MAX4787 Maxim, MAX4787 Datasheet - Page 7

• Manufacturer Part Number: MAX4787
• Description: The MAX4785-MAX4788 family of switches feature internal current limiting to prevent host devices from being damaged due to faulty load conditions
• Manufacturer: Maxim

Table 1. MAX4785–MAX4788 Switch Truth Table
The MAX4785 and MAX4787 feature a latched fault out-
put, FLAG. Whenever a fault is activated, it latches
FLAG output low and also turns the switch off. FLAG is
an open-drain output transistor and requires an exter-
nal pullup resistor from FLAG to IN. During shutdown
(ON is low), the pulldown on FLAG output is released in
order to limit power dissipation. FLAG goes low when
any of the following conditions occur:
• The die temperature exceeds the thermal-shutdown
• The device is in current limit for more than the fault-
• V
When the forward or reverse current-limit threshold is
exceeded, t
The timer resets if the overcurrent condition disappears
before t
is started immediately after t
during that time, the switch is latched off. At the end of
t
exists, the cycle is repeated. If the fault has been
removed, the switch stays on.
The autoretry feature saves system power in the case
of an overcurrent or short-circuit condition. During
t
the current limit. During t
the current through the switch is zero. Instead of
observing the full load current, the switch sees the
equivalent load current times duty cycle or I
I
t
cycle is 12%, which results in an 88% power savings
over the switch being on the entire time. The duty cycle
is consistent across the process and devices.
When the forward or reverse current-limit threshold is
exceeded, t
RETRY
BLANK
BLANK
LOAD
temperature limit of +150°C.
blanking period.
High
High
High
Low
IN
ON
is below UVLO threshold.
, the switch is turned on again. If the fault still
, when the switch is on, the supply current is at
✕
BLANK
= 37ms and typical t
t
BLANK
Undervoltage lockout
BLANK
BLANK
has elapsed. A retry time delay, t
Autoretry (MAX4786/MAX4788)
Latch Off (MAX4785/MAX4787)
Current limit
/(t
_______________________________________________________________________________________
Thermal
FAULT
BLANK
timer begins counting (Figure 2).
timer begins counting. The timer
X
RETRY
50mA/100mA Current-Limit Switches
+ t
RETRY
BLANK
RETRY
, when the switch is off,
OFF
OFF
OFF immediately (t
OFF after t
ON during t
MAX4786 and MAX4788; cycle repeats until fault is removed
FLAG Indicator
= 259ms, the duty
). With a typical
has elapsed and
BLANK
BLANK
SUPPLY
RETRY
period has elapsed
period, OFF during t
BLANK
SWITCH ON/OFF
=
,
period does not apply)
resets if the overcurrent condition disappears before
t
current condition continues up to the end of the blank-
ing time. Reset the switch by either toggling ON (Figure
3a) or cycling the input voltage below UVLO, typically
2V (Figure 3b).
The MAX4785–MAX4788 feature 14ms (min) fault
blanking. Fault blanking allows current-limit faults,
including momentary short-circuit faults that occur
when hot swapping a capacitive load, and also ensures
that no fault is issued during power-up. When a load
transient causes the device to enter current limit, an
internal counter starts. If the load-transient fault persists
beyond the fault-blanking timeout, the MAX4785 and
MAX4787s’ FLAG asserts low. Load-transient faults less
than t
Only current-limit faults are blanked.
A thermal fault and input voltage drops below the
UVLO threshold cause FLAG to assert immediately and
do not wait for the blanking time.
The MAX4785–MAX4788 have a thermal shutdown fea-
ture to protect the devices from overheating. The switch
turns off and FLAG goes low immediately (no fault blank-
ing) when the junction temperature exceeds +150°C.
Figure 2. MAX4786/MAX4788 Autoretry Fault Blanking Diagram
BLANK
CURRENT
SWITCH
STATUS
LOAD
OFF
ON
BLANK
RETRY
has elapsed. The switch is shut off if the over-
t
BLANK
period for the
do not cause a FLAG output assertion.
t
RETRY
t
BLANK
SUPPLY CURRENT MODE
See the Autoretry section
Thermal Shutdown
Fault Blanking
Shutdown
Shutdown
Latch off
Latch off
t
BLANK
CURRENT
CONDITION
REMOVED
LIMIT
SHORT
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