Evaluation circuit EVAL-LT8350-AZ is a 40V synchronous buck-boost converter featuring the LT8350. It drives up to 2.5A load at 12V output when VIN is between 9V and 40V and will run down to 3VIN with reduced output current. EVAL-LT8350-AZ runs at 350kHz switching frequency with spread spectrum frequency modulation (SSFM) disabled. When enabled, SSFM spreads the switching frequency of the LT8350 from fSW to fSW +25% for reduced EMI emission.
The LT8350 has an operating input voltage range of 3V to 40V. It has internal, synchronous 42V MOSFETs at buck-side and 20V MOSFETs at boost-side for high efficiency and small size. It has an adjustable switching frequency between 200kHz and 2MHz. The LT8350 can be synchronized to an external source, programmed with SSFM enabled for low EMI, or set to normal operation.
The LT8350’s integrated LOADTG high side PMOS driver assists with disconnecting load when a fault is triggered. LOADTG turns off the PMOS when FB < 0.25V or FB > 1.1V or ISP–ISN > 0.75V. LOADEN can be used directly to turn off LOADTG and all power switches.
The LT8350 can regulate output current using ISP and ISN. Maximum load current can be limited when LT8350 is used for a constant voltage regulator. It can be also used for a LED driver or a battery charger where constant current regulation is required. Output current can be adjusted by placing a controllable DC voltage on the CTRL pin.
The output current can be monitored through the ISMON output pin. ISMON can be used to improve load transient response by injecting load current to VC. The load current injection is described in the following sections.
Undervoltage lockout can be adjusted on EVAL-LT8350-AZ with a few simple resistor choices.
Small ceramic input and output capacitors are used to save space and cost. The board is designed with tiny, high frequency capacitors placed near the VIN and VOUT pins for a reduction in radiated EMI.
There is an inductor EMI filter and a small ferrite bead EMI filter on the input and output of EVAL-LT8350-AZ. These filters, combined with proper board layout and SSFM, are effective in reducing EMI to pass CISPR25 class 5 conducted EMI. Please follow the recommended layout and four-layer PCB thickness of EVAL-LT8350-AZ for low EMI applications.
The LT8350 data sheet gives a complete description of the part, operation, and applications information. The data sheet must be read in conjunction with the demo manual for EVAL-LT8350-AZ. The LT8350RV is assembled in a 32-lead laminate package with QFN footprint (LQFN) with thermally enhanced exposed ground pads. Proper board layout is essential for maximum thermal performance.
