max5079eudt Maxim Integrated Products, Inc., max5079eudt Datasheet - Page 10

• Manufacturer Part Number: max5079eudt
• Description: Max5079 Oring Mosfet Controller With Ultra-fast 200ns Turn-off
• Manufacturer: Maxim Integrated Products, Inc.

The MAX5079 ORing MOSFET controller drives an
external n-channel MOSFET and replaces ORing
diodes in high-reliability redundant power-management
systems or multiple paralleled power supplies. The
device has an internal charge pump to drive the high-
side n-channel ORing MOSFET. Additional features
include an adjustable undervoltage lockout threshold
(UVLO), output overvoltage detector (OVI/OVP), input
power-good detector (PGOOD), and two programma-
ble reverse voltage detectors to detect both fast and
slow rises in the reverse voltage across the ORing
MOSFET. The input power-supply range is from 2.75V
to 13.2V or down to 1V when an auxiliary supply of at
least 2.75V is available.
This section describes a detailed startup sequence and
behavior of the MAX5079 under different conditions of
V
is equal to or greater than 2.75V and V
the UVLO threshold of 0.66V. Operation with V
to 1V is possible as long as V
2.75V.
When V
to V
pump delivers 2mA to charge the gate capacitance of
the external MOSFET connected to GATE. The constant
gate-charge current prevents large inrush currents from
the input supply. During turn-on, the MAX5079 will
ignore the reverse voltage at IN with respect to BUS.
This is necessary to avoid the unintentional turn-off of
the ORing MOSFET as the momentary inrush current
causes V
Figure 2 shows the MAX5079 in an ORing configuration
with three parallel power supplies (PS1, PS2, and PS3)
and three MAX5079s (U1, U2, and U3) provided by out-
puts V
must be carefully considered to ensure proper function-
ality of the MAX5079 ICs.
1) V
ORing MOSFET Controller with
Ultra-Fast 200ns Turn-Off
10
BUS
All three power supplies are turned ON simulta-
neously. V
V
a. When V
BUS
OUT3
IN
______________________________________________________________________________________
begin charging from V
diode. When V
threshold, the MAX5079 (U1) charge pump turns
on, and U1 monitors the positive potential from
V
pump brings GATE (U1) to 5.5V above V
7.5V above V
and V
OUT1
OUT1
and the charge pump turns on. The charge
UVLO
IN
is zero with a discharged capacitor (C
.
to dip.
, V
IN
to V
OUT1
crosses the UVLO threshold, V
. The MAX5079 powers up whenever V
OUT2
OUT1
BUS
turns on, the bus capacitors (C
IN
, and V
. When V
UVLO
comes up before V
Detailed Description
depending on the magnitude of
Operational Description
(U1) rises above the UVLO
OUT3
UVLO
OUT1
OUT1
. The following events
≥ 0.6V and V
through N1’s body
≥ V
BUS
UVLO
the charge
OUT2
GATE
IN
exceeds
IN
AUXIN
(U1) (or
down
BUS
rises
BUS
and
IN
).
≥
)
2) PS1 and PS2 are on and sharing the load when
b. V
c. V
PS3 is hot-inserted. PS3 will take the same
course as discussed in 1b above.
a. If V
b. If V
V
This results in a less than 10µs turn-on time for the
FDB7045L used in the MAX5079 evaluation kit. The
fast turn-on is needed to assure that N1 is ON
before the rising V
value. If the MOSFET is not turned on before V
reaches its steady state, V
to the shorting of the 0.7V (forward drop) of N1’s
body diode. A higher V
charge the charge-pump capacitor to 5V (or 7V),
while a lower V
MOSFET turns on at V
soft-start time of the power supply is large enough
(> 5ms) to avoid V
V
overloading of V
current (using the current-sharing circuit) or delay
the loading of the BUS until all three power supplies
are up and running.
at IN (U2). V
old, the MAX5079 (U2) charge pump turns on and
U2 monitors the V
voltage difference becomes positive, GATE (U2)
begins sourcing 2mA into N2’s gate capacitance.
During turn-on, the reverse voltage turn-off circuit
is momentarily disabled. If V
V
of PS2 will try to increase V
from V
enough not to cause any overshoot before N2
turns on and starts sharing the current.
as U2. Eventually V
to equilibrium and sharing equal currents.
increase to the new level determined by V
The external load-sharing controller circuit of PS1
and PS2 will increase V
current sharing.
cuit of PS3 will increase V
of current. This causes V
V
tive, GATE (U3) begins sourcing 2mA into N3’s
gate capacitance. Again, the reverse voltage turn-
off circuit is disabled momentarily, as discussed
before. The load-sharing circuit of PS3’s controller
will adjust V
IN
BUS
OUT2
OUT1
OUT3
BUS
), by sourcing 2mA into N1’s gate capacitance.
OUT3
OUT3
. When this voltage difference becomes posi-
to overshoot. Care must be taken to avoid the
, the external load-sharing controller circuit
OUT2
turns on and U3 follows the same sequence
turns on and begins increasing the voltage
is higher than V
is lower than V
OUT3
. Assume V
UVLO
IN
OUT1
(U1) will take longer. Typically the
so as to share the load current.
OUT2
OUT1
(U2) crosses the UVLO thresh-
OUT1
OUT1
by either limiting the source
GS
to V
racing ahead and causing
, V
IN
OUT1
BUS
OUT2
BUS
OUT3
reaches its steady-state
OUT3
BUS
= 2.5V. Ensure that the
OUT2
(U1) can more quickly
BUS
OUT2
, the load-sharing cir-
, the BUS voltage will
’s rise time is slow
and V
OUT2
to force the sharing
to increases above
may overshoot due
, and V
voltage. When this
to source current
OUT2
is lower than
OUT3
to force
OUT3
reach
OUT1
.