MAX8535/MAX8536/MAX8585
ORing MOSFET Controllers with Fastest
Fault Isolation for Redundant Power Supplies
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Detailed Description
Critical loads often employ parallel-connected power
supplies with redundancy to enhance system reliability.
The MAX8535/MAX8536/MAX8585 are highly integrated
but inexpensive MOSFET controllers that provide isolation
and redundant power capability in high-reliability sys-
tems. The MAX8535/MAX8585 is used in 12V systems
and has an internal charge pump to drive the gates of the
n-channel pass elements to V
CC
+ 10V. The MAX8536 is
used in 3.3V and 5V systems, with a charge pump output
of V
CC
+ 5V.
During startup, the MAX8535/MAX8536/MAX8585 moni-
tor the voltage drop across external MOSFETs. Once
V
CC
approaches or exceeds the bus voltage, the
MOSFETs are turned on. The MAX8535/MAX8536/
MAX8585 feature a dual-purpose TIMER input. A single
external resistor from TIMER to ground sets the turn-on
speed of the external MOSFETs. Optionally, the TIMER
input can be used as a logic-enable pin. Once the
device is turned on, the MAX8535/MAX8536/MAX8585
monitor the load, protecting against overvoltage, under-
voltage, and reverse-current conditions.
Overvoltage and undervoltage fault thresholds are
adjustable and can be disabled. The current-limit trip
points are set by the external MOSFETs’ R
DS(ON)
,
reducing component count. An open-drain logic-low
fault output indicates if an overvoltage, undervoltage, or
reverse-current fault occurs.
V
CC
V
CC
is the power-supply input for the MAX8535/
MAX8536/MAX8585 and the input to the internal charge
pump that drives the gate of the external MOSFETs. The
MAX8535/MAX8536/MAX8585 monitor V
CC
at all times.
V
CC
connects directly to the power supply (Silver Box or
DC-DC power modules). During startup, the device
turns on when V
CC
rises above the undervoltage thresh-
old V
CCOK
. After V
CC
exceeds V
CCOK
and V
CC
is
greater than (CS - 0.4V), the charge pump turns on, dri-
ving GATE high and turning on the external MOSFETs.
TIMER
The MAX8535/MAX8536/MAX8585 provide a program-
mable-frequency charge pump and shutdown function
through TIMER. Slowing down the charge-pump fre-
quency allows a user to program soft-start. Connecting
a resistor from TIMER to GND sets the charge-pump fre-
quency from 100kHz to 500kHz. Connecting TIMER to a
logic high sets charge-pump operation to a maximum
frequency of 550kHz. Pulling TIMER to GND shuts down
the charge pump and turns off the external MOSFET.
Reducing the charge-pump frequency increases the
I
REVERSE
startup blank time (see the Reverse-Current
Fault section).
GATE
GATE is the output of the internal charge pump that
drives the external MOSFETS. During startup, the GATE
voltage ramps up according to the charge-pump fre-
quency. At 250kHz, the GATE drive current for the
MAX8535/MAX8585 is 25µA and the GATE drive current
Pin Description
PIN NAME FUNCTION
1 GATE Gate Drive Output. Bypass GATE with a 0.01µF capacitor to ground.
2 GND Ground
3V
CC
Power-Supply Input. Bypass V
CC
with a 0.1µF capacitor to ground.
4 UVP
Undervoltage Input. Connect a resistor-divider from the V
CC
to GND with the center point connected
to UVP. Leave high impedance if not used. UVP fault threshold must be set above V
CCOK
threshold.
5 TIMER
Timer Input. Connect a resistor from TIMER to ground to select the charge-pump operating
frequency. The charge-pump frequency is proportional to the TIMER output current. TIMER can
source up to 100µA. If pulled low (<0.5V), the gate drive is disabled. If pulled high (above 1.25V), the
charge pump operates at 550kHz.
6 OVP
Overvoltage Input. Connect a resistor-divider from the CS to GND with the center point connected to
OVP. Connect to GND if not used.
7 FAULT
Open-Drain Fault Output. FAULT is low during a fault, high impedance during normal operation.
Connect a pullup resistor of 50kΩ or higher value to a voltage rail.
8CS
Current-Sensing Input. Connect CS to the positive side of the system bus. Bypass with 1nF capacitor
to GND.
