MAX4785 Ver la hoja de datos (PDF) - Maxim Integrated
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MAX4785 Datasheet PDF : 10 Pages
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50mA/100mA Current-Limit Switches
Table 1. MAX4785–MAX4788 Switch Truth Table
ON
Low
High
High
High
FAULT
X
Undervoltage lockout
Thermal
Current limit
SWITCH ON/OFF
OFF
OFF
OFF immediately (tBLANK period does not apply)
OFF after tBLANK period has elapsed
ON during tBLANK period, OFF during tRETRY period for the
MAX4786 and MAX4788; cycle repeats until fault is removed
SUPPLY CURRENT MODE
Shutdown
Shutdown
Latch off
Latch off
See the Autoretry section
FLAG Indicator
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
temperature limit of +150°C.
• The device is in current limit for more than the fault-
blanking period.
• VIN is below UVLO threshold.
Autoretry (MAX4786/MAX4788)
When the forward or reverse current-limit threshold is
exceeded, tBLANK timer begins counting (Figure 2).
The timer resets if the overcurrent condition disappears
before tBLANK has elapsed. A retry time delay, tRETRY,
is started immediately after tBLANK has elapsed and
during that time, the switch is latched off. At the end of
tRETRY, the switch is turned on again. If the fault still
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
tBLANK, when the switch is on, the supply current is at
the current limit. During tRETRY, when the switch is off,
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 ISUPPLY =
ILOAD ✕ tBLANK/(tBLANK + tRETRY). With a typical
tBLANK = 37ms and typical tRETRY = 259ms, the duty
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.
Latch Off (MAX4785/MAX4787)
When the forward or reverse current-limit threshold is
exceeded, tBLANK timer begins counting. The timer
tBLANK
tRETRY
tBLANK
LOAD
CURRENT
tBLANK
CURRENT
LIMIT
SHORT
CONDITION
REMOVED
ON
SWITCH
STATUS
OFF
Figure 2. MAX4786/MAX4788 Autoretry Fault Blanking Diagram
resets if the overcurrent condition disappears before
tBLANK has elapsed. The switch is shut off if the over-
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).
Fault Blanking
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 tBLANK do not cause a FLAG output assertion.
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.
Thermal Shutdown
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.
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