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, no current
flows through the switch. Instead of observing the full
load current, the switch sees the equivalent load current,
multiplied by the duty cycle or ISUPPLY = ILOAD x tBLANK
/ (tBLANK + tRETRY). With a typical tBLANK = 37ms and
typical tRETRY = 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.
Latcho (MAX4826/MAX4828/MAX4830)
When the forward- or reverse-current-limit threshold is
exceeded, the 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 blanking
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-
transient faults less than tBLANK do not cause FFLG
assertion. 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 recommend-
ed for lower voltage applications.
Output Capacitance
Connect a 0.1µF capacitor from OUT to GND. This capac-
itor helps prevent inductive parasitics from pulling OUT
negative during turn-off, thus preventing the MAX4826–
MAX4831 from tripping erroneously. If the load capaci-
tance 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 maximum capacitive
load value that can be driven from OUT is obtained by the
following formula:
FWD_MIN BLANK_MIN
MAX
CV
×
<
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 maximum
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:
IN_MAX OUT_MAX BLANK
MAX
P 88mW
tt
××
+
where,
VIN_MAX = 5.5V, IOUT_MAX = 240mA, tBLANK = 37ms,
and tRETRY = 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.
www.maximintegrated.com Maxim Integrated
│
8
MAX4826–MAX4831 50mA/100mA Current-Limit Switches
with NO-LOAD Flag in µDFN