Overcurrent Cutout Switches

Over-current sensors using external low-resistance sense resistors are fairly common. However, the members of a new family of ICs from Maxim (www.maxim-ic.com) feature an internal sensor resistor and a switch for disconnecting the load if the current limit is exceeded. The members of this IC family are listed in the table.


Overcurrent Cutout Switches Circuit diagram:

Overcurrent Cutout Switches
 
There are two sorts of over-current switches in the family. The types listed in the ‘Latching’ column store any occurrence of an over current condition and indicate it at the FLAG output until they are switched off and then on again by a pulse on the ON input. The types in the ‘Auto-Retry’ column automatically attempt to reconnect the load after a delay time. When the delay time expires, they check whether an over-current recurs, and if necessary they immediately switch off again.  The auto-retry types do not have a /FLAG output.

They switch on for approximately 40 ms every 300 ms (typical) to measure the current. During this 40-ms ‘blanking time’, the IC checks whether the current is less than the selected limit level. The latching types have the same time delay before the switch opens and the FLAG output is asserted. The FLAG output can act as signal for a microcontroller or simply drive an LED. In the latter case, the input voltage must be greater than the forward voltage of the LED. R1 must be dimensioned for the desired current through the LED. Capacitors C1 and C2 provide decoupling and prevent false triggering of the IC by spurious voltage spikes.

Overcurrent Cutout Switches


The MAX47xx family of ICs operates over a supply voltage range of +2.3–5.5 V. The ICs have under voltage lockout (UVLO) and reliably switch off when the current exceeds the type-specific limit, even if the current flows in the reverse direction (from the load to the input). The table indicates the possible range of the over-current threshold for each type. For instance, a given MAX2791 might switch off at a current as low as 250 mA. How-ever, other examples of the same type will not switch off until the current reaches 350 mA. The same threshold values apply to reverse currents. An over temperature cutout circuit protects the IC against thermal destruction.

The latching types come in a 5-pin SMD package, while the auto-retry types without a /FLAG output manage with only four pins. The 50-mA and 100-mA versions fit into the tiny SC70 package. The types for higher current levels require an SOT23 or SOT143 package.  There are also other Maxim ICs with similar functions, such as the MAX4795–MAX4798 series with typical cutoff thresholds of 450 mA and 500 mA. Finally, there are the MAX4772 and MAX4773, which have a programmable threshold that can be set to 200 mA or 500 mA using a Select input. However, the IC types mentioned in this paragraph require a different circuit arrangement than what is shown here.

Author : Gregor Kleine – Copyright : Elektor

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