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M
450mA/500mA Current-Limit Switches
_______________________________________________________________________________________
7
Autoretry (MAX4796/MAX4798)
When the forward or reverse current-limit threshold is
exceeded, t
BLANK
timer begins counting (Figure 2).
The timer resets if the overcurrent condition disappears
before t
BLANK
has elapsed. A retry time delay, t
RETRY
,
is started immediately after t
BLANK
has elapsed and
during that time, the switch is latched off and
FLAG
asserts. At the end of t
RETRY
, 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 and
FLAG
deasserts.
The autoretry feature saves system power in the case
of an overcurrent or short-circuit condition. During
t
BLANK
, when the switch is on, the supply current is at
the current limit. During t
RETRY
, 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 I
SUPPLY
=
I
LOAD
t
BLANK
/(t
BLANK
+ t
RETRY
). With a typical
t
BLANK
= 37ms and typical t
RETRY
= 555ms, the duty
cycle is 6%, which results in a 94% power savings over
the switch being on the entire time. The duty cycle is
consistent across the process and devices.
Latchoff (MAX4795/MAX4797)
When the forward or reverse current-limit threshold is
exceeded, t
BLANK
timer begins counting. The timer
resets if the overcurrent condition disappears before
t
BLANK
has elapsed. The switch is shut off and
FLAG
asserts if the overcurrent condition continues up to the
end of the blanking time. Reset the switch by either tog-
gling ON (Figure 3) or cycling the input voltage.
Fault Blanking
The MAX4795
–
MAX4798 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,
FLAG
asserts low. Load-tran-
sient faults less than t
BLANK
do not cause a
FLAG
out-
put assertion. Only current-limit faults are blanked.
A thermal fault causes
FLAG
to assert immediately and
does not wait for the blanking time.
Thermal Shutdown
The MAX4795
–
MAX4798 have a thermal-shutdown fea-
ture to protect the devices from overheating. The switch
turns off and
FLAG
goes low immediately (no fault
blanking) when the junction temperature exceeds
+150
°
C. The switches with autoretry turn back on when
the device temperature drops approximately 15
°
C. The
switches with latchoff require ON cycling.
Applications Information
Input Capacitor
To limit the input voltage drop during momentary output
short-circuit conditions, connect a capacitor from IN to
GND. A 0.1μF ceramic capacitor is adequate for most
applications; however, higher capacitor values further
reduce the voltage drop at the input and are recom-
mended for lower voltage applications.
LOAD
CURRENT
SWITCH
STATUS
ON PIN
VOLTAGE
t
BLANK
t
BLANK
OFF
ON
(a)
LOAD
CURRENT
SWITCH
STATUS
IN PIN
VOLTAGE
OFF
ON
t
BLANK
t
BLANK
FLAG PIN
STATUS
FLAG PIN
STATUS
Figure 3. MAX4795/MAX4797 Latch-Off Fault Blanking