MAX4792 查看數據表（PDF） - Maxim Integrated
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MAX4792 Datasheet PDF : 11 Pages
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200mA/250mA/300mA Current-Limit Switches
Table 1. MAX4789–MAX4794 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
MAX4790, MAX4792, and MAX4794; cycle repeats until fault
is removed
SUPPLY CURRENT MODE
Shutdown
Shutdown
Latch off
Latch off
See the Autoretry section
off. FLAG is an open-drain output transistor and requires
an external pullup resistor from FLAG to IN. During shut-
down (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 (MAX4790/MAX4792/MAX4794)
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 cur-
rent limit. During tRETRY, when the switch is off, the cur-
rent 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 con-
sistent across the process and devices.
Latch Off (MAX4789/MAX4791/MAX4793)
When the forward or reverse current-limit threshold is
exceeded, tBLANK timer begins counting. The timer
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-
tBLANK
tRETRY
tBLANK
LOAD
CURRENT
tBLANK
CURRENT
LIMIT
SHORT
CONDITION
REMOVED
ON
SWITCH
STATUS
OFF
Figure 2. MAX4790/MAX4792/MAX4794 Autoretry Fault
Blanking Diagram
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 MAX4789–MAX4794 feature 14ms (min) fault blank-
ing. 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 MAX4789/MAX4791/
MAX4793 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 MAX4789–MAX4794 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.
The switch turns back on when the device temperature
drops approximately 15°C.
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