs by limiting the gate
charge. Case B uses the current-limiting feature and
turns on the MOSFETs as fast as possible while still
preventing a high inrush current. The output voltage
ramp-up time (t
ON
) is determined by the longer of the
two timings, case A and case B. Set the MAX5906–
MAX5909 startup timer t
START
to be longer than t
ON
to
guarantee enough time for the output voltage to settle.
Case A: Slow Turn-On (Without Current Limit)
There are two ways to turn on the MOSFETs without
reaching the fast-comparator current limit:
If the board capacitance (C
BOARD
) is small, the
inrush current is low.
If the gate capacitance is high, the MOSFETs turn
on slowly.
In both cases, the turn-on time is determined only by the
charge required to enhance the MOSFET. The small
gate-charging current of 100µA effectively limits the out-
put voltage dV/dt. Connecting an external capacitor
between GATE and GND extends turn-on time. The time
required to charge/discharge a MOSFET is as follows:
where:
C
GATE
is the external gate to ground capacitance
(Figure 4)
∆V
GATE
is the change in gate voltage
t
CVQ
I
GATE GATE GATE
GATE
=
×∆ +
R
t
pF
TIM
START
=
×128 800
R
VmV
A
LIM
TH
=
− 25
025. µ
MAX5904–MAX5909
Low-Voltage, Dual Hot-Swap Controllers/Power
Sequencers
______________________________________________________________________________________ 17
COMPONENT MANUFACTURER PHONE WEBSITE
Dale-Vishay 402-564-3131 www.vishay.com
Sense Resistors
IRC 704-264-8861 www.irctt.com
International Rectifier 310-233-3331 www.irf.com
Fairchild 888-522-5372 www.fairchildsemi.com
MOSFETs
Motorola 602-244-3576 www.mot-sps.com/ppd
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, D2PAK, 50V
FDS6670A 10mΩ, 8SO, 30V
NDS8426A 13.5mΩ, 8 SO, 20V
FDB8030L
Fairchild
4.5mΩ, D2PAK, 30V
Table 2. Recommended n-Channel
MOSFETs
Table 3. Component Manufacturers