Datasheet
MAX8535/MAX8536/MAX8585
ORing MOSFET Controllers with Fastest
Fault Isolation for Redundant Power Supplies
______________________________________________________________________________________ 13
for the MAX8536 is 12µA. Increasing the charge-pump
frequency increases the GATE drive current. Adding a
resistor from GATE to the gate of the external MOSFETs
further increases turn-on and turn-off times.
CS
The voltage drop across the external MOSFETs is mea-
sured between the V
CC
and CS inputs. CS connects to
the positive side of the system bus. The voltage drop
across the CS and V
CC
determines operation modes.
I
FORWARD
is defined as V
CC
- CS > 0.01V. I
REVERSE
is
defined as CS - V
CC
> 0.03V in all except the
MAX8585. In the MAX8585, I
REVERSE
= 0V.
FAULT Conditions
The MAX8535/MAX8536/MAX8585 contain a versatile
FAULT output that signals overvoltage, undervoltage,
or reverse-current conditions. During a FAULT condi-
tion, the charge pump shuts down and the GATE dis-
charges to ground.
Undervoltage Fault
The MAX8535/MAX8536/MAX8585 turn off the external
MOSFET if the input voltage falls below the UVP thresh-
old. If UVP is left unconnected, the undervoltage input
is disabled. Set the undervoltage threshold to any value
above V
CCOK
. When the input voltage rises above the
UVP threshold, FAULT clears and the MOSFET turns
back on.
Overvoltage Fault
The MAX8535/MAX8536/MAX8585 contain an
adjustable OVP feature. A resistor-divider from the CS
system bus to the OVP input pin sets the overvoltage
threshold. When the OVP level is exceeded and the
part is in the I
FORWARD
condition (defined as V
CC
>
CS + 0.01V), the MAX8535/MAX8536/MAX8585 turn off
the external MOSFET and a fault is latched. If there is
no I
FORWARD
condition, an OVP detection has no
effect. In this way, only the input supply, which is caus-
ing the overvoltage condition, is turned off in a redun-
dant power system application. An overvoltage fault is
a latching fault condition, and requires V
CC
or TIMER
to be cycled to reset the part.
Reverse-Current Fault
The MAX8535/MAX8536 contain a reverse-current pro-
tection feature. If, after the 500ms (typ) startup blank
time, an I
REVERSE
condition is detected, the MAX8535/
MAX8536 turn off the external MOSFET and a fault is
latched. A reverse-current fault forces the MAX8535/
MAX8536 to latch off. Cycle V
CC
or TIMER to exit a
latched fault condition. Startup blanking time allows the
incoming power supply to connect to the system bus at
V
BUS
- 0.4V. Reducing charge-pump frequency
increases the startup blanking time. The MAX8585
does not latch this fault.
Applications Information
Selecting the Timer Resistor
To set the frequency of the internal charge-pump oper-
ation, connect a resistor from TIMER to GND.
Determine the frequency by using the equation:
Pull TIMER above 1.5V for maximum charge-pump fre-
quency. Pull TIMER below 0.5V to disable the charge
pump. Leave TIMER unconnected for a 500kHz
charge-pump frequency.
Selecting the Gate Capacitor
and Gate Resistor
The charge pump uses an internal monolithic transfer
capacitor to charge the external MOSFET gates.
Frequency A
V
R
kHz A
TIMER
=×
⎛
⎝
⎜
⎞
⎠
⎟
5 100
125
µµ-
.
/
Table 1. MAX8535/MAX8536/MAX8585 Fault Modes
FAULT MODE PIN CONDITIONS GATE PIN FAULT PIN LATCHING
V
CC
UVLO V
CC
< V
CCOK
Low
High impedance
No
UVP pin undervoltage protection UVP < 1.25V Low Low No
OVP pin overvoltage protection
OVP > 1.25V
V
CC
> CS + 0.01V
Low Low Yes
Reverse-current protection
( M AX 8535/M AX 8536)
V
CC
< CS - 0.03V
Gate ON for t > 0.5s
Low Low Yes
Reverse-current protection
( M AX 8585)
V
CC
< CS
Gate ON for t > 0.032s
Low Low No
V
C C
i nter nal ( M AX 8535/M AX 8585)
over vol tag e p r otecti on
V
CC
> 14.5V Low Low No