MAX8535/MAX8536/MAX8585
ORing MOSFET Controllers with Fastest
Fault Isolation for Redundant Power Supplies
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Normally, the external MOSFET’s gate capacitance is
sufficient to serve as a reservoir capacitor. If the
MOSFETs are located at a significant distance from the
MAX8535/MAX8536/MAX8585, place a local bypass
capacitor (0.01µF, typ) across GATE and GND. For
slower turn-on times, add a small capacitor between
GATE and GND and a series resistor between GATE
and the gate of the MOSFETs.
Set the UVP Fault Threshold
To set the undervoltage lockout threshold, use a resistor-
divider connected between V
CC
and GND, with the
center node of the divider connected to UVP. For
example, use a 10kΩ resistor (R4 in Figure 4) from UVP
to GND and calculate the other resistor (R3) using:
where V
UVLO
is the desired undervoltage lockout volt-
age and V
UVP
is the UVP reference threshold specified
in the Electrical Characteristics (1.25V, typ). To defeat
the UVP, leave UVP unconnected.
Set the OVP Fault Threshold
To set the OVP threshold, use a resistor-divider con-
nected between CS and GND, with the center node of
the divider connected to OVP. For example, use a
10kΩ resistor (R6 in Figure 4) from OVP to GND and
calculate the other resistor, R5, using:
where V
OVLO
is the desired overvoltage lockout voltage
and V
OVP
is the OVP reference threshold specified in
the Electrical Characteristics (1.25V, typ). To defeat the
OVP, connect the OVP input to GND.
MOSFET Selection
The MAX8535/MAX8536/MAX8585 drive n-channel
MOSFETs. The most important feature of the MOSFETs
is R
DS(ON)
. As load current flows through the external
MOSFET, a voltage (V
DS
) is generated from drain-to-
source due to the MOSFET’s on-resistance, R
DS(ON)
.
The MAX8535/MAX8536/MAX8585 monitor V
DS
of the
MOSFETs at all times. The MAX8535/MAX8536/
MAX8585 determine the state of the monitored power
supply by measuring the voltage drop across the exter-
nal MOSFETs. With two external MOSFETs, the equa-
tion becomes:
V
DSTOTAL
= R
DS(ON)1
x I
LOAD
+ R
DS(ON)2
x I
LOAD
Selecting a MOSFET with a low R
DS(ON)
allows more
current to flow through the MOSFETs before the
MAX8535/MAX8536/MAX8585 detect reverse-current
(I
REVERSE
) and forward-current (I
FORWARD
) conditions.
Using a Single MOSFET
Single MOSFETs can be used if the OVP function is not
needed. Connect the source of the MOSFET to V
CC
and the drain of the MOSFET to CS.
Layout Guidelines
Keep all traces as short as possible and maximize the
high-current trace width to reduce the effect of undesir-
able parasitic inductance. The MOSFET generates a
fair amount of heat because of the high currents
involved. In order to dissipate the heat generated by
the MOSFET, make the power traces very wide with a
large amount of copper area, and place the MAX8535/
MAX8536/MAX8585 as close as possible to the drain of
the external MOSFET. A more efficient way to achieve
good power dissipation on a surface-mount package is
to lay out two copper pads directly under the MOSFET
package on both sides of the board. Connect the two
pads to the ground plane through vias and use
enlarged copper mounting pads on the topside of the
board. Use a ground plane to minimize impedance and
inductance. Refer to the MAX8535 Evaluation Kit data
sheet for an example of a PC board layout.
In addition to the usual high-power considerations,
bypassing prevent false faults by:
1) Bypass V
CC
with a 0.1µF capacitor to ground and
bypassing CS with a 1nF capacitor to ground.
2) Making the traces connecting UVP and OVP as
short as possible.
3)Kelvin connecting V
CC
and CS to the external
MOSFET.
