Overvoltage Protection Controllers
with Reverse Polarity Protection
Detailed Description
The MAX4864L/MAX4865L/MAX4866L/MAX4867 provide
up to +28V overvoltage and negative voltage protection
for low voltage systems. When the input voltage exceeds
the overvoltage trip level, the MAX4864L/MAX4865L/
MAX4866L/MAX4867 turn off a low-cost external n-channel
MOSFET to prevent damage to the protected compo-
nents. The devices also drive an external p-channel
MOSFET to protect against negative voltage inputs. An
internal charge-pump (see the
Functional Diagram),
drives the MOSFET GATEN for a simple, robust solution.
On power-up, the device waits for 50ms before driving
GATEN high. The open-drain FLAG output is kept at a
high impedance for an additional 50ms after GATEN goes
high before deasserting. The FLAG output asserts high
immediately to an overvoltage fault.
output voltage is a function of input voltage, as shown
in the
Typical Operating Characteristics.
MAX4864L/MAX4865L/MAX4866L/MAX4867
GATEP Driver
When the input voltage drops below ground, GATEP
goes high turning the external p-channel MOSFET off.
When the input voltage goes above ground, GATEP
pulls low and turns on the p-channel MOSFET. An inter-
nal clamp protects the p-channel MOSFET by insuring
that the GATEP-to-IN voltage does not exceed +16V
when the input (IN) rises to +28V.
Device Operation
The MAX4864L/MAX4865L/MAX4866L/MAX4867 have
an on-board state machine to control device operation.
A flowchart is shown in Figure 5. On initial power-up, if
V
IN
< UVLO or if V
IN
> OVLO, GATEN is held at 0V and
FLAG is high.
If UVLO < V
IN
< OVLO, the device enters startup after a
50ms internal delay. The internal charge pump is
enabled, and GATEN begins to be driven above V
IN
by
the internal charge pump. FLAG is held high during
startup until the FLAG blanking period expires, typically
50ms after the GATEN starts going high. At this point,
the device is in its on-state.
At any time if V
IN
drops below UVLO, FLAG is driven
high and GATEN is driven to ground.
Undervoltage Lockout (UVLO)
The MAX4864L/MAX4865L/MAX4866L have a fixed
+2.85V typical UVLO level, and the MAX4867 has
+2.5V UVLO level. When V
IN
is less than the UVLO, the
GATEN driver is held low and FLAG is asserted.
Overvoltage Lockout (OVLO)
The MAX4864L has a +7.4V typical OVLO threshold;
the MAX4865L has +6.35V typical OVLO threshold; the
MAX4866L has a +5.8V typical OVLO threshold; and
the MAX4867 has a +4.65V typical OVLO threshold.
When V
IN
is greater than OVLO, the GATEN driver is
held low and FLAG is asserted.
FLAG Output
The open-drain FLAG output is used to signal to the
host system when there is a fault with the input voltage.
On power-up, FLAG is held high for 50ms after GATEN
turns on, before deasserting. FLAG asserts immediately
to overvoltage and undervoltage faults. When the fault
condition is removed, FLAG deasserts 50ms after
GATEN turns on. Connect a pullup resistor from FLAG
to the logic I/O voltage of the host system.
STANDBY
GATEN = 0
FLAG = HIGH
V
IN
> UVLO
TIMER STARTS
COUNTING
t = 50ms
V
IN
< UVLO
OVLO CHECK
GATEN = 0
FLAG = HIGH
V
IN
> OVLO
STARTUP
GATEN DRIVEN HIGH
FLAG = HIGH
t = 50ms
ON
GATEN HIGH
FLAG = LOW
GATEN Driver
An on-chip charge pump is used to drive GATEN
above IN, allowing the use of a low-cost n-channel
MOSFET. The charge pump operates from the internal
+5.5V regulator.
The actual GATEN output voltage tracks approximately
two times V
IN
until V
IN
exceeds +5.5V, or the OVLO trip
level is exceeded, whichever comes first. The
MAX4864L has a +7.4V typical OVLO, therefore GATEN
remains relatively constant at approximately +10.5V for
+5.5V < V
IN
< +7.4V. The MAX4866L has a +5.8V typi-
cal OVLO, but this can be as low as +5.5V. The GATEN
V
IN
> OVLO
Figure 5. State Diagram
7
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