Rainbow Electronics MAX5079 User Manual
Page 12
MAX5079
ORing MOSFET Controller with
Ultra-Fast 200ns Turn-Off
12
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5) PS1, PS2, PS3 are present and PS1 goes open.
PS1’s output capacitors discharge and V
OUT1
drops
below V
BUS
. The MAX5079 (U1) senses a negative
potential from V
OUT1
to V
BUS
. Depending upon how
fast PS1’s output capacitor discharges, N1 is turned off
due to the crossing of the fast- or slow-comparator
reverse voltage threshold. N1’s gate is discharged with
a 2A sink current into GATE (U1), turning off N1 and
isolating PS1 from the BUS. The load-sharing circuit of
PS2 and PS3 will increase PS2 and PS3’s load current
until the total BUS current requirement is satisfied.
6) PS1, PS2, PS3 are present and providing BUS
current. PS1 loses its feedback signal and goes
into an overvoltage condition.
V
BUS
increases and PS1 is loaded heavily. The current
share circuit forces V
OUT2
and V
OUT3
higher and they
will eventually saturate at their current-sharing voltage
range. Now only PS1 has a positive voltage at IN (U1)
with respect to BUS. All OVI inputs will sense the over-
voltage, but only OVP (U1) will be asserted and latched
low. GATE (U1) is pulled to PGND and remains low as
long as V
OVI
≥ 0.6V. When V
OVI
drops below 0.6V, OVP
remains low, however, U1 tries to turn on N1 unless
V
OUT1
is actively kept below the undervoltage lockout.
Use OVP (U1) to either drive the cathode of an opto-
coupler to shutdown PS1 from the primary, or fire an
SCR connected between IN (U1) and PGND.
Internal and External
Undervoltage Lockout
The internal undervoltage lockout monitors V
IN
and
V
AUXIN
and keeps the MAX5079 off until either voltage
reaches 2.75V. Once powered and V
IN
and/or V
AUXIN
increase above 2.75V, the external UVLO is monitored.
The external undervoltage lockout feature monitors the
UVLO input and keeps the MAX5079 off (GATE shorted
to PGND) until V
UVLO
is greater than 0.66V. Connect a
resistive divider from IN to UVLO to GND or from
AUXIN to UVLO to GND to set the external undervolt-
age lockout threshold. We advise setting the external
UVLO
≥ 2.75V when AUXIN is not present.
Charge Pump
The MAX5079 has an internal charge pump that pumps
the gate-drive voltage (V
GATE
) high enough to fully
enhance the n-channel ORing MOSFET. The charge
pump is divided into two stages, a voltage doubler run-
ning at 70kHz using an external charge-pump capaci-
tor (C
EXT
), and a voltage tripler running at 1MHz using
an internal capacitor.
Connect an external capacitor (C
EXT
) between C+ and
C-. C
EXT
is charged from the higher of V
IN
or V
AUXIN
.
When the rising V
IN
becomes greater then V
BUS
(V
UVLO
> 0.66V), C
EXT
is discharged through GATE into the
external MOSFET’s gate capacitance. The charge-
pump output is controlled by an internal regulator. The
charge-pump output at GATE sources typically 2mA.
This provides enough current drive to turn on a typical
ORing MOSFET in less than 10µs. When (V
GATE
- V
IN
)
reaches the target value (depending on V
IN
) the charge
pump is switched off (see the Electrical Characteristics
table). Choose C
EXT
equal to 10 times the ORing
MOSFET gate capacitance. Too low of a capacitance
will delay the turn-on of the ORing MOSFET, while too
high of a capacitance can cause excessive ripple at
V
IN
. Bypass IN to GND with a 1µF ceramic capacitor to
avoid ripple at IN caused by the charge-pump switch-
ing. A clamp is placed internally between GATE and IN
to prevent (V
GATE
- V
IN
) from exceeding 11V. When V
IN
is less than 5V, the charge pump (tripler) will increase
V
GATE
to 3x’s V
IN
to further reduce the R
DSON
of the
ORing MOSFET. The internal charge-pump booster
(voltage tripler) section is operational only when V
IN
and
V
AUXIN
are low and is turned off when V
IN
exceeds 5V.
When an additional supply is connected to AUXIN and
(V
AUXIN
- V
IN
) > 5V, both charge pumps are completely
disabled. In this case, the charging of the ORing gate
comes entirely from V
AUXIN
. In this case, the charge-
pump flying capacitor can be eliminated and C+, C-
can be left floating.
GATE Drive and Gate Pulldown
The MAX5079’s charge pump provides bias to charge
the ORing MOSFET gate above IN (the MOSFET’s
source). GATE source current and the turn-on speed
depends upon the value of C
EXT
(connected between
C+ and C-). Typically GATE can source up to 2mA with
C
EXT
= 0.1µF. This enables V
GATE
to rise to over 2V
above V
IN
in less than 10µs for an ORing MOSFET gate
capacitance of up to 10nF. With V
IN
< 5V, 12V MOSFETs
can be used for better R
DSON
characteristics. The
MAX5079 automatically selects the gate-drive voltage for