DESCRIPTION

Evaluation circuit EVAL-LT8350-AZ is a 40V synchronous

buck-boost converter featuring the LT®8350. 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

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 this 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.