MAX4826–MAX4831
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. 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.
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 cur-
rent limit. During t
RETRY
, when the switch is off, no cur-
rent flows through the switch. Instead of observing the
full load current, the switch sees the equivalent load
current, multiplied by the duty cycle or I
SUPPLY
= I
LOAD
x t
BLANK
/ (t
BLANK
+ t
RETRY
). With a typical t
BLANK
=
37ms and typical t
RETRY
= 518ms, the duty cycle is 6%
which results in a 94% power savings, as opposed to
the switch being on the entire time. The duty cycle is
consistent across the process and devices.
Latchoff (MAX4826/MAX4828/MAX4830)
When the forward- or reverse-current-limit threshold is
exceeded, the t
BLANK
timer begins counting. The timer
resets if the overcurrent condition disappears before
t
BLANK
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 MAX4826–MAX4831 feature 14ms (min) fault blank-
ing. Fault blanking allows current-limit faults, including
momentary short-circuit faults that occur when hot swap-
ping a capacitive load. Fault blanking also ensures that
no fault is issued during power-up. When a load transient
causes the device to enter the current limit, an internal
counter starts. If the load-transient fault persists beyond
the fault-blanking timeout, FFLG asserts low. Load-tran-
sient faults less than t
BLANK
do not cause FFLG asser-
tion. Only current-limit faults are blanked.
A thermal fault and input voltage drops below the UVLO
threshold cause FFLG to assert immediately. These
faults do not wait for the blanking time.
Thermal Shutdown
The MAX4826–MAX4831 have a thermal-shutdown fea-
ture to protect the devices from overheating. The switch
turns off and FFLG goes low immediately (no fault
blanking) when the junction temperature exceeds
+150°C. The switches with the autoretry feature turn
back on when the device temperature drops approxi-
mately 15°C. The switches with the latchoff feature
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.
Output Capacitance
Connect a 0.1µF capacitor from OUT to GND. This
capacitor helps prevent inductive parasitics from pulling
OUT negative during turn-off, thus preventing the
MAX4826–MAX4831 from tripping erroneously. If the
load capacitance is too large, current may not have
enough time to charge the capacitance, and the device
assumes that there is a faulty load condition. The maxi-
mum capacitive load value that can be driven from OUT
is obtained by the following formula:
Layout and Thermal Dissipation
To optimize the switch response time to output short-
circuit conditions, it is very important to keep all traces
as short as possible to reduce the effect of undesirable
parasitic inductance. Place input and output capacitors
as close as possible to the device (no more than 5mm).
IN and OUT pins must be connected with short traces
to the power bus.
During normal operation, the power dissipation is small
and the package temperature change is minimal. If the
output is continuously shorted to ground at the maxi-
mum supply voltage, the operation of the switches with
the autoretry option does not cause problems because
the total power dissipated during the short is scaled by
the duty cycle:
where,
V
IN_MAX
= 5.5V, I
OUT_MAX
= 240mA, t
BLANK
= 37ms,
and t
RETRY
= 518ms.
Attention must be given to the MAX4826/MAX4828/
MAX4830 where the latchoff condition must be manually
reset by toggling ON from high to low. If the latchoff time
duration is not sufficiently high, it is possible for the
device to reach the thermal shutdown threshold and
never be able to turn the device on until it cools down.
