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Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram

Switching Power Supply Voltages with Transistors

Transistors are commonly employed to switch power supply voltages. MOSFETs are particularly popular due to their low ‘on’ resistance and their availability for handling large currents. However, both discrete transistors and MOSFETs lack built-in protective functions like current limiting and overtemperature protection.

Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram

Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram 2

Addressing the Issue with MIC2545A from Micrel

The MIC2545A by Micrel offers a solution to this challenge. This MOSFET switch is equipped with programmable current limiting, along with undervoltage and overtemperature safeguards. It operates effectively within a voltage range of +2.7 V to +5.5 V. Featuring a remarkably low typical ‘on’ resistance of only 35 mΩ, this IC can handle switching tasks up to 2.5 A and comes in DIP8, SO8, or TSSOP14 packages. Additionally, it incorporates a soft-start circuit, which restricts the initial switch-on current for the first two milliseconds. The integrated charge pump generates the requisite gate voltage for controlling the MOSFET.

Configurable Current Limiting and Versatility

The current limiting level can be easily established using an external resistor connected between the ILIM pin and ground. The resistance value can be computed using a straightforward formula: Rset = 230 / Ilim, where Ilim represents the current in amperes, and Rset is in ohms. For a maximum current range between 0.5 and 2.5 A, the resistance value falls within the spectrum of 460 Ω to 92 Ω. In case of a short circuit, the current is restricted to approximately 1.6 times Ilim. The MIC2454A operation is controlled through an Enable input. To cater to a broad range of applications, it is accessible in two different versions.

MIC2454A Versions and Power Consumption

The MIC2454A-1 activates the MOSFET when the Enable input is set to High (Vin > 2.4 V), whereas the MIC2454A-2 version triggers the MOSFET with a Low Enable input (Vin < 0.8 V). During operation, the IC typically consumes 90 µA when the switch is active, dropping to less than 1 µA in the switched-off state. This low power usage makes it suitable for battery-operated devices, eliminating the need for a mechanical battery switch.

Operating State Indication and Error Conditions

The high-side switch’s operating status is denoted by an open-gate flag output. An error condition, whether it’s overcurrent, undervoltage, or overtemperature, is signified by a low resistance at this output, pulling an external pull-up resistor to ground.

Overtemperature Protection and MIC2549A Derivative

When the chip temperature reaches around 130°C, the overtemperature cutout is triggered. The switch can be reactivated once the temperature falls below 120°C. In cases where preserving the overtemperature state and preventing automatic reactivation after an overtemperature incident is necessary, the MIC2549A derivative can be employed. It includes a flip-flop that requires resetting through deactivating the Enable signal before re-enabling the switch. The MIC2549A comes in two versions: MIC2549-1 with active high Enable and MIC2549-2 with active low Enable.

Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram 3

Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram 4

Switch ICs with Adjustable Current Limiting Schematic Circuit Diagram 5

Enhancing Switch-On Current with MIC2545A

The MIC2545A introduces a noteworthy feature enabling an increase in the switch-on current for the subsequent assembly. By incorporating a parallel series RC combination with Rset, the effective resistance connected to the ILIM pin experiences a temporary reduction immediately after switch-on. During the charging period of the capacitor (corresponding to the time constant of the RC combination, t = RC), the two resistors are in parallel, boosting the current limit value. Once the capacitor is charged, only the standard resistor remains effective.

Adjusting Current Limiting Level with Transistor Control

Another intriguing possibility involves altering the current limiting level using a transistor controlled, for instance, by a reset IC or a supply voltage monitoring IC. This approach enables the limitation of the switch-on current to a lower level. When the input voltage falls below the required threshold, the current limiting level switches to a lower value since the RESET signal is Low, and the FET is cut off, rendering only one of the two resistors active. Once the input voltage reaches the acceptable level, the RESET signal goes High, activating the FET. Both resistors are now connected in parallel, elevating the current limiting level. For additional details, further information can be found at www.micrel.com.


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