Rainbow Electronics MAX5929 - Manual (Page 16)

MAX5927/MAX5929
age. t D,UVLO ensures that the board is fully inserted
into the backplane and that the input voltages are
stable. The MAX5927/MAX5929 include a UVLO glitch
filter, t
D,GF , to reject all input voltage noise and tran-
sients. Bringing all input supplies below the UVLO
threshold for longer than t
D,GF
reinitiates t
D,UVLO
and
the startup period, t START . See Figure 8 for an example
of automatic turn-on function.
Latched and Autoretry Fault Management
The MAX5929L always latch the external MOSFETs off
when an overcurrent fault is detected, and the
MAX5929A are always in autoretry mode. The
MAX5927 can be configured to either latch the external
MOSFETs off or to autoretry (see Table 2). Toggling
ON_ below 0.875V for at least 100µs clears the
MAX5929L or MAX5927 (LATCH = FLOAT) fault and
reinitiates the startup period. Similarly, the MAX5929A
or MAX5927 (LATCH = GND) turn the external
MOSFETs off when an overcurrent fault is detected,
then automatically restart after the autoretry delay that
is internally set to 64 times t
START .
Status Outputs (STAT_)
The status (STAT_) outputs are open-drain outputs that
assert when hot swap is successful and t
START has
elapsed. STAT_ deasserts if ON_ is low or if the chan-
nel is turned off for any fault condition.
The polarity of the STAT_ outputs is selected using POL
for the MAX5927 (see Table 3). Tables 4 and 5 contain
the MAX5927/MAX5929 truth tables.
Applications Information
Component Selection
N-Channel MOSFETs
Select the external MOSFETs according to the applica-
tion’s current levels. Table 6 lists recommended com-
ponents. 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 autoretry mode (see
the MOSFET Thermal Considerations section).
Using these devices in latched mode allows the use of
MOSFETs with lower power ratings. A MOSFET typical-
ly withstands single-shot pulses with higher dissipation
than the specified package rating. Table 7 lists some
recommended MOSFET manufacturers.
Low-Voltage, Quad, Hot-Swap
Controllers/Power Sequencers
16 ______________________________________________________________________________________
LATCH FAULT MANAGEMENT
Floating Fault condition latches MOSFETs off
Low Autoretry mode
Table 2. Selecting Fault Management
Mode (MAX5927)
POL STAT_
Low Asserts low
Floating Asserts high (open drain)
Table 3. Selecting STAT_ Polarity
(MAX5927)
Figure 8. Automatic Turn-On when Input Voltages are Above
their Respective Undervoltage Lockout Threshold (Provided
t
D , UVLO Requirement is Met)
MAX5927
MAX5929
V
1
ON1
ON2
ON3
ON4
GND
GND
ON1
ON2
ON3
ON4
REMOVABLE CARD BACKPLANE
V 2
V
3
V 4