Rainbow Electronics MAX14523C - Manual (Page 9)

MAX14523A/MAX14523AL/MAX14523B/MAX14523C
250mA to 1.5A, Adjustable
Current-Limit Switches
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The autoretry feature saves system power in case of an
overcurrent or short-circuit condition. During t
BLANK
time, when the switch is on, the supply current is held
at the current limit. During time t
RETRY , when the switch
is off, the current through the switch is zero. Thus, the
average output current is much less than the pro-
grammed current limit. Calculate the average output
current using the following equation:
I LOAD = I LIM [(t BLANK )/(t BLANK + t RETRY )]
With a typical t
BLANK
= 17.5ms and typical t
RETRY
=
560ms, the duty cycle is 3%, resulting in a 97% power
savings over the switch being on the entire time.
Latchoff (MAX14523B)
When the forward current reaches the current thresh-
old, the t
BLANK timer begins counting (Figure 3). FLAG
asserts if an overcurrent condition is present for greater
than t
BLANK time. The timer resets if the overcurrent
condition disappears before t BLANK has elapsed.
The switch turns off if the overcurrent condition contin-
ues beyond the blanking time. Reset the switch by
either toggling the control logic (ON or ON) or cycling
the input voltage. If the die temperature exceeds
+170°C (typ) due to self heating, the MAX14523B goes
into thermal shutdown until the die temperature
drops by approximately 15°C (see the
Thermal
Shutdown section).
Continuous Current Limit (MAX14523C)
When the forward current reaches the forward-current
threshold, the MAX14523C limits the output current to
the programmed current limit. FLAG asserts if the cur-
rent limit is present for t
BLANK and deasserts when the
overload condition is removed. In this mode, if die tem-
perature exceeds +170°C (typ) due to self heating, the
MAX14523C goes into thermal shutdown until the die
temperature drops by approximately 15°C (see the
Thermal Shutdown section).
Switch-On/Off Control
The ON input for the MAX14523A/MAX14523B/
MAX14523C and ON input for the MAX14523AL control
the switch, see Table 1. Toggle ON for the MAX14523B
to reset the fault condition once the short-circuit current
is detected and the device shuts down.
Reverse-Current Protection
The MAX14523A/MAX14523AL/MAX14523B/MAX14523C
feature a reverse-current protection circuit that limits
the backflow current to 2µA when the reverse-current
protection circuitry has tripped. The switch turns off and
FLAG asserts without waiting for t
BLANK to elapse. The
switch turns back on and FLAG deasserts when V IN -
V OUT drops below the reverse-current threshold by
95mV (typ).
FLAG Indicator
FLAG is an open-drain fault indicator output and
requires an external pullup resistor to a DC supply.
FLAG goes low when any of the following conditions
occur:
• The blanking time has elapsed
• The reverse-current protection has tripped
• The die temperature exceeds +170°C
• SETI is connected to ground.
Thermal Shutdown
Thermal-shutdown circuitry protects the devices from
overheating. The switch turns off and FLAG goes low
immediately when the junction temperature exceeds
+170°C (typ). The switch turns on again after the
device temperature drops by approximately 15°C (typ).
Applications Information
Setting the Current Limit/Threshold
A resistor from SETI to ground programs the current
limit/threshold value for the MAX14523A/MAX14523AL/
MAX14523B/MAX14523C. Table 2 lists various current
limit/thresholds set by different resistor values at SETI.
Leaving SETI unconnected selects a 0 current
limit/threshold. Connecting SETI to ground asserts
FLAG.
Use the following formula to calculate the current limit:
R k
V
I mA
k
SETI
LIM
( )
( )
( )
. ( ) Ω Ω = −
141400
2 48
MAX14523A
MAX14523B
MAX14523C
MAX14523AL
ON ON
SWITCH STATUS
0 1 Off
1 0 On
Table 1. Switch Truth Table