Step-up Switching Regulator with Integrated Current Limit Schematic Circuit Diagram

LT1618: A Step-Up Switching Regulator with Current Limit

Linear Technology, through its LT1618 model available at www.linear.com, offers a step-up switching regulator equipped with a current limit mechanism. This feature simplifies the process of safeguarding a switching regulator that lacks short-circuit protection. The input voltage remains consistently connected to the output circuit through an inductor and a diode, ensuring continuous functionality.

Limiting Input Current (Figure 1) for Circuit Protection

In Figure 1, it is possible to restrict the current at the input, thereby regulating the overall current consumed by the entire circuit. Conversely, as depicted in Figure 2, the output current can be limited. This flexibility empowers the creation of constant current sources operating at voltages surpassing the input voltage. In the illustrated circuit, the step-up switching regulator maintains a nominal output voltage of approximately 22 V.

The output voltage can be calculated using the formula:

Vout = 1.263 V (1+R1/R2)

The output current can be set via R3 as follows:

Imax = Vsense / R3

where Vsense = 50 mV

The IADJ input can be set to a voltage between 0 V and +1.58 V resulting in a linear reduction of the limit current.

The sense voltage of 50 mV across R3 for maximum current is reduced as follows:

Vsens = 0.04 (1.263 V – 0.8 VIADJ )

Hence, for a fixed value of R3, the VIADJ input allows the current limit to be adjusted.

Proper Placement of Sense Resistor R3 in the First Circuit

In the initial circuit configuration, it’s crucial to note that the sense resistor R3 is positioned between the input electrolytic capacitor and the inductor. If R3 is placed before the capacitor, it disrupts the effective control of the inductor current, leading to operational issues.

LT1618 Specifications and Operating Parameters

The LT1618 is designed to function within input voltage ranges spanning from +1.6 V to +18 V. Its output voltage is required to fall within the range of Vin and +35 V. When a switching current of 1 A flows through pin SW to ground, the circuit can anticipate an output current of approximately 100 mA. The IC operates at a switching frequency of around 1.4 MHz and is conveniently packaged in a compact 10-pin MSOP package, enhancing its practical usability.

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