Buck Converter with Resistor-Based Current Sensing
Created: Sep 28, 2016
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Buck converters are electrical circuit that effectively transfer energy from a DC voltage source to a load. It is transferred through electronic switches to energy storage devices and subsequently switched from storage into the load. The switches are transistors and diodes while the storage devices are inductors and capacitors. This process of energy transfer results in an output voltage that is related to the input voltage by the duty ratios of the switches. This buck converter uses MAX17558 dual-output synchronous step down controller that operates from a 4.5V to 60V wide input supply range with programmable output voltage ranging from 0.8V to 24V.
This design is a buck converter with resistor based current sensing that has a high efficiency output of 5V and 3.3V . A typical current-sensing circuit using a discrete resistor is incorporated in the circuit that detects and converts current to an easily measured output voltage proportional to the current through the measured path. The two controllers of the IC can be independently shut down and enabled using the EN1 and EN2 pins. Pulling either of these pins below 1.25V (typ) shuts down the corresponding controller. Pulling both EN1 and EN2 below 0.7V disables both controllers and most internal circuits, including the VCCINT LDOs. In this state, the device draws only 10µA (typ) of quiescent current. The startup of each controller’s output voltage is controlled by the voltage on the relevant SS pin for that controller. VCCINT output voltage powers the gate drivers and internal control circuitry. VCCINT should be decoupled to PGND with at least a 4.7µF low-ESR ceramic capacitor. High-side gate drivers are powered from the bootstrap capacitors connected between BST_ and LX_. The bootstrap capacitor normally gets charged to VCCINT during each switching cycle through an external Schottky diode, when the low-side MOSFET turns on. Also, low-side external MOSFET gate drivers are powered from VCCINT and under normal operating conditions, the lowside gate-driver output (DL) is always the complement of the high-side gate-driver output (DH). On each controller, dedicated circuitry monitors the DH and DL outputs and prevents either gate-drive signal from turning on until the other gate-drive signal is fully off.Thus, allowing DH to turn on only when DL has been turned off.
This high efficiency buck converter has various applications that can be used in industrial power supplies and distributed DC power systems. It can also be integrated for motion control, programmable logic controllers and computerized numerical control.